A) Basics of Electrial and Electronics

8/13/2019 A) Basics of Electrial and Electronics

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INDEX

S. No. CONTENTS Page No.

1.0 Basics of electricity -------------------------------------------------------------------- 51.1 Current flow ----------------------------------------------------------------------------- 61.2 Sources of electricity ------------------------------------------------------------------- 71.3 Electromagnetic induction------------------------------------------------------------- 8

1.4 Electrical circuits ----------------------------------------------------------------------- 91.5 Ohms Laws------------------------------------------------------------------------------ 9

1.5.1 Resistor ---------------------------------------------------------------------------------- 11

1.5.2 Voltage----------------------------------------------------------------------------------- 11

1.5.3 Voltage and current -------------------------------------------------------------------- 12

1.5.4 Inductance------------------------------------------------------------------------------- 13

1.5.5

Capacitance ---------------------------------------------------------------------------- 14

1.6 Frequency ------------------------------------------------------------------------------ 15

1.7 Power and power factor ---------------------------------------------------------------- 15

1.8 Force ------------------------------------------------------------------------------------- 17

1.9 Torque------------------------------------------------------------------------------------ 18

2.0 Speed ------------------------------------------------------------------------------------ 19

2.0.1 Linear speed----------------------------------------------------------------------------- 19

2.0.2 Angular speed -------------------------------------------------------------------------- 20

2.0.3 Acceleration ---------------------------------------------------------------------------- 20

2.0.4 Law of inertia---------------------------------------------------------------------------- 20


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2.0.5 Friction -------------------------------------------------------------------------------------- 21

2.1 Work ---------------------------------------------------------------------------------------- 21

2.2 Alternating current and direct current -------------------------------------------------- 22

2.2.1 Poly phase AC circuit --------------------------------------------------------------------- 23

2.2.2 3 phase power system--------------------------------------------------------------------- 28

2.3 Control circuit ------------------------------------------------------------------------------ 35

2.4 Power circuit with star delta connections ----------------------------------------------- 36

2.5 Electrical motors --------------------------------------------------------------------------- 37

2.5.1 Working ------------------------------------------------------------------------------------ 37

2.5.2 Classifications of motor ------------------------------------------------------------------ 38

2.6 DC motor components -------------------------------------------------------------------- 39

2.6.1 Advantages of DC motors ---------------------------------------------------------------- 40

2.6.2 Types of DC motors ---------------------------------------------------------------------- 40

2.6.2.1 Self excited DC motors ------------------------------------------------------------------- 40

2.6.2.2 DC compound motor ---------------------------------------------------------------------- 41

2.6.2.3 Separately excited DC motors ----------------------------------------------------------- 42

2.7 AC motor ----------------------------------------------------------------------------------- 43

2.7.1 Synchronous motor ------------------------------------------------------------------------ 44

2.7.2 Induction motor ---------------------------------------------------------------------------- 53

2.7.3 Servo motor --------------------------------------------------------------------------------- 56

2.7.4 Stepper motor ------------------------------------------------------------------------------ 58


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2.7.4.1 Variable reluctance stepper ---------------------------------------------------------- 61

2.8 Semiconductor device ---------------------------------------------------------------- 64

Figures:

1.0 Fig -----------Bound and free electrons ------------------------------------------------- 51.1 Fig -----------Electron flow --------------------------------------------------------------- 6

1.2 Fig -----------Current flow -------------------------------------------------------------- 7

1.3 Fig -----------Battery---------------------------------------------------------------------- 8

1.4 Fig -----------Current draw in parallel circuit------------------------------------------- 10

1.5 Fig -----------Self excited DC motor --------------------------------------------------- 411.6 Fig -----------DC compound motor ----------------------------------------------------- 42

1.7 Fig -----------Separately excited DC motor -------------------------------------------- 43

1.8 Fig -----------Synchronous motor running with step alternator --------------------- 45

1.9 Fig -----------Sine wave drive synchronous motor ----------------------------------- 46

2.0 Fig ---------- Addition of field decrease speed ---------------------------------------- 47

2.1 Fig -----------One winding 12 pole synchronous motor ----------------------------- 47

2.2 Fig ----------- Represented by permission of Westclox theory ----------------------- 48

2.3 Fig ----------- 3 phase four pole synchronous motor ----------------------------------- 49

2.4 Fig ----------- Electronic synchronous motor ------------------------------------------ 50

2.5 Fig ----------- Phase locked loop controls in synchronous motor Speed------------- 50

2.6 Fig ----------- Motor ripple torque and mechanical analog --------------------------- 51

2.7 Fig ----------- Winding distributed in belt produce a more sinusoidal field ------ 51


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2.8 Fig --------- PWM approximate sine wave------------------------------------------------- 52

2.9 Fig --------- Stepper motor VS servo motor------------------------------------------------- 58

3.0 Fig Unipolar drive of center tapped coil at(b) emulates AC current in single coil at (a) 59

3.1 Fig ---------Stepper speed characteristics --------------------------------------------------- 60

3.2 Fig ---------Three and four phase variable reluctance stepper motor ------------------- 61

3.3 Fig ---------Stepping sequence for variable reluctance stepper ------------------------- 62

3.4 Fig ---------Variable reluctance stepper motor -------------------------------------------- 63

3.5 Fig --------- Variable reluctance stepper drives lead screws ----------------------------- 63

Questions ?----------------------------------------------------------------------------------------------


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BASICS OF ELECTRICALS AND ELECTRONICS

Electricity generation is the process of generatingelectrical power from other sources

ofprimary energy.

The fundamental principles of electricity generation were discovered during the 1820s

and early 1830s by the British scientistMichael Faraday.His basic method is still used today:

electricity is generated by the movement of a loop of wire, or disc of copper between the poles of

amagnet.

Forelectric utilities,it is the first process in the delivery of electricity to consumers. The

other processes, electricitytransmission ,distribution,and electrical power storage and recoveryusingpumped-storage methods are normally carried out by theelectric power industry.

