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Unit III
INDUCTION MOTOR
An induction motor or asynchronous motor is a type of alternating current motor where
power is supplied to the rotor by means of electromagnetic induction
An electric motor converts electrical power to mechanical power in its rotor (rotating part)
There are several ways to supply power to the rotor In a DC motor this power is supplied to the
armature directly from a DC source while in an induction motor this power is induced in the
rotating device An induction motor is sometimes called a rotating transformer because the stator
(stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the
secondary side Unlike the normal transformer which changes the current by using time varying
flux induction motors use rotating magnetic fields to transform the voltage The primary sides
current creates an electromagnetic field which interacts with the secondary sides electromagnetic
field to produce a resultant torque thereby transforming the electrical energy into mechanical
energy Induction motors are widely used especially polyphase induction motors which are
frequently used in industrial drives
Induction motors are now the preferred choice for industrial motors due to their rugged
construction absence of brushes (which are required in most DC motors) andmdashthanks to modern
power electronicsmdashthe ability to control the speed of the motor
Principle of operation and comparison to synchronous motors
3-phase power supply provides a rotating magnetic field in an induction motor The basic
difference between an induction motor and a synchronous AC motor is that in the latter a current is
supplied into the rotor (usually DC) which in turn creates a (circular uniform) magnetic field
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around the rotor The rotating magnetic field of the stator will impose an
electromagnetic torque on
the still magnetic field of the rotor causing it to move (about a shaft) and rotation of the rotor is
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produced It is called synchronous because at steady state the speed of the rotor is the same as the
speed of the rotating magnetic field in the stator
By way of contrast the induction motor does not have any direct supply onto the rotor instead a
secondary current is induced in the rotor To achieve this stator windings are arranged around the
rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern
which sweeps past the rotor This changing magnetic field pattern induces current in the rotor
conductors These currents interact with the rotating magnetic field created by the stator and in
effect causes a rotational motion on the rotor
However for these currents to be induced the speed of the physical rotor must be less than the
speed of the rotating magnetic field in the stator or else the magnetic field will not be moving
relative to the rotor conductors and no currents will be induced If by some chance this happens
the rotor typically slows slightly until a current is re-induced and then the rotor continues as
before This difference between the speed of the rotor and speed of the rotating magnetic field in
the stator is called slip It is unitless and is the ratio between the relative speed of the magnetic
field as seen by the rotor (the slip speed) to the speed of the rotating stator field Due to this an
induction motor is sometimes referred to as an asynchronous machine
Construction
The stator consists of wound poles that carry the supply current to induce a magnetic field that
penetrates the rotor In a very simple motor there would be a single projecting piece of the stator
(a salient pole) for each pole with windings around it in fact to optimize the distribution of the
magnetic field the windings are distributed in many slots located around the stator but the
magnetic field still has the same number of north-south alternations The number of poles can
vary between motor types but the poles are always in pairs (ie 2 4 6 etc)
Induction motors are most commonly built to run on single-phase or three-phase power but two-
phase motors also exist In theory two-phase and more than three phase induction motors are
possible many single-phase motors having two windings and requiring a capacitor can actually be
viewed as two-phase motors since the capacitor generates a second power phase 90 degrees from
the single-phase supply and feeds it to a separate motor winding Single-phase power is more
widely available in residential buildings but cannot produce a rotating field in the motor (the field
merely oscillates back and forth) so single-phase induction motors must incorporate some kind of
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starting mechanism to produce a rotating field They would using the simplified analogy of salient
poles have one salient pole per pole number a four-pole motor would have four salient poles
Three-phase motors have three salient poles per pole number so a four-pole motor would have
twelve salient poles This allows the motor to produce a rotating field allowing the motor to start
with no extra equipment and run more efficiently than a similar single-phase motor
There are three types of rotor
Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor It is made up of bars of either solid copper (most
common) or aluminum that span the length of the rotor and those solid copper or aluminium strips
can be shorted or connected by a ring or some times not ie the rotor can be closed or semiclosed
type The rotor bars in squirrel-cage induction motors are not straight but have some skew to
reduce noise and harmonics
Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to
slip rings When these slip rings are shorted the rotor behaves similarly to a squirrel-cage rotor
they can also be connected to resistors to produce a high-resistance rotor circuit which can be
beneficial in starting
Solid core rotor
A rotor can be made from a solid mild steel The induced current causes the
rotation
Speed control
The synchronous rotational speed of the rotor (ie the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency of
the supply voltage Under load the induction motors speed varies according to size of the load As
the load is increased the speed of the motor decreases increasing the slip which increases the
rotors field strength to bear the extra load Before the development of economical semiconductor
power electronics it was difficult to vary the frequency to the motor and induction motors were
