- 1. 1 READING MATERIAL FOR III SEM DIPLOMA EX STUDENTS OF RGPV
AFFILIATED COLLEGES SUBJECT ELECTRICAL MACHINES I Professor MD Dutt
Addl General Manager (Retd) BHARAT HEAVY ELECTRICALS LIMITED
Professor(Retd) in EX Department Bansal Institute of Science and
Technology KOKTA ANANAD NAGAR BHOPAL Presently Head of The
Department ( EX) Shri Ram College Of Technology Thuakheda BHOPAL
Sub Code BE 305 Subject Electrical Machines I UNIT III D.C
MOTORS
2. 2 RGPV DIPLOMA EX BRANCH SEMESTER: THIRD SCHEME: Jul.09
COURSE CODE: 305 PAPER CODE: 6232 NAME OF COURSE: ELECTRICAL
MACHINE I COMMON WITH PROGRAM (S): E01 Syllabus UNIT III D. C.
Motors - Principle, production of back EMF, torque equation.
Classification, characteristics and applications of motors. D. C.
motor starters speed control, losses and efficiency. Brake test,
Swinburne test. Simple numerical. . INDEX S No Topic Page 1
Principle 3 2 Production of back EMF 3 3 Torque equation 4,5 4
Classification 6,7 5 Characteristics 7,8,9,10,11 6 Application of
DC motors 11,12 7 DC motor starters speed control
12,13,14,15,16,17,18,19 8 Losses and efficiency 19,20,21,22,23 9
Break test 23 3. 3 10 Swinburne test 23,24,25,26 Basic principle of
operation of a D.C. Motor When a conductor carrying current is put
in a magnetic field, a force is produced in it. Let us consider one
such conductor placed in a slot of armature and it is acted upon
the magnetic field from the north pole of the motor. By applying
L.H.S it is found that the conductor has tendency to move to the
L.H.S. Since the conductor is placed on the slot at circumference
of rotor , The force acts in a tangential direction of rotor, Thus
a torque is developed on the rotor Similar torques are produced on
all the rotor conductors .Since the rotor is free to move, It
starts rotating.BACK EMF When the motor of armature rotates, its
conductors cut the magnetic flux, Therefore the e.m.f. rotation Er
is induced in them. In case of a motor, the e.m.f is known as BACK
E.M.F or counter e.m.f. The back e.m.f opposes the applied 4. 4
voltage , Since the back E.M.F is induced due to generating action
it magnitude is given Eb = PNZ/60A TORQUE OF A DC MACHINE When the
machine is operating as a motor the torque is transferred to the
shaft of the rotor and drives the mechanical load, The expression
for the torque is same for the generator or motor. The Voltage
equation of DC machine is V= E+IaRa Multiplying with Ia We get V Ia
= EIa +Ia Ra VIa = Electrical input to the armature Ia Ra = copper
loss in the armature We also know that Input = output + losses EIa
= Electrical equivalent of gross mechanical power developed by
armature av = average electromagnetic torque developed by armature
in Newton Mtrs (Nm) Mechanical power developed by the armature 5. 5
Pm = av = 2av Pm = EIa = av = 2av But E = nP Z/A Therefore (nP Z/A)
Ia =2av av = (PZ/2 A ) X Ia This is called Torque equation For a
given machine the value of PZ A are constant Therefore PZ/2 A is
also constant = k av = k Ia EQUIVALENT DIAGRAM aaat fff RIEV RIV
==== ==== 6. 6 CLASSIFICATION OF DC MOTORS A DC motor whose field
winding is supplied itself is called self excited DC machine, A DC
motor whose field winding is supplied separately excited DC motor
In a self excited DC machine the field coils are connected in
parallel with armature winding . DC motors are classified as
follows:- i) Shunt excited DC motor ii) Series excited DC motor
iii) Compound DC motor iv) Separately excited DC Motor i)Shunt
excited DC motor :- In a shunt excited DC motor the field coil
winding is connected parallel to the armature winding Ish = V/ Rsh
Ia = Il Ish Eb = V Ia Ra -2Vb 7. 7 ii) SERIES EXCITED DC MOTOR In
case of series excited DC motor the line current passes through
series field winding and also armature current. Ia = Il = Ise Eb =
V Ia(Ra +Rse) -2 Vb COMPOUND DC MOTOR:- There are two type of
compound DC motors i) Cumulative compound motor ii) Differential
compound motor In the compound wound DC motor the field is produced
by the shunt field as well as series field. Generally shunt field
is stronger than series field. When the series field assists the
shunt field it is called cumulative compound wound DC motor. When
series field opposes the shunt field the motor is called
differentially wound DC motor. 8. 8 Cumulative compound dc motor
Differential compound DC motor iii)Separately Excited DC Motor :-
In separately excited DC motor there are two sources of DC supply
is required one for Armature and other one is for field winding .