Electricity is most often generated at apower stationby electromechanicalgenerators,

primarily driven byheat engines fueled by chemicalcombustion ornuclear fissionbut also by

other means such as thekinetic energy of flowing water and wind. Other energy sources include

solar photovoltaic andgeothermal power.

1.0 What is Electricity?

Everything that has substance and takes up space, whether it is solid, liquid, or gaseous,is made up of very small particles called atoms.

Fig:1.0

Scientists believe all atoms have negatively charged particles known as electrons, which

revolve around a central core, or nucleus. This nucleus is believed to be positively charged, andto contain other particles known as protons and neutrons. Electrons in the inner orbits are known
http://en.wikipedia.org/wiki/Electrical_powerhttp://en.wikipedia.org/wiki/Primary_energyhttp://en.wikipedia.org/wiki/Michael_Faradayhttp://en.wikipedia.org/wiki/Magnethttp://en.wikipedia.org/wiki/Electric_utilityhttp://en.wikipedia.org/wiki/Electric_power_transmissionhttp://en.wikipedia.org/wiki/Electric_power_distributionhttp://en.wikipedia.org/wiki/Pumped-storage_hydroelectricityhttp://en.wikipedia.org/wiki/Electric_power_industryhttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Heat_enginehttp://en.wikipedia.org/wiki/Combustionhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Geothermal_powerhttp://en.wikipedia.org/wiki/Geothermal_powerhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Combustionhttp://en.wikipedia.org/wiki/Heat_enginehttp://en.wikipedia.org/wiki/Electrical_generatorhttp://en.wikipedia.org/wiki/Power_stationhttp://en.wikipedia.org/wiki/Electric_power_industryhttp://en.wikipedia.org/wiki/Pumped-storage_hydroelectricityhttp://en.wikipedia.org/wiki/Electric_power_distributionhttp://en.wikipedia.org/wiki/Electric_power_transmissionhttp://en.wikipedia.org/wiki/Electric_utilityhttp://en.wikipedia.org/wiki/Magnethttp://en.wikipedia.org/wiki/Michael_Faradayhttp://en.wikipedia.org/wiki/Primary_energyhttp://en.wikipedia.org/wiki/Electrical_power


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as bound electrons. Those in the outer orbit are called free electrons, and can be easily forced outof their orbits, flowing from one atom to another.

1.1 Current flow:

The electron theory states that current flow is the organized, forced movement of free

electrons in a specific direction. Continuous current flow in only one direction is known as DCor Direct Current.

Fig: 1.1

Electrons flow

Flow that alternates back and forth is called Alternating Current.A good conductor is a

material that has many free electrons, such as copper. Lead and gold have large numbers of freeelectrons. Good conductors readily transmit electricity. The force that causes electrons to move

from one atom to another is called electromotive force (EMF).

Electromotive force is caused by a difference in electrical potential and is measured in

volts, also referred to as voltage. Some older cars and heavy equipment had their positive battery

terminal connected to the chassis or frame and the negative side of the circuit was switched.

Practically no production vehicle uses this configuration today.


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Fig: 1.2

Current flow

When a switch is closed, current flows from the battery negative post to chassis ground, through

the load and switch, and to the positive terminal. Even though the ground has the excess of

electrons, the positive post is the one that is generally regarded as hot.

Electron theory supposes current flow to be from negative to positive, while

Conventional Theory supposes the current flows from positive to negative. Thus, the current inautomotive circuits is usually traced from the source (battery) to the load (bulb, motor, etc.) and

then to ground.

1.2 Sources of Electricity:

A battery is a chemical source of electricity. It contains a number of positive plates and an equalnumber of negative plates.


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Fig: 1.3Battery

The positive and negative plates are immersed in an electrolyte solution composed of

water and sulphuric acid. When the battery is charged, a chemical reaction of the acid on the

plates results in an excess of electrons collecting on the negative plates. If connected between the

plates, current (electrons) will flow through the conductor from one plate to the other.

1.3 Electromagnetic Induction:

When a conductor connected to a closed circuit is passed through a magnetic field, current is

produced in the conductor. Alternators, generators, and various inductive sensors use this

principle to do their work.


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1.4 Electrical Circuits:

An electrical circuit is a complete path for current flow. Basically defined as being from a power

source to a load component that has resistance and uses electricity to do its work, and finally aground path back to the negative terminal of the power source.

1.5 Ohms Law:

"One microampere flowing in one ohm causes a one microvolt potential drop."

Georg Simon Ohm

Ohms Law is the basic rule for the relationships between voltage, current, and resistance.

1. CURRENT FLOW IS DIRECTLY PROPORTIONAL TO VOLTAGE

2. CURRENT FLOW IS INVERSELY PROPORTIONAL TO RESISTANCE

1. CURRENT FLOW IS DIRECTLY PROPORTIONAL TO VOLTAGE: Simply put, this

means that an increase in VOLTAGE will cause an INCREASE in current flow. This is true

because voltage is, in effect, electrical pressure. The higher the voltage, the higher the

pressure, thus the higher the current flow. Remember: 1volt pushes 1amp through 1ohm.

2. CURRENT FLOW IS INVERSELY PROPORTIONAL TO RESISTANCE This statement

means that an increase in RESISTANCE will cause a DECREASE in current flow.


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The unit of the measurement to determine current flow is the ampere (abbreviated amp).Amp).

Fig: 1.4

One ampere is defined as the movement of one coulomb of electrons of past a given point

in one second. A coulomb is coulomb billion electrons. Current is thus a measure of the rate of

electrical flow. It can also be known as amperage or draw, and is measured with an ammeter.When loads are connected parallel, as in the illustration on the left, each draws current

Independently of the other. Important: total current draw in a parallel circuit equals thesum of the individual current draws.


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1.5.1 Resistance:

Resistance is an opposition to current flow offered by a load or a resistor. Even

conductors have some resistance; for example, a piece of 22 gauge copper wire 60 feet long hasone ohm of resistance. Larger diameter wires have less resistance. Heat generally causes

generally resistance to increase in a conductor or connection.

When resistors are in series, the total, resistance is the sum of the individual resistances 6+6 = 12

When resistors are in parallel, the total, resistance is equals the source voltage divided by the

Combined current draw.