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mainly used in fixed speed applications As an induction motor has no brushes and is easy to
control many older DC motors are now being replaced with induction motors and accompanying
inverters in industrial applications
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase induction motor is to connect its terminals to the line This
method is often called direct on line and abbreviated DOL
In an induction motor the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor
and the rotor current depends on this emf When the motor is started the rotor speed is zero The
synchronous speed is constant based on the frequency of the supplied AC voltage So the slip
speed is equal to the synchronous speed the slip ratio is 1 and the induced emf in the rotor is
large As a result a very high current flows through the rotor This is similar to a transformer with
the secondary coil short circuited which causes the primary coil to draw a high current from the
mains
When an induction motor starts DOL a very high current is drawn by the stator in the order of 5
to 9 times the full load current This high current can in some motors damage the windings in
addition because it causes heavy line voltage drop other appliances connected to the same line
may be affected by the voltage fluctuation To avoid such effects several other strategies are
employed for starting motors
Wye-Delta starters
An induction motors windings can be connected to a 3-phase AC line in two
different ways
wye in US star in Europe where the windings are connected from phases of the supply to
the neutral
delta (sometimes mesh in Europe) where the windings are connected
between phases of
the supply
A delta connection of the machine winding results in a higher voltage at each winding compared to
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a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
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shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
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positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
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order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
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maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
C
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In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
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Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
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R
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
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as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
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around the rotor The rotating magnetic field of the stator will impose an
electromagnetic torque on
the still magnetic field of the rotor causing it to move (about a shaft) and rotation of the rotor is
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produced It is called synchronous because at steady state the speed of the rotor is the same as the
speed of the rotating magnetic field in the stator
By way of contrast the induction motor does not have any direct supply onto the rotor instead a
secondary current is induced in the rotor To achieve this stator windings are arranged around the
rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern
which sweeps past the rotor This changing magnetic field pattern induces current in the rotor
conductors These currents interact with the rotating magnetic field created by the stator and in
effect causes a rotational motion on the rotor
However for these currents to be induced the speed of the physical rotor must be less than the
speed of the rotating magnetic field in the stator or else the magnetic field will not be moving
relative to the rotor conductors and no currents will be induced If by some chance this happens
the rotor typically slows slightly until a current is re-induced and then the rotor continues as
before This difference between the speed of the rotor and speed of the rotating magnetic field in
the stator is called slip It is unitless and is the ratio between the relative speed of the magnetic
field as seen by the rotor (the slip speed) to the speed of the rotating stator field Due to this an
induction motor is sometimes referred to as an asynchronous machine
Construction
The stator consists of wound poles that carry the supply current to induce a magnetic field that
penetrates the rotor In a very simple motor there would be a single projecting piece of the stator
(a salient pole) for each pole with windings around it in fact to optimize the distribution of the
magnetic field the windings are distributed in many slots located around the stator but the
magnetic field still has the same number of north-south alternations The number of poles can
vary between motor types but the poles are always in pairs (ie 2 4 6 etc)
Induction motors are most commonly built to run on single-phase or three-phase power but two-
phase motors also exist In theory two-phase and more than three phase induction motors are
possible many single-phase motors having two windings and requiring a capacitor can actually be
viewed as two-phase motors since the capacitor generates a second power phase 90 degrees from
the single-phase supply and feeds it to a separate motor winding Single-phase power is more
widely available in residential buildings but cannot produce a rotating field in the motor (the field
merely oscillates back and forth) so single-phase induction motors must incorporate some kind of
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starting mechanism to produce a rotating field They would using the simplified analogy of salient
poles have one salient pole per pole number a four-pole motor would have four salient poles
Three-phase motors have three salient poles per pole number so a four-pole motor would have
twelve salient poles This allows the motor to produce a rotating field allowing the motor to start
with no extra equipment and run more efficiently than a similar single-phase motor
There are three types of rotor
Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor It is made up of bars of either solid copper (most
common) or aluminum that span the length of the rotor and those solid copper or aluminium strips
can be shorted or connected by a ring or some times not ie the rotor can be closed or semiclosed