The field windings are made from thin wires with more number of
turns because the field current in this case is very small.
OPERATING CHARACTERISTICS OF DC MOTORS In both cases of shunt or
separately excited D.C motors . the field is supplied from a
constant voltage so that the field current is constant. It is graph
between two dependent quantities. SPEED ARMATURE CURRENT
CHARECTERISTICS:- In a shunt motor, Ish =VRsh, if V is constant Rsh
will also be constant, Hence the flux is constant but at no load
the flux decreases slightly due to armature reaction. If the effect
of armature Reaction is neglected the flux will remain constant.
The motor speed is given by N= V-Ia RA If Is constant N is
proportional to V-Ia RA 9. 9 This is the equation of straight line
with a negative slope. That is the speed N of motor decreases
linearly with increase of current. Since the IaRa at full load is
very small compare to V the drop in speed from noload to full load
is very small. For all practical purposes the shunt motor is taken
a constant speed motor. TORQUE / ARMATURE CURRENT CHARECTERISTICS:-
IF the flux is constant in a shunt motor, the torque would increase
linearly with armature current Ia, However for larger Ia , the net
flux decreases due to the demagnetizing effect of armature
reaction. In view of this , the torque current characteristics
deviates from straight line as illustrated here below av = k Ia
SPEED TORQUE CHARECTERISTICS::- The speed torque characteristics is
also called the mechanical characteristics and under steady state
conditions it can be obtained as follows 10. 10 m = Vt Ia Ra Ka It
is seen from the characteristics that with increase in torque the
speed drops. DC SERIES MOTOR SPEED CURRENT CHARECTERISTICS::- If
the armature reaction and saturation is neglected, the main flux is
directly proportional to the armature current Ia , therefore it can
be written as K Ia where K is constant m o = Vt/ Ka = Vt/Ka K Ia
Since m o is inversely proportional to Ia, the no load speed of
series motor becomes dangerously due to small no load current . In
view of this, the series motor must always be started and operate
under load mechanically coupled with it. 11. 11 TORQUE CURRENT
CHARECTERISTICS::- We know that = KIa which shows that the torque
is proportional to the Ia and therefore Torque current
characteristics is PARABOLA SPEED TORQUE CHARECTERISTICS::- The
speed verses Torque curve is HYPERBOLA. It is seen from the curve
that the increase in Torque e does not change the speed drop
drastically DC COMPOUND MOTORS:- SPEED CURRENT CHARECTERISTICS::-
With increase in Ia and se increases. With increase in Ia the speed
drops at faster rate in a cumulative compound motor. 12. 12 TORQUE
CURRENT CHARECTERISTICS At no load the value of Ia = 0, se=0 and
e=0. As the armature current rises with load the shunt field sh
remains almost constant but series field se rises, as a result
motor torque e SPEED TORQUE CHARECTERISTICS With the increase in
motor torque e , armature current rises, with this field flux se
rises, Consequently speed drops in cumulatively compound DC motor.
APPLICATION OF DC MACHINES:- Presently the DC generators are
overtaken by AC to DC rectifiers which are static equipments. Thus
DC generator are superseded by rectified ac supply. The main
application of DC machines are follows SERIES MOTOR:- These motors
are required where high starting torque is required and speed can
vary like traction, cranes. SHUNT MOTOR :- These motors are used
where constant speed is required and starting condition are not
severe for example, Lifts, Fans, Blowers, Lathe etc. 13. 13
COMPOUND MOTORS These motors are used where high starting torque
and fairly constant speed is required, presses, shears, conveyors,
elevators, rolling machines. Small DC machines ( In fractional KW )
are used primarily as control device like tacho generator for speed
sensing and servomotor for positioning and tracking. STARTING OF DC
MOTORS:- At the time of starting the motor speed is zero. Therefore
back emf is also zero. Vt = Ea +Ia Ra Ea = 0 Vt = 0 + IaRa for
shunt motor Vt = 0+ Ia (Ra +Rs) for series motor and compound motor
With the rated voltage is applied the starting armature current is
= Vt/Ra for shunt motor , = Vt/ Ra+Rs for series motor and compound
motor Since the value of Ra and Rs is very small the motor draws
large starting armature current from supply means. A 10Kw ,250V
shunt motor having armature resistance 0.2 wiil draw 250/0.2 =1250A
where as the rate current is 40A. Such heavy in rush of currents
may result the following 1. Detrimental sparking at commutator 2.