1.5.2 Voltage:

Electricity can be compared with water flowing through a pipe. A force is required to getwater to flow through a pipe. This force comes from either a water pump or gravity. Voltage is

the force that is applied to a conductor that causes electric current to flow. Electrons are negative

and are attracted by positive charges. They will always be attracted from a source having an

excess of electrons, thus having a negative charge, to a source having a deficiency of electrons,giving it a positive charge. The force required to make electricity flow through a conductor is

called a difference in potential, electromotive force (EMF), or voltage. Voltage is designated by

the letter E, or the letter V. The unit of measurement for voltage is a volt which is also

designated by the letter V.


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Electrons are negative and are attracted by positive charges. They will always be attractedfrom a source having an excess of electrons, thus having a negative charge, to a source having a

deficiency of electrons, giving it a positive charge. The force required to make electricity flow

through a conductor is called a difference in potential, electromotive force (EMF), or voltage.Voltage is designated by the letter E, or the letter V. The unit of measurement for voltage is

volts which are also designated by the letter V.

1.5.3 Voltage and Current:

Peak value: The sine wave illustrates how voltage and current in an AC circuit rises and

falls with time. The peak value of a sine wave occurs twice each cycle, once at the positive

maximum value and once at the negative maximum value. The value of the voltage or currentbetween the peak positive and peak negative values is called the peak-to-peak value. The

instantaneous value is the value at any one particular time. It can be in the range of anywherefrom zero to the peak value.

Peak to Peak value:The value of the voltage or current between the peak positive and peak

negative values is called the peak-to-peak value.


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Instantaneous value: The instantaneous value is the value at any one particular time. It can be

in the range of anywhere from zero to the peak value.

1.5.4 Inductance:

The circuits studied to this point have been resistive. Resistance and voltage are not theonly circuit properties that effect current flow, however. Inductance is the property of an Electric

circuit that opposes any change in electric current. Resistance opposes current flow; inductance

opposes change in current flow. Inductance is designated by the letter .L...The Unit ofmeasurement for inductance is the henry (h).

Inductance is usually indicated symbolically on an electrical drawing by one of two ways.

A curled line or a filled rectangle can be used. Inductors are coils of wire. They may be wrapped


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around core. The inductance of a coil is determined by the number of turns in the coil, thespacing between the turns, the coil diameter, and the core material, the number of layers of

Windings, the type of winding, and the shape of the coil. Examples of inductors are transformers,chokes, and motors.

1.5.5 Capacitance:

Capacitance and capacitors Capacitance is a measure of a circuit ability to store an

electrical charge. A device manufactured to have a specific amount of capacitance is called a

capacitor. A capacitor is made up of a pair of conductive plates separated by a thin layer of

insulating material. Another name for the insulating material is dielectric material. When avoltage is applied to the plates, electrons are forced onto one plate. That plate has an excess of

electrons while the other plate has a deficiency of electrons. The plate with an excess of electrons

is negatively charged. The plate with a deficiency of electrons is positively charged.

Direct current cannot flow through the dielectric material because it is an insulator. Capacitorshave a capacity to hold a specific quantity of electrons. The capacitance of a capacitor. Depends

on the area of the plates, the distance between the plates, and the material of the dielectric. The

unit of measurement for capacitance is farads, abbreviated .F. Usually Capacitors are rated in mF

(microfarads), or pF (Pico farads).

Capacitor symbol:

Capacitance is usually indicated symbolically on an electrical drawing by a combination of astraight line with a curved line, or two straight lines.


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1.6 Frequency:

The number of cycles per second made by voltage induced in the armature is the

frequency of the generator. If the armature rotates at a speed of 60 revolutions per second, thegenerated voltage will be 60 cycles per second. The accepted term for cycles per second is hertz.

The standard frequency in the United States is 60 hertz. The following illustration shows 15

cycles in 1/4 second which is equivalent to 60 cycles in one second.

1.7 Power and power factor:

Power consumed by a resistor is dissipated in heat and not returned to the source. This is

true power. True power is the rate at which energy is used. Current in an AC circuit rises to peak

values and diminishes to zero many times a second. The energy stored in the magnetic field of aninductor, or plates of a capacitor, is returned to the source when current changes direction. Power

in an AC circuit is the vector sum of true power and reactive power. This is called apparentpower. True power is equal to apparent power in a purely resistive circuit because voltage and

current are in phase. Voltage and current are also in phase in a circuit containing equal values of

inductive reactance and capacitive reactance. If voltage and current are 90 degrees out of phase,

as would be in a purely capacitive or purely inductive circuit, the average value of true power isequal to zero. There are high positive and negative peak values of power, but when added

together the result is zero. Power is the rate of doing work.

True and apparent power:

Power =force x distance / timePower= work / time

The formula for apparent power is:P = EI

Apparent power is measured in volt-amps (VA).


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True power is calculated from another trigonometric function, the cosine of the phase angle

(costita). The formula for true power is:

P = EI costita {Note: True power is measured in watts.}

In a purely resistive circuit, current and voltage are in phase. There is a zero degree angle

displacement between current and voltage. The cosine of zero is one. Multiplying a value by one

does not change the value. In a purely resistive circuit the cosine of the angle is ignored .In a

purely reactive circuit, either inductive or capacitive, current or voltage are 90 degrees out ofphase. The cosine of 90 is zero. Multiplying a value times zero results in a zero product. No

power is consumed in a purely reactive circuit.

E.g.: Calculating apparent power in a simple R-L-C circuit:

In the following 120 volt circuit, it is equal to 84.9 milliamps. Inductive reactance is 100 W and

capacitive reactance is 1100 W. The phase angle is -45 degrees. By referring to a trigonometrictable, the cosine of -45 degrees is found to be .7071.