type The rotor bars in squirrel-cage induction motors are not straight but have some skew to
reduce noise and harmonics
Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to
slip rings When these slip rings are shorted the rotor behaves similarly to a squirrel-cage rotor
they can also be connected to resistors to produce a high-resistance rotor circuit which can be
beneficial in starting
Solid core rotor
A rotor can be made from a solid mild steel The induced current causes the
rotation
Speed control
The synchronous rotational speed of the rotor (ie the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency of
the supply voltage Under load the induction motors speed varies according to size of the load As
the load is increased the speed of the motor decreases increasing the slip which increases the
rotors field strength to bear the extra load Before the development of economical semiconductor
power electronics it was difficult to vary the frequency to the motor and induction motors were
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mainly used in fixed speed applications As an induction motor has no brushes and is easy to
control many older DC motors are now being replaced with induction motors and accompanying
inverters in industrial applications
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase induction motor is to connect its terminals to the line This
method is often called direct on line and abbreviated DOL
In an induction motor the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor
and the rotor current depends on this emf When the motor is started the rotor speed is zero The
synchronous speed is constant based on the frequency of the supplied AC voltage So the slip
speed is equal to the synchronous speed the slip ratio is 1 and the induced emf in the rotor is
large As a result a very high current flows through the rotor This is similar to a transformer with
the secondary coil short circuited which causes the primary coil to draw a high current from the
mains
When an induction motor starts DOL a very high current is drawn by the stator in the order of 5
to 9 times the full load current This high current can in some motors damage the windings in
addition because it causes heavy line voltage drop other appliances connected to the same line
may be affected by the voltage fluctuation To avoid such effects several other strategies are
employed for starting motors
Wye-Delta starters
An induction motors windings can be connected to a 3-phase AC line in two
different ways
wye in US star in Europe where the windings are connected from phases of the supply to
the neutral
delta (sometimes mesh in Europe) where the windings are connected
between phases of
the supply
A delta connection of the machine winding results in a higher voltage at each winding compared to
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a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
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shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
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positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
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order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
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maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
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Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
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as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
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produced It is called synchronous because at steady state the speed of the rotor is the same as the
speed of the rotating magnetic field in the stator
By way of contrast the induction motor does not have any direct supply onto the rotor instead a
secondary current is induced in the rotor To achieve this stator windings are arranged around the
rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern
which sweeps past the rotor This changing magnetic field pattern induces current in the rotor
conductors These currents interact with the rotating magnetic field created by the stator and in
effect causes a rotational motion on the rotor
However for these currents to be induced the speed of the physical rotor must be less than the
speed of the rotating magnetic field in the stator or else the magnetic field will not be moving
relative to the rotor conductors and no currents will be induced If by some chance this happens
the rotor typically slows slightly until a current is re-induced and then the rotor continues as
before This difference between the speed of the rotor and speed of the rotating magnetic field in
the stator is called slip It is unitless and is the ratio between the relative speed of the magnetic
field as seen by the rotor (the slip speed) to the speed of the rotating stator field Due to this an
induction motor is sometimes referred to as an asynchronous machine
Construction
The stator consists of wound poles that carry the supply current to induce a magnetic field that
penetrates the rotor In a very simple motor there would be a single projecting piece of the stator
(a salient pole) for each pole with windings around it in fact to optimize the distribution of the
magnetic field the windings are distributed in many slots located around the stator but the
magnetic field still has the same number of north-south alternations The number of poles can
vary between motor types but the poles are always in pairs (ie 2 4 6 etc)
Induction motors are most commonly built to run on single-phase or three-phase power but two-
phase motors also exist In theory two-phase and more than three phase induction motors are
possible many single-phase motors having two windings and requiring a capacitor can actually be
viewed as two-phase motors since the capacitor generates a second power phase 90 degrees from
the single-phase supply and feeds it to a separate motor winding Single-phase power is more
widely available in residential buildings but cannot produce a rotating field in the motor (the field
merely oscillates back and forth) so single-phase induction motors must incorporate some kind of
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starting mechanism to produce a rotating field They would using the simplified analogy of salient
poles have one salient pole per pole number a four-pole motor would have four salient poles
Three-phase motors have three salient poles per pole number so a four-pole motor would have