Damage of armature winding and detritions of insulation due to
overheating 3. High starting torque and quick acceleration which
may damage the rotating parts of the rotor 14. 14 4. Large dips in
supply voltage. In view of this , the armature current must be
limited to a value that can be commutated safely, by inserting a
suitable external resistance in the armature circuit. As the motor
accelerates the back EMF is generated in the armature and reduces
the armature current to a small value. The external resistance
inserted must be gradually reduced as the rotor accelerates. SHUNT
AND COMPOUND STARTERs A Primary function of starter is to limit
starting current in the armature circuit during starting or
accelerating time. The simplest type of starter is however modified
to include few protective devices such as over current release, no
volt release etc. THREE POINT STARTERS FOR DC SHUNT MOTOR The
handle H is kept at OFF position by spring S . For starting the
motor , the handle H is moved manually and when makes contact with
the resistance stud 1 it is in the START position. In position the
field winding receives the full supply voltage., but the armature
current is limited by the graded resistance R = ( R1+R2+R3+R4). The
starter handle is then gradually moved from stud to stud , allowing
the speed of the motor to build up until it reaches the RUN
position. In this position (a) the motor attains the full speed ,
(b) The supply is directly across the both winding of motor(c) The
resistance R is completely cut out. The handle H is held in RUN
position by an electromagnet energized by a no volt 15. 15 Coil
NVC. The no no volt trip coil is connected in series with field
winding of the motor. In the event of switching off, or when the
supply voltage falls below a predetermined value, or complete
failure of supply while the motor is running. NVC is de energized,
This results in release of the handle. . which is then pulled back
to the OFF position. By the action of spring. The current to the
motor is cut off, and the motor is not started without resistance R
in the armature circuit.. The NVC also provides protection against
an open circuit in the field winding. The NVC is called no-volt or
under voltage protection of motor. Without this protection, the
supply voltage might be restored with the handle in the RUN
position, Consequently full line voltage may be applied directly to
the armature resulting in a very large current. The other
protective device incorporated in the starter is overload
protection. Overload protection is provide by overload trip coil
OLC and the NVC. The overload coil is a small electromagnet. It
carries the armature and for normal values of armature current the
magnetic pull of OLC is insufficient to attract the strip P When
the armature current exceeds the normal rated value ( that the
motor is overloaded) P is attracted by the electromagnet of OLC and
closes the contact aa, Thus NVC is short circuited , This results
in release of handle H which returns to OFF position and the motor
supply is cut off. DRAW BACK OF THREE POINT STARTER The motor with
large speed variation with armature voltage control suffers . To
increase the speed of the motor the field is to be decreased,
therefore the current through the shunt field is reduced. The field
current may become very low. The very low field current may not
develop sufficient electromagnetic hold to over come the force of
the spring 16. 16 . FOUR POINT STARTER The schematic connection
diagram is given herewith. The basic difference in the circuit of
Four Point starter compare to 3 point starter is that , The holding
coil is removed from the shunt field circuit and is connected
directly across the line with a current limiting resistance r in
series. Such arrangements form three parallel circuits. 1)
Armature, starting resistance and overload release. 2) A variable
resistance and shunt field winding 3) Holding coil and current
limiting resistance. With this arrangement, a change in field
current for variation of speed of the motor, does not affect the
current through the holding coil, because the two circuits are
independent of each other. Now a days automatic push button
starters are used. In such starters the ON push button is preset at
the time of starting limiting the armature current. The resistors
are gradually disconnected by an automatic controlling arrangement
until full line voltage is available to the armature circuit. By
the pressing of OFF button, the circuit is disconnected. 17. 17
SPEED CONTROL OF DC MOTORS The speed of DC motor depends on the
following equation:- N = ( V-IaRa)/ From the above equation it is
clear that the speed is dependent on supply voltage V armature
resistance and field flux which is produced by field current. These
three factors are used for controlling speed of a DC motor. 1
Variation of armature resistance in the armature circuit. 2
Variation of field flux 3 Variation of armature voltage. ARMATURE
RESISTANCE CONTROL :-In this method a variable series resistor Re
is connected in the armature circuit. In the case of shunt motor
the field is 18. 18 directly connected to the supply and therefore
flux Is not affected by this. In the case of series motor the flux
gets affected due to change in Ia and RA since Re resistor carries
full load armature current it must be designed keeping into this
consideration Disadvantages:- i) A large amount of power is wasted
in external resistance Re ii) Control is limited to give speeds
below the normal rated speed. iii)For a given value of Re the speed
reduction is not constant but varies with motor load. VARIATION OF
FIELD FLUX In a shunt motor this is done by connecting a variable
resister Re in series with field winding. The resister Re is called
the field regulator.. the Re reduces the field current by virtue of
this the reduces, decrease in flux causes increase in speed.