The apparent power consumed by the circuit is:

P =EI

P=VA= 120x .0849

=10. 2VA

The true power consumed by the circuit is:

P= EI costita

P=120x.0849x.7071P=7.2watts


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Another formula for true power is:

P =I^2xR

P=.0849^2x1000

P=7.2watts

Power factor:

Power factor is the ratio of true power to apparent power in an AC circuit. Power factor isexpressed in the following formula:

PF=PT/PA

Power factor can also be expressed using the formulas for true power and apparent power. The

value of EI cancels out because it is the same in the numerator and denominator Power factor isthe cosine of the angle.

PF=EI costita/ EI

PF= costita

In a purely resistive circuit, where current and voltage are in phase, there is no angle of

displacement between current and voltage. The cosine of a zero degree angle is one. The power

factor is one. This means that all energy delivered by the source is consumed by the circuit and

dissipated in the form of heat. In a purely reactive circuit, voltage and current are 90 degreesapart. The cosine of a 90 degree angle is zero. The power factor is zero. This means the circuit

returns all energy it receives from the source to the source. In a circuit where reactance and

resistance are equal, voltage and current are displaced by 45 degrees. The cosine of a 45degreeangle is .7071. The power factor is .7071. This means the circuit has used approximately 70% of

the energy supplied by the source and returned approximately 30%.

1.8 Force:

In simple terms, a force is a push or a pull. Force may be caused by electromagnetism,

gravity, or a combination of physical means.

Net Force:

Net force is the vector sum of all forces that act on an object, including friction and

gravity. When forces are applied in the same direction they are added. For example, if two 10 lb

forces were applied in the same direction the net force would be 20 lb.


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If 10 lb of force were applied in one direction and 5 lb of force applied in the opposite direction,the net force would be 5 lb and the object would move in the direction of the greater force.

If 10 lb of force were applied equally in both directions, the net force would be zero and theobject would not move.

1.9 Torque:

Torque is a twisting or turning force that tends to cause an object to rotate. A force

applied to the end of a lever, for example, causes a turning effect or torque at the pivot point.

Torque () is the product of force and radius (lever distance).

Torque () = Force x Radius


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In the English system torque is measured in pound-feet (lb-ft) or pound-inches (lb-in). If 10 lbs

of force were applied to a lever 1 foot long, for example, there would be 10 lb-ft of torque.

An increase in force or radius would result in a corresponding increase in torque. Increasing the

radius to 2 feet, for example, results in 20 lb-ft of torque.

2.0 Speed:

An object in motion travels a given distance in a given time. Speed is the ratio of the distance

traveled to the time it takes to travel the distance.

Speed= Distance/Time

2.0.1 Linear Speed:

The linear speed of an object is a measure of how long it takes the object to get from point A to

point B. linear speed is usually given in a form such as meters per second (m/s). For example, if

The distance between point A and point B were 10 meters, and it took 2 seconds to travel thedistance, the speed would be 5m/s.


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2.0.2 Angular (Rotational) Speed:

The angular speed of a rotating object is a measurement of how long it takes a given

point on the object to make one complete revolution from its starting point. Angular speed is

generally given in revolutions per minute (RPM). An object that makes ten complete revolutionsin one minute, for example, has a speed of 10 RPM.

2.0.3 Acceleration:

An object can change speed. An increase in speed is called acceleration. Acceleration occursonly when there is a change in the force acting upon the object. An object can also change from ahigher to a lower speed. This is known as deceleration (negative acceleration). A rotating object,

for example, can accelerate from 10 RPM to 20 RPM, or decelerate from 20 RPM to 10 RPM.

2.0.4 Law of Inertia:

Mechanical systems are subject to the law of inertia. The law of inertia states that an

object will tend to remain in its current state of rest or motion unless acted upon by an externalforce. This property of resistance to acceleration/deceleration is referred to as the


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Moment of inertia. The English system of measurement is pound-feet squared (lb-ft2).If

we look at a continuous roll of paper, as it unwinds; we know that when the roll is stopped, it

would take a certain amount of force to overcome the inertia of the roll to get it rolling. The forcerequired to overcome this inertia can come from a source of energy such as a motor. Once

rolling, the paper will continue unwinding until another force acts on it to bring it to a stop.

2.0.5 Friction:

A large amount of force is applied to overcome the inertia of the system at rest to start it

moving. Because friction removes energy from a mechanical system, a continual force must be

applied to keep an object in motion. The law of inertia is still valid, however, since the forceapplied is needed only to compensate for the energy lost. Once the system is in motion, only the

energy required to compensate for various losses need be applied to keep it in motion. In the

previous illustration, for example: these losses include:

Friction within motor and driven equipment bearings

Wind age losses in the motor and driven equipment

Friction between material on winder and rollers

2.1 Work:

Whenever a force of any kind causes motion, work is accomplished. For example, work

is accomplished when an object on a conveyor is moved from one point to another.


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Work is defined by the product of the net force (F) applied and the distance (d) moved. If

twice the force is applied, twice the work is done. If an object moves twice the distance, twicethe work is done.

W = F x d

2.2 Alternating current and Direct current:

Most students of electricity begin their study with what is known as direct current (DC),

which is electricity flowing in a constant direction, and/or possessing a voltage with constant

polarity. DC is the kind of electricity made by a battery (with positive and negative terminals), or

the kind of charge generated by rubbing certain types of materials against each other. As usefuland as easy to understand as DC is, it is not the only "kind" of electricity in use. Certain sources

of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages

alternating in polarity, reversing positive and negative over time. Either as a voltage switchingpolarity or as a current switching direction back and forth, this "kind" of electricity is known as

Alternating Current (AC).

Whereas the familiar battery symbol is used as a generic symbol for any DC voltage

source, the circle with the wavy line inside is the generic symbol for any AC voltage source. Onemight wonder why anyone would bother with such a thing as AC. It is true that in some cases

AC holds no practical advantage over DC. In applications where electricity is used to dissipate

Energy in the form of heat, the polarity or direction of current is irrelevant, so long as there isenough voltage and current to the load to produce the desired heat (power dissipation). However,

with AC it is possible to build electric generators, motors and power distribution systems that are

far more efficient than DC, and so we and AC used predominately across the world in highpower applications. To explain the details of why this is so, a bit of background knowledge aboutAC is coils with the turning of a shaft, AC voltage will be produced across the wire coils as that

shaft is rotated, in accordance with Faraday's Law of electromagnetic induction. This is the basic

operating principle of an AC generator, also known as an alternator.