twelve salient poles This allows the motor to produce a rotating field allowing the motor to start
with no extra equipment and run more efficiently than a similar single-phase motor
There are three types of rotor
Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor It is made up of bars of either solid copper (most
common) or aluminum that span the length of the rotor and those solid copper or aluminium strips
can be shorted or connected by a ring or some times not ie the rotor can be closed or semiclosed
type The rotor bars in squirrel-cage induction motors are not straight but have some skew to
reduce noise and harmonics
Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to
slip rings When these slip rings are shorted the rotor behaves similarly to a squirrel-cage rotor
they can also be connected to resistors to produce a high-resistance rotor circuit which can be
beneficial in starting
Solid core rotor
A rotor can be made from a solid mild steel The induced current causes the
rotation
Speed control
The synchronous rotational speed of the rotor (ie the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency of
the supply voltage Under load the induction motors speed varies according to size of the load As
the load is increased the speed of the motor decreases increasing the slip which increases the
rotors field strength to bear the extra load Before the development of economical semiconductor
power electronics it was difficult to vary the frequency to the motor and induction motors were
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mainly used in fixed speed applications As an induction motor has no brushes and is easy to
control many older DC motors are now being replaced with induction motors and accompanying
inverters in industrial applications
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase induction motor is to connect its terminals to the line This
method is often called direct on line and abbreviated DOL
In an induction motor the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor
and the rotor current depends on this emf When the motor is started the rotor speed is zero The
synchronous speed is constant based on the frequency of the supplied AC voltage So the slip
speed is equal to the synchronous speed the slip ratio is 1 and the induced emf in the rotor is
large As a result a very high current flows through the rotor This is similar to a transformer with
the secondary coil short circuited which causes the primary coil to draw a high current from the
mains
When an induction motor starts DOL a very high current is drawn by the stator in the order of 5
to 9 times the full load current This high current can in some motors damage the windings in
addition because it causes heavy line voltage drop other appliances connected to the same line
may be affected by the voltage fluctuation To avoid such effects several other strategies are
employed for starting motors
Wye-Delta starters
An induction motors windings can be connected to a 3-phase AC line in two
different ways
wye in US star in Europe where the windings are connected from phases of the supply to
the neutral
delta (sometimes mesh in Europe) where the windings are connected
between phases of
the supply
A delta connection of the machine winding results in a higher voltage at each winding compared to
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a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
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shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
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positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
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order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
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maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
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Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
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as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
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starting mechanism to produce a rotating field They would using the simplified analogy of salient
poles have one salient pole per pole number a four-pole motor would have four salient poles
Three-phase motors have three salient poles per pole number so a four-pole motor would have
twelve salient poles This allows the motor to produce a rotating field allowing the motor to start
with no extra equipment and run more efficiently than a similar single-phase motor
There are three types of rotor
Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor It is made up of bars of either solid copper (most
common) or aluminum that span the length of the rotor and those solid copper or aluminium strips
can be shorted or connected by a ring or some times not ie the rotor can be closed or semiclosed
type The rotor bars in squirrel-cage induction motors are not straight but have some skew to
reduce noise and harmonics
Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to
slip rings When these slip rings are shorted the rotor behaves similarly to a squirrel-cage rotor
they can also be connected to resistors to produce a high-resistance rotor circuit which can be
beneficial in starting
Solid core rotor
A rotor can be made from a solid mild steel The induced current causes the
rotation
Speed control
The synchronous rotational speed of the rotor (ie the theoretical unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency of
the supply voltage Under load the induction motors speed varies according to size of the load As
the load is increased the speed of the motor decreases increasing the slip which increases the
rotors field strength to bear the extra load Before the development of economical semiconductor
power electronics it was difficult to vary the frequency to the motor and induction motors were
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
mainly used in fixed speed applications As an induction motor has no brushes and is easy to
control many older DC motors are now being replaced with induction motors and accompanying
inverters in industrial applications
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase induction motor is to connect its terminals to the line This
method is often called direct on line and abbreviated DOL
In an induction motor the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor
and the rotor current depends on this emf When the motor is started the rotor speed is zero The