Increase in flux causes decrease in speed. By this method the speed
can be attained above normal rated speed. The variation in field
current is done by two methods i) By connecting a variable resistor
parallel to the field winding. This is called divertor method ii)
The second method is tapped field control. 19. 19 ADVANTAGES I)
This is very easy and convenient method II) Since the field current
Ish is very small , the power loss in shunt field is also very
small ARMATURE VOLTAGE CONTROL Ward Leonard system of speed control
is based on tis method. In this system M is the motor whose speed
is to be controlled and G is the separately excite DC generator,
The generator is driven by 3 Phase induction motor or synchronous
motor. The combination of ac motor and DC generator is called MG
Set. By changing generator field current the generated voltage is
changed this voltage increases the speed of the motor. With the
armature voltage control constant torque variable speed is
obtained. 20. 20 ADVANTAGES OF WARD LEONARD DERIVES 1) Smooth
control of DC motor on both directions is possible 2) It has
inherent regenerative breaking capacity 3) When load is
intermittent like rolling mills, IM is used with flywheel to take
care of intermittent loading, in case of WARD LEONARD this is
possible without flywheel. 4) By using over excited Synchronous
motor a drive the system power factor can be improved DISADVANTAGES
OF WARD LEONARD DERIVES 1) Higher initial cost due to MG set 2)
Larger size and weight, require more floor area 3) Frequent
maintenance and produces more noise. 4) Lower efficiency and higher
total losses. 21. 21 SOLID STATE CONTROL Rotating MG sets are now a
days replaced with solid state convertors to control the speed of
DC motors, The convertors are controlled rectifiers or choppers. In
case of AC supply, controlled rectifiers are used to convert fixed
ac supply voltage into variable D voltage. When the supply is D.C
Choppers are used to obtain variable DC voltage from the fixed DC
voltage supply. LOSSES IN A DC MACHINES and efficiency Following
are the losses in the DC machines 1) Electrical or copper loss ( IR
Losses) 2) Core losses or Iron losses 3) Brush Losses 4) Mechanical
losses 5) Stray load losses ELECTRICAL LOSSES:- Windings having
resistance consumes certain losses, these are termed as copper
losses because mostly windings are made of copper. i) Armature
copper loss IaRa ( Ia is armature current) ii) Shunt field copper
loss IshRsh iii) Copper loss in the series field IseRse iv) Copper
loss in the interpole winding which are in series with armature
IaRi 22. 22 v) Compound machines both series field and shunt field
copper losses are also there vi) Copper losses are there in the
compensating winding CORE LOSES:- The core losses are the
hysteresis losses and Eddy current losses. Since the machine
usually operates at constant flux density and speed, these losses
are almost constant. These losses are about 20% of Full load
losses. BRUSH LOSES:- There is a power loss at the brush contact
with commutator and the carbon brushes. This loss can me measured
by the voltage drop at the brush contact and armature current. Pbd
= Vbd Ia The voltage drop is more or less remains constant over a
wide range of Ia and it is assumed 2V ( approx) MECHANICAL LOSSES:-
The losses associated with mechanical effect are called mechanical
losses. These consists of friction losses at bearing and windage
losses ( fan losses) . the fans are used to take away the heat
produced due to IR losses and iron losses inside the machine. STRAY
LOAD LOSSES:- These are miscellaneous losses which are due to the
following reasons:- 1) Distortion of flux due to armature reaction
23. 23 2) Short circuit currents in the coils due to commutation.