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2.2.1 Poly Phase AC Circuit:

Single-phase power systems:

Depicted above is a very simple AC circuit. If the load resistor's power dissipation were

substantial we might call this a "power circuit" or "power system" instead of regarding it as just aregular circuit. The distinction between a "power circuit" and a "regular circuit" may seem

arbitrary, but the practical concerns are definitely not.

One such concern is the size and cost of wiring necessary to deliver power from the AC

source to the load. Normally, we do not give much thought to this type of concern, if we're

merely analyzing a circuit for the sake of learning about the laws of electricity. However, in thereal world it can be a major concern. If we give the source in the above circuit a voltage value

and also give power dissipation values to the two load resistors, we can determine the wiring

needs for this particular circuit:

I =P/E

I =10 kW/120 V

(For each load resistor)

I total = Iload#1 + Iload#2I total = 166.67 A

I = 83.33 AI total = (83.33 A) + (83.33 A)

P total = (10 kW) + (10 kW)

P total = 20 kW


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83.33 amps for each load resistor add up to 166.66 amps total circuit current. This is no small

Amount of current, and would necessitate copper wire conductors of at least 1/0 gage. Such wire

Is well over 1/4 inches in diameter, weighing over 300 pounds per thousand feet? Bear in mindthat Copper is not cheap either! It would be in our best interest to and ways to minimize such

costs if we were designing a power system with long conductor lengths.

One way to do this would be to increase the voltage of the power source and use loads

built to dissipate 10 kW each at this higher voltage. The loads, of course, would have to have

greater Resistance values to dissipate the same power as before (10 kW each) at a greater voltage

than before. The advantage would be less current required, permitting the use of smaller, lighter,and cheaper wire:

I =P/EI =10 kW/120 V

(For each load resistor)

I total = Iload#1 + Iload#2I total = 166.67 A

I = 83.33 A

I total = (83.33 A) + (83.33 A)

P total = (10 kW) + (10 kW)P total = 20 kW

Now our total circuit current is 83.33 amps, half of what it was before. We can now usenumber gage wire, which weighs less than half of what 1/0 gage wire does per unit length. This

is a considerable reduction in system cost with no degradation in performance. This is why

power distribution system designers elect to transmit electric power using very high voltages

(many thousands of volts): to capitalize on the savings realized by the use of smaller, lighter,cheaper wire. However, this solution is not without disadvantages. Another practical concern

with power circuits is the danger of electric shock from high voltages. Again, this is not usually

the sort of thing we concentrate on while learning about the laws of electricity, but it is a veryvalid concern in the real world, especially when large amounts of power are being dealt with.

The gain in efficiency realized by stepping up the circuit voltage presents us with increased

danger of electric shock. Power distribution companies tackle this problem by stringing their


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power lines along high poles or towers, and insulating the lines from the supporting

structures with large, porcelain insulators.

At the point of use (the electric power customer), there is still the issue of what voltage to

use for powering loads. High voltage gives greater system efficiency by means of reducedconductor current, but it might not always be practical to keep power wiring out of reach at the

point of use the way it can be elevated out of reach in distribution systems. This trade between

efficiency and danger is one that European power system designers have decided to risk, all their

households and appliances operating at a nominal voltage of 240 volts instead of 120 volts as itis in North America. That is why tourists from America visiting Europe must carry small step-

down transformers for their portable appliances, to step the 240 VAC (volts AC) power down to

a more suitable 120 VAC. Is there any way to realize the advantages of both increased efficiency

and reduced safety hazard at the same time? One solution would be to install step-downtransformers at the end-point of power use, just as the American tourist must do while in Europe.

However, this would be expensive and inconvenient for anything but very small loads (where thetransformers can be built cheaply) or very large loads (where the expense of thick copper wireswould exceed the expense of a transformer). An alternative solution would be to use a higher

voltage supply to provide power to two lower voltage loads in series. This approach combines

the efficiency of a high-voltage system with the safety of a low-voltage system:

Notice the polarity markings (+ and -) for each voltage shown, as well as the unidirectional

arrows for current. For the most part, I've avoided labeling "polarities" in the AC circuits we've

been analyzing, even though the notation is valid to provide a frame of reference for phase. In

later sections of this chapter, phase relationships will become very important, so I'm introducingthis notation early on in the chapter for your familiarity.

The current through each load is the same as it was in the simple 120 volt circuit, but thecurrents are not additive because the loads are in series rather than parallel. The voltage across

each load is only 120 volts, not 240, so the safety factor is better. Mind you, we still have a full

240 volts across the power system wires, but each load is operating at a reduced voltage. If


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Anyone is going to get shocked; the odds are that it will be from coming into contact with

the conductors of a particular load rather than from contact across the main wires of a powersystem. Theresonly one disadvantage to this design: the consequences of one load failing open,

or being turned (assuming each load has a series on/off switch to interrupt current) are not good.

Being a Series circuit, if either load were to open, current would stop in the other load as well.For this reason, we need to modify the design a bit:

Instead of a single 240 volt power supply, we use two 120 volt supplies (in phase with each

other) in series to produce 240 volts, then run a third wire to the connection point between the

loads to handle the eventuality of one load opening. This is called a split-phase power system.Three smaller wires are still cheaper than the two wires needed with the simple parallel design,

so we're still ahead on efficiency. The astute observer will note that the neutral wire only has to

carry the difference of current between the two loads back to the source. In the above case, withperfectly "balanced" loads consuming equal amounts of power, the neutral wire carries zero

current.