synchronous speed is constant based on the frequency of the supplied AC voltage So the slip
speed is equal to the synchronous speed the slip ratio is 1 and the induced emf in the rotor is
large As a result a very high current flows through the rotor This is similar to a transformer with
the secondary coil short circuited which causes the primary coil to draw a high current from the
mains
When an induction motor starts DOL a very high current is drawn by the stator in the order of 5
to 9 times the full load current This high current can in some motors damage the windings in
addition because it causes heavy line voltage drop other appliances connected to the same line
may be affected by the voltage fluctuation To avoid such effects several other strategies are
employed for starting motors
Wye-Delta starters
An induction motors windings can be connected to a 3-phase AC line in two
different ways
wye in US star in Europe where the windings are connected from phases of the supply to
the neutral
delta (sometimes mesh in Europe) where the windings are connected
between phases of
the supply
A delta connection of the machine winding results in a higher voltage at each winding compared to
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
WWWVIDYARTHIPLUSCOM
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positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
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order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
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maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
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Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
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as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
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mainly used in fixed speed applications As an induction motor has no brushes and is easy to
control many older DC motors are now being replaced with induction motors and accompanying
inverters in industrial applications
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase induction motor is to connect its terminals to the line This
method is often called direct on line and abbreviated DOL
In an induction motor the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor
and the rotor current depends on this emf When the motor is started the rotor speed is zero The
synchronous speed is constant based on the frequency of the supplied AC voltage So the slip
speed is equal to the synchronous speed the slip ratio is 1 and the induced emf in the rotor is
large As a result a very high current flows through the rotor This is similar to a transformer with
the secondary coil short circuited which causes the primary coil to draw a high current from the
mains
When an induction motor starts DOL a very high current is drawn by the stator in the order of 5
to 9 times the full load current This high current can in some motors damage the windings in
addition because it causes heavy line voltage drop other appliances connected to the same line
may be affected by the voltage fluctuation To avoid such effects several other strategies are
employed for starting motors
Wye-Delta starters
An induction motors windings can be connected to a 3-phase AC line in two
different ways
wye in US star in Europe where the windings are connected from phases of the supply to
the neutral
delta (sometimes mesh in Europe) where the windings are connected
between phases of
the supply
A delta connection of the machine winding results in a higher voltage at each winding compared to
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a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
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shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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WWWVIDYARTHIPLUSCOM V+TEAM
Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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WWWVIDYARTHIPLUSCOM
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In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
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Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
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a wye connection (the factor is ) A wye-delta starter initially connects the motor in wye which
produces a lower starting current than delta then switches to delta when the motor has reached a
set speed Disadvantages of this method over DOL starting are
Lower starting torque which may be a serious issue with pumps or any devices with
significant breakaway torque
Increased complexity as more contactors and some sort of speed switch or
timers are
needed
Two shocks to the motor (one for the initial start and another when the
motor switches
from wye to delta)
Variable-frequency drives
Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors
A VFD can easily start a motor at a lower frequency than the AC line as well as a lower voltage
so that the motor starts with full rated torque and with no inrush of current The rotor circuits
impedance increases with slip frequency which is equal to supply frequency for a stationary rotor
so running at a lower frequency actually increases torque
Resistance starters
A resistance starter and its 4MW 11kV induction motor driving a ball mill
This method is used with slip ring motors where the rotor poles can be accessed by way of the slip
rings Using brushes variable power resistors are connected in series with the poles During start-
up the resistance is large and then reduced to zero at full speed
At start-up the resistance directly reduces the rotor current and so rotor heating is reduced Another
important advantage is the start-up torque can be controlled As well the resistors generate a phase
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
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Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
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shift in the field resulting in the magnetic force acting on the rotor having a favorable angle
Series Reactor starters
In series reactor starter technology an impedance in the form of a reactor is introduced in series
with the motor terminals which as a result reduces the motor terminal voltage resulting in a
reduction of the starting current the impedance of the reactor a function of the current passing
through it gradually reduces as the motor accelerates and at 95 speed the reactors are bypassed