These losses are difficult to find out, However they are taken as 1
% of full load power output. EFFICIENCY InputPower Losses
InputPower LossesInputPower InputPower OutputPower ==== ==== ==== 1
24. 24 Let us assume that the R = Total resistance I = Output
current Ish = Current through the shunt field Ia armature current I
+Ish V is the terminal voltage Power loss in the shunt field = V
Ish Mechanical Losses = Friction losses at bearings+ friction
losses at commutator + windage losses Stray losses Core losses
mechanical losses and shunt field copper losses are considered as
combined fixed losses. = Output / Input = VI / ( VI + IaRat +Pk
=VbdIa Ia = I + Ish Since Ish compare to I is very small we can
consider Ia I 25. 25 = VI/ ( VI+ IRat +Vbd I +Pk) LOAD for Maximum
efficiency Ifl = Full load current at maximum efficiency Im =
current at maximum efficiency Im Rat = Pk Im = Pk/ Rat Current at
Maximum = F.L Current X( Pk/F.L Copper loss) TESTING OF DC
MACHINES:- Testing of Dc machines can be done by following three
methods:- 1) Direct testing 2) Swinburnes Test DIRECT TESTING :-
This method is suitable for small machines. In direct testing
method DC machine is subjected to rated load and the entire out put
is wasted. The ration of out put power to the input power gives the
efficiency. For DC generators the out put power is wasted in
resistors, the out put voltage and current gives the out put power.
26. 26 For Dc motors break test is carried out . the belt
tightening hand wheels H1 and H2 help in So adjusting the load on
the pulley. S1 S2 are the spring balance. These spring balances are
calibrated in Kg. The motor out put is given by =(S1 S2)r X 9081
Watts S1 S2 are the spring balance tight side and slack side
readings, r is the effective radius of the pulley in meters = (
outside pulley diameter + belt thickness) is the ( 2) is the motor
speed in rad/sec If Vt is the motor terminal voltage and Il is the
line current, then power input to the motor VtIl watts The
efficiency % = { ( S1-S2) 9.81 X100 }/ VtIl For series motor the
break should be tight enough before motor is switched on
Disadvantages 1) The spring balance are not steady 2) The friction
torque , at particular setting of hand wheel H1 and H2 does not
remain constant. 27. 27 SWINBERNs TEST The machine is run as a
motor at rated voltage and speed. Connection diagram is as follows
Let V = supply voltage I0 = No load current Ish = Shunt Field
current Therefore No load armature current Ia0 = Io- Ish NO load
input VIo No load power input supplies the following losses i) Iron
loss in the core ii) Friction losses at bearing and commutator iii)
Windage losses iv) Armature cupper loss at no load When the machine
is loaded the resistance of the armature and field resistance
changes due to temperature rise and due to IR losses. Let the
resistance at the room temperature tOc is made by passing current
through armature and field from a low voltage DC supply. Let the
rise be 50 degree the hot resistance Rt1 = R0 + 0t1 R(t1=50) = R0{1
+ 0 (t1+5050)} 0 = temperature coefficient of resistance at zero
degree temperature 28. 28 Stray losses = iron loss +friction loss +
windage loss = input at no load =VI0 Pf Pa0 = Ps Pc = no load input
no load armature copper loss Pc = Ps +Pf By knowing the constant
losses of the machine its efficiency can be calculated at any other
load. Let I be the load current Motor input VI Armature cupper loss
= IaRa +Pc = {VI (IaRa +Pc)}/ VI Efficiency when running as
generator Ia = I + Ish Out put = VI 29. 29 gen = VI/ { VI +( I+Ish)
Ra +Pc} ADVANTAGES i) It is convenient and economical, since power
requirement is very less ii) The efficiency at any load can be
determined as the constant losses are known DISADVANTAGES i) This
cannot be done on series machines. ii) No account is taken for iron
loss from no load to full load. At full load due to the armature
reaction iron losses increases.