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Notice how the neutral wire is connected to earth ground at the power supply end. This is

a common feature in power systems containing "neutral" wires, since grounding the neutral wire

Ensures the least possible voltage at any given time between any "hot" wire and earth ground. Anessential component to a split-phase power system is the dual AC voltage source. Fortunately,

designing and building one is not difficult. Since most AC systems receive their power from a

step- down transformer anyway (stepping voltage down from high distribution levels to a user-level voltage like 120 or 240), that transformer can be built with a center-tapped secondary

winding:

If the AC power comes directly from a generator (alternator), the coils can be similarlycenter-tapped for the same effect. The extra expense to include a center-tap connection in a

transformer or alternator winding is minimal. Here is where the (+) and (-) polarity markings

really become important. This notation is often used to reference the phasingsof multiple AC

voltage sources, so it is clear whether they are aiding ("boosting") each other or opposing("bucking") each other. If not for these polarity markings, phase relations between multiple AC

sources might be very confusing. Note that the split-phase sources in the schematic (each one

120 volts 6 0o), with polarity marks (+) to (-) just like series-aiding batteries can alternatively berepresented as such:


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To mathematically calculate voltage between "hot" wires, we must subtract voltages; because

their polarity marks show them to be opposed to each other:

If we mark the two sources' common connection point (the neutral wire) with the same

polarity mark (-), we must express their relative phase shifts as being 180o apart. Otherwise,

we'd be denoting two voltage sources in direct opposition with each other, which would give 0

volts between the two "hot" conductors. Why am I taking the time to elaborate on polarity marksand phase angles? It will make more sense in the next section! Power systems in American

households and light industry are most often of the split-phase variety, providing so-called

120/240 VAC power. The term "split-phase" merely refers to the split- voltage supply in such asystem. In a more general sense, this kind of AC power supply is called single phase because

both voltage waveforms are in phase, or in step, with each other. The term "single phase" is a

counterpoint to another kind of power system called "poly phase" which we are about to

investigate in detail. Apologies for the long introduction leading up to the title-topic of thischapter. The advantages of poly phase power systems are more obvious if one first has a good

understanding of single phase systems.

2.2.2 Three-Phase Power Systems:

Split-phase power systems achieve their high conductor efficiency and low safety risk by

splitting up the total voltage into lesser parts and powering multiple loads at those lesser

voltages, while drawing currents at levels typical of a full-voltage system. This technique, by the

way, works just as well for DC power systems as it does for single-phase AC systems. Suchsystems are usually referred to as three-wire systems rather than split-phase because "phase" is a

concept restricted to AC. But we know from our experience with vectors and complex numbers

that AC voltages don't always add up as we think they would if they are out of phase with each

other. This principle, applied to power systems, can be put to use to make power systems witheven greater conductor efficiencies and lower shock hazard than with split-phase. Suppose that

we had two sources of AC voltage connected in series just like the split-phase system we saw

before, except that each voltage source was 120o out of phase with the other:


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Since each voltage source is 120 volts, and each load resistor is connected directly in

parallel with its respective source, the voltage across each load must be 120 volts as well. Givenload currents of 83.33 amps, each load must still be dissipating 10 kilowatts of power. However,

voltage between the two "hot" wires is not 240 volts because the phase difference between thetwo sources is not 180o. Instead, the voltage is:

Nominally, we say that the voltage between "hot" conductors is 208 volts (rounding up), andThus the power system voltage is designated as 120/208. If we calculate the current through the

"neutral" conductor, we and that it is not zero, even with balanced load resistances. Kirchhoff'sCurrent Law tells us that the currents entering and exiting the node between the two loads mustbe zero:


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So, we find that the "neutral" wire is carrying full 83.33 amps, just like each "hot" wire. Note

that we are still conveying 20 kW of total power to the two loads, with each load's "hot Wire

carrying 83.33 amps as before. With the same amount of current through each "hot" wire, wemust use the same gage copper conductors, so we haven't reduced system cost over the split-

phase 120/240 system. However, we have realized a gain in safety, because the overall voltage

between the two "hot" conductors is 32 volts lower than it was in the split-phase system (208volts instead of 240 volts). The fact that the neutral wire is carrying 83.33 amps of current raises

an interesting possibility: since its carrying current anyway, why not use that third wire as

another "hot" conductor, powering another load resistor with a third 120 volt source having a

phase angle of 240o. That way, we could transmit more power (another 10 kW) without having

to add any more conductors. Let's see how this might look:

A full mathematical analysis of all the voltages and currents in this circuit wouldnecessitate the use of a network theorem, the easiest being the Superposition Theorem. I'll spare

you the long, Drawn-out calculations because you should be able to intuitively understand that


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the three voltage sources at three different phase angles will deliver 120 volts each to a balanced

triad of load resistors. For proof of this, we can use SPICE to do the math for us:


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Sure enough, we get 120 volts across each load resistor, with (approximately) 208 volts

between any two "hot" conductors and conductor currents equal to 83.33 amps. At that current

and voltage, each load will be dissipating 10 kW of power. Notice that this circuit has no"neutral" conductor to ensure stable voltage to all loads if one should open. What we have here is

a situation similar to our split-phase power circuit with no "neutral" conductor: if one load

should happen to fail open, the voltage drops across the remaining load(s) will change. To ensureload voltage stability in the event of another load opening, we need a neutral wire to connect the

source node and load node together:

So long as the loads remain balanced (equal resistance, equal currents), the neutral wire

will not have to carry any current at all. It is there just in case one or more load resistors should

fail open (or be shut or through a disconnecting switch). This circuit we've been analyzing withthree voltage sources is called a poly phase circuit. The prepare "poly" simply means "more than

one," as in "polytheism" (belief in more than one deity), polygon" (a geometrical shape made of

multiple line segments: for example, pentagon and hexagon), and "polyatomic" (a substancecomposed of multiple types of atoms). Since the voltage sources are all at different phase angles

(in this case, three different phase angles), this is a "poly phase" circuit more specifically, it is a

three-phase circuit, and the kind used predominantly in large power distribution systems.