by a suitable bypass method which enables the motor to run at full voltage and full speed Air core
series reactor starters or a series reactor soft starter is the most common and recommended method
for fixed speed motor starting The applicable standards are [IEC 289] AND [IS 5553 (PART 3) ]
Single Phase induction motor
In a single phase induction motor it is necessary to provide a starting circuit to start rotation of the
rotor If this is not done rotation may be commenced by manually giving a slight turn to the rotor
The single phase induction motor may rotate in either direction and it is only the starting circuit
which determines rotational direction
For small motors of a few watts the start rotation is done by means of a single turn of heavy copper
wire around one corner of the pole The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field which imparts to
the field sufficient rotational character to start the motor Starting torque is very
low and efficiency
is also reduced Such shaded-pole motors are typically used in low-power applications with low or
zero starting torque requirements such as desk fans and record players
Larger motors are provided with a second stator winding which is fed with an out -of-phase current
to create a rotating magnetic field The out-of-phase current may be derived by feeding the
winding through a capacitor or it may derive from the winding having different values of
inductance and resistance from the main winding
In some designs the second winding is disconnected once the motor is up to speed usually either
by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a
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positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
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WWWVIDYARTHIPLUSCOM V+TEAM
Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
positive temperature coefficient thermistor which after a few seconds of operation heats up and
increases its resistance to a high value reducing the current through the second winding to an
insignificant level Other designs keep the second winding continuously energised during running
which improves torque
Control of speed in induction motor can be obtained in 3 ways
1scalar control
2vector control
3direct torque control
Rotating magnetic field
Description
A symmetric rotating magnetic field can be produced with as few as three coils The three coils
will have to be driven by a symmetric 3-phase AC sine current system thus each phase will be
shifted 120 degrees in phase from the others For the purpose of this example the magnetic field is
taken to be the linear function of the coils current
Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis
Magnetic fields add as vectors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of
resulting rotating magnetic field
The result of adding three 120-degrees phased sine waves on the axis of the motor is a single
rotating vector The rotor has a constant magnetic field The N pole of the rotor will move toward
the S pole of the magnetic field of the stator and vice versa This magneto-mechanical attraction
creates a force which will drive rotor to follow the rotating magnetic field in a synchronous
manner
A permanent magnet in such a field will rotate so as to maintain its alignment with the external
field This effect was utilized in early alternating current electric motors A rotating magnetic field
can be constructed using two orthogonal coils with a 90 degree phase difference in their AC
currents However in practice such a system would be supplied through a three-wire arrangement
with unequal currents This inequality would cause serious problems in the standardization of the
conductor size In order to overcome this three-phase systems are used where the three currents
are equal in magnitude and have a 120 degree phase difference Three similar coils having mutual
geometrical angles of 120 degrees will create the rotating magnetic field in this
case The ability of
the three phase system to create the rotating field utilized in electric motors is one of the main
reasons why three phase systems dominate in the world electric power supply systems
Rotating magnetic fields are also used in induction motors Because magnets degrade with time
induction motors use short-circuited rotors (instead of a magnet) which follow the rotating
magnetic field of a multicoiled stator In these motors the short circuited turns of the rotor develop
eddy currents in the rotating field of stator which in turn move the rotor by Lorentz force These
types of motors are not usually synchronous but instead necessarily involve a degree of slip in
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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o
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m
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o
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
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If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
order that the current may be produced due to the relative movement of the field and the rotor
3-φmotor runs from 1-φ power but does not start
The single coil of a single phase induction motor does not produce a rotating magnetic field but a
pulsating field reaching maximum intensity at 0o
and 180o
electrical (Figure below)
Single phase stator produces a nonrotating pulsating magnetic field
Another view is that the single coil excited by a single phase current produces two counter rotating
magnetic field phasors coinciding twice per revolution at 0o
(Figure above-a) and 180o
(figure
e) When the phasors rotate to 90o
and -90o
they cancel in figure b At 45o
and -45o
(figure c)
they are partially additive along the +x axis and cancel along the y axis An analogous situation
exists in figure d The sum of these two phasors is a phasor stationary in space but alternating
polarity in time Thus no starting torque is developed
However if the rotor is rotated forward at a bit less than the synchronous speed It will develop
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WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
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WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
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in
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u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
maximum torque at 10 slip with respect to the forward rotating phasor Less torque will be