Let's survey the advantages of a three-phase power system over a single-phase system of

equivalent load voltage and power capacity. A single-phase system with three loads connected

directly in parallel would have a very high total current (83.33 times 3, or 250 amps):


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This would necessitate 3/0 gage copper wire (very large!), at about 510 pounds per

thousand feet, and with a considerable price tag attached. If the distance from source to load was

1000 feet, we would need over a half-ton of copper wire to do the job. On the other hand, wecould build a split-phase system with two 15 kW, 120 volt loads:

Our current is half of what it was with the simple parallel circuit, which is a great

improvement. We could get away with using number 2 gage copper wire at a total mass of about

600 pounds, configuring about 200 pounds per thousand feet with three runs of 1000 feet eachbetween source and loads. However, we also have to consider the increased safety hazard of

having 240 volts present in the system, even though each load only receives 120 volts. Overall,

there is greater potential for dangerous electric shock to occur. When we contrast these two

examples against our three-phase system, the advantages are quite clear. First, the conductorcurrents are quite a bit less (83.33 amps versus 125 or 250 amps), permitting the use of much

thinner and lighter wire. We can use number 4 gage wire at about 125 pounds per thousand feet,

which will total 500 pounds (four runs of 1000 feet each) for our example circuit. This representsa significant cost savings over the split-phase system, with the additional benefit that the

maximum voltage in the system is lower (208 versus 240).One question remains to be answered:

how in the world do we get three AC voltage sources whose phase angles are exactly 120o apart?

Obviously we can't center-tap a transformer or alternator winding like we did in the split-phasesystem, since that can only give us voltage waveforms that are either in phase or 180o out of

phase. Perhaps we could configure out some way to use capacitors and inductors to create phase

shifts of 120o, but then those phase shifts would depend on the phase angles of our loadimpedances as well (substituting a capacitive or inductive load for a resistive load would change

everything!).


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The best way to get the phase shifts we're looking for is to generate it at the source:

construct the AC generator (alternator) providing the power in such a way that the rotating

magnetic field passes by three sets of wire windings, each set spaced 120o apart around thecircumference of the machine:

Together, the six "pole" windings of a three-phase alternator are connected to comprise threewinding pairs, each pair producing AC voltage with a phase angle 120o shifted from either of

the other two winding pairs. The interconnections between pairs of windings (as shown for thesingle-phase alternator: the jumper wire between windings 1a and 1b) have been omitted from

the three-phase alternator drawing for simplicity. In our example circuit, we showed the three

voltage sources connected together in a "Y" configure-duration (sometimes called the "star"configuration), with one lead of each source tied to a common Point (the node where we attached


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the "neutral" conductor). The common way to depict this connection scheme is to draw the

windings in the shape of a "Y" like this:

2.3 Control circuit:


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2.4 Power circuit with star delta connection:


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2.5 Electrical Motors:

Electromechanical device that converts electrical energy to mechanical energy Mechanical energy used to e.g.

Rotate pump impeller, fan, blower Drive compressors Lift materials

Motors in industry: 70% of electricalload2.5.1 Working:


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Types of loads:

2.5.2 Classifications Of Motors:

Motor

loads

Description Examples

Constant

torque

loads

Output power varies

but torque is

constant

Conveyors, rotary

kilns, constant-

displacement pumps

Variable

torque

loads

Torque varies with

square of operation

speed

Centrifugal pumps,

fans

Constant

power

loads

Torque changes

inversely with speed

Machine tools


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2.6 DC MotorsComponents

Direct-Current motors, as the name implies, use a direct-unidirectional current. A DC motor is

shown in the figure and has three main components:

F ield pole.Simply put, the interaction of two magnetic fields causes the rotation in a DCmotor. The DC motor has field poles that are stationary and an armature that turns on

bearings in the space between the field poles. A simple DC motor has two field poles: a

north pole and a south pole. The magnetic lines of force extend across the opening

between the poles from north to south. For larger or more complex motors there are oneor more electromagnets. These electromagnets receive electricity from an outside power

source and serve as the field structure.

Armature. When current goes through the armature, it becomes an electromagnet. Thearmature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For thecase of a small DC motor, the armature rotates in the magnetic field established by the

poles, until the north and south poles of the magnets change location with respect to thearmature. Once this happens, the current is reversed to switch the south and north polesof the armature.

Commutator. This component is found mainly in DC motors. Its purpose is to overturnthe direction of the electric current in the armature. The commutator also aids in the

transmission of current between the armature and the power source.


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2.6.1 Advantages of DCmotors:

1.The main advantage of DC motors is speed control, which does not affect the quality of power

supply. It can be controlled by adjusting:

the armature voltageincreasing the armature voltage will increase the speed the field currentreducing the field current will increase the speed.

2.DC motors are available in a wide range of sizes, but their use is generally restricted to a fewlow speed, low-to-medium power applications like machine tools and rolling mills because of

problems with mechanical commutation at large sizes. Also, they are restricted for use only in

clean, non-hazardous areas because of the risk of sparking at the brushes.

3.DC motors are also expensive relative to AC motors.

The relationship between speed, field flux and armature voltage is shown in the followingequation

Back electromagnetic force: E = KN

Torque: T = KIa

E = electromagnetic force developed at armature terminal (volt)

= field flux which is directly proportional to field current

N = speed in RPM (revolutions per minute)

T = electromagnetic torqueIa = armature currentK = an equation constant

2.6.2 Types of DC Motors:

Self-excited DC motor: Series,Shunt,Compound motors Separately excited DC motor: field current supplied from a separate force

2.6.2.1 Self-excited DC Motor: Series Motor

In a series motor, the field winding (shunt field) is connected in series with the armaturewinding (A) as shown in the figure. The field current is therefore equal to the armature

current.

The following can be said about the speed of a series motor Speed is restricted to 5000 RPM It must be avoided to run a series motor with no load because the motor will

accelerate uncontrollably


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Series motors are suited for applications requiring a high starting torque, such as cranesand hoists.

12

Self-excited DC motor: series motor

Field winding in series

with armature winding Field current =

armature current

Speed restricted to

5000 RPM

Avoid running with

no load: speed

uncontrolled

Suited for high

starting torque:

cranes, hoists

Fig:1.5

Self-excited DC motor

2.6.2.2 DC Compound Motor:

A DC compound motor is a combination of shunt and series motor. In a compound motor, thefield winding (shunt field) is connected in parallel and in series with the armature winding (A) as

shown in the figure.