developed above or below 10 slip The rotor will see 200 - 10 slip with respect to the counter
rotating magnetic field phasor Little torque (see torque vs slip curve) other than a double freqency
ripple is developed from the counter rotating phasor Thus the single phase coil will develop
torque once the rotor is started If the rotor is started in the reverse direction it will develop a
similar large torque as it nears the speed of the backward rotating phasor
Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of
steel laminations typical of poly-phase induction motors
Permanent-split capacitor motor
One way to solve the single phase problem is to build a 2-phase motor deriving 2-phase power
from single phase This requires a motor with two windings spaced apart 90o
electrical fed with
two phases of current displaced 90o
in time This is called a permanent-split capacitor motor in
Figure below
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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m
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
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in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Permanent-split capacitor induction motor
This type of motor suffers increased current magnitude and backward time shift as the motor
comes up to speed with torque pulsations at full speed The solution is to keep the capacitor
(impedance) small to minimize losses The losses are less than for a shaded pole motor This
motor configuration works well up to 14 horsepower (200watt) though usually applied to
smaller motors The direction of the motor is easily reversed by switching the capacitor in series
with the other winding This type of motor can be adapted for use as a servo motor described
elsewhere is this chapter
Single phase induction motor with embedded stator coils
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
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m
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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m
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t
o
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
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in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Single phase induction motors may have coils embedded into the stator as shown in Figure
above for larger size motors Though the smaller sizes use less complex to build concentrated
windings with salient poles
C
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o
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m
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WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
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ct
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n
m
o
t
o
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
e
s
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s
t
a
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c
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p
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m
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in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
In Figure below a larger capacitor may be used to start a single phase induction motor via the
auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed
Moreover the auxiliary winding may be many more turns of heavier wire than used in a resistance
split-phase motor to mitigate excessive temperature rise The result is that more starting torque is
available for heavy loads like air conditioning compressors This motor configuration works so
well that it is available in multi-horsepower (multi-kilowatt) sizes
Capacitor-start induction motor
C
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m
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in
d
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ct
io
n
m
o
t
o
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A variation of the capacitor-start motor (Figure below) is to start the motor with a relatively large
capacitor for high starting torque but leave a smaller value capacitor in place after starting to
improve running characteristics while not drawing excessive current The additional complexity of
the capacitor-run motor is justified for larger size motors
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
e
s
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s
t
a
n
c
e
s
p
l
i
t
-
p
h
a
s
e
m
o
t
o
r
in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
Capacitor-run motor induction motor
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be
two + to + (or - to -) series connected polarized electrolytic capacitors Such AC rated electrolytic
capacitors have such high losses that they can only be used for intermittent duty (1 second on 60
seconds off) like motor starting A capacitor for motor running must not be of electrolytic
construction but a lower loss polymer type
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
e
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s
t
a
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c
e
s
p
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s
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m
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o
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in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
R
e
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a
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in
d
u
ct
io
n
m
o
t
o
r
If an auxiliary winding of much fewer turns of smaller wire is placed at 90o
electrical to the
main winding it can start a single phase induction motor (Figure below) With lower
inductance and higher resistance the current will experience less phase shift than the main
winding About 30o
of phase difference may be obtained This coil produces a moderate
starting torque which is
disconnected by a centrifugal switch at 34 of synchronous speed This simple
(no capacitor)
arrangement serves well for motors up to 13 horsepower (250 watts) driving
easily started loads
Resistance split-phase motor induction motor
This motor has more starting torque than a shaded pole motor (next section) but not as much
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
WWWVIDYARTHIPLUSCOM
WWWVIDYARTHIPLUSCOM V+TEAM
as a two phase motor built from the same parts The current density in the auxiliary winding is
so high during starting that the consequent rapid temperature rise precludes frequent restarting
or slow starting loads