For this reason this motor has a good starting torque and a stable speed. The higher the percentage of compounding (i.e. percentage of field winding connected in

series), the higher the starting torque this motor can handle.

For example, compounding of 40-50% makes the motor suitable for hoists and cranes,but standard compound motors (12%) are not.


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13

DC compound motor

Field winding in

series and

parallel with

armature winding

Good torque and

stable speed

Higher %

compound in

series = high

starting torque

Suited for high

starting torque if high

% compounding:

cranes, hoists

Fig:1.6

DC compound motor

2.6.2.3 Separately Excited DC Motor:

If field is supplied from a separate source it is called as separately excited DC motor.

In a shunt motor, the field winding (shunt field) is connected in parallel with the armaturewinding (A). The total line current is therefore the sum of field current and armature

current

The following can be said about the speed of shunt motors: The speed is practically constant independent of the load (up to a certain torque

after which speed decreases) and therefore suitable for commercial applicationswith a low starting load, such as machine tools

Speed can be controlled by either inserting a resistance in series with the armature(decreasing speed) or by inserting resistance in the field current (increasing speed)


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11

Separately excited DC motor: field currentsupplied from a separate force

Self-excited DC motor: shunt motor

ld winding parallelth armature winding

rrent = field currentrmature current

Speed constant

independent of load

up to certain torque

Speed control:

insert resistance

in armature orfield current

(Rodwell Int.

Corporation, 1999)

Fig:1.7

Separately excited DC motor

2.7 AC Motors:

Electrical current reverses direction

Two parts: stator and rotor Stator: stationary electrical component Rotor: rotates the motor shaft

Speed difficult to control two types

Synchronous motor Induction motor


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Alternating current (AC) motors use an electrical current, which reverses its direction atregular intervals.

An AC motor has two basic electrical parts: a "stator" and a "rotor". The stator is in thestationary electrical component. The rotor is the rotating electrical component, which in

turn rotates the motor shaft.

The main advantage of DC motors over AC motors is that speed is more difficult tocontrol for AC motors. To compensate for this, AC motors can be equipped with variable

frequency drives but the improved speed control comes together with a reduced power

quality.

There are two types of AC motors: synchronous (see figure) and induction. The maindifference between the synchronous motor and the induction motor is that the rotor of the

synchronous motor travels at the same speed as the rotating magnetic field.

2.7.1 Synchronous Motor:

A synchronous motor is an AC motor, which runs at constant speed fixed by frequency ofthe system.

It requires direct current (DC) for excitation and has low starting torque, and synchronousmotors are therefore suited for applications that start with a low load, such as air

compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they

are often used in systems that use a lot of electricity.

This motor rotates at a synchronous speed, which is given by the following equationSingle phase synchronous motors are available in small sizes for applications requiring

precise timing such as time keeping, (clocks) and tape players. Though battery powered quartz

regulated clocks are widely available, the AC line operated variety has better long term accuracyover a period of months. This is due to power plant operators purposely maintaining the long

term accuracy of the frequency of the AC distribution system. If it falls behind by a few cycles,

they will make up the lost cycles of AC so that clocks lose no time.

Above 10 Horsepower (10 kW) the higher efficiency and leading power factor make

large synchronous motors useful in industry. Large synchronous motors are a few percent more

efficient than the more common induction motors. Though, the synchronous motor is morecomplex.

Since motors and generators are similar in construction, it should be possible to use agenerator as a motor, conversely, use a motor as a generator. A synchronous motor is similar to

an alternator with a rotating field. The figure below shows small alternators with a permanent

magnet rotating field. This could either be two paralleled and synchronized alternators driven by

a mechanical energy sources, or an alternator driving a synchronous motor. Or, it could be twomotors, if an external power source were connected. The point is that in either case the rotors


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Must run at the same nominal frequency, and be in phase with each other. That is, they must be

synchronized. The procedure for synchronizing two alternators is to (1) open the switch, (2)drive both alternators at the same rotational rate, (3) advance or retard the phase of one unit until

both AC outputs are in phase, (4) close the switch before they drift out of phase. Once

synchronized, the alternators will be locked to each other, requiring considerable torque to breakone unit loose (out of synchronization) from the other.

Fig: 1.8Synchronous motor running with step alternator

If more torque in the direction of rotation is applied to the rotor of one of the above

rotating alternators, the angle of the rotor will advance (opposite of (3)) with respect to themagnetic field in the stator coils while still synchronized and the rotor will deliver energy to the

AC line like an alternator. The rotor will also be advanced with respect to the rotor in the otheralternator. If a load such as a brake is applied to one of the above units, the angle of the rotor willlag the stator field as at (3), extracting energy from the AC line, like a motor. If excessive torque

or drag is applied, the rotor will exceed the maximum torque angle advancing or lagging somuch that synchronization is lost. Torque is developed only when synchronization of the motor

is maintained.

In the case of a small synchronous motor in place of the alternator right, it is not

necessary to go through the elaborate synchronization procedure for alternators. However, the

synchronous motor is not self-starting and must still be brought up to the approximate alternatorelectrical speed before it will lock (synchronize) to the generator rotational rate. Once up to

speed, the synchronous motor will maintain synchronism with the AC power source and developtorque.


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Fig: 2.0Addition of field poles decreases speed

A 2-pole (pair of N-S poles) alternator will generate a 60 Hz sine wave when rotated at 3600 rpm

(revolutions per minute). The 3600 rpm corresponds to 60 revolutions per second. A similar 2-pole permanent magnet synchronous motor will also rotate at 3600 rpm. A lower speed motor

may be constructed by adding more pole pairs. A 4-pole motor would rotate at 1800 rpm, a 12-

pole motor at 600 rpm. The style of construction shown is for illustration. Higher efficiency

higher torque multi-pole stator synchronous motors actually have multiple poles in the rotor.

Fig: 2.1

One winding 12 pole synchronous motor

Rather than wind 12-coils for a 12-pole motor, wind a single coil with twelve inter

digitated steel poles pieces. Though the polarity of the coil alternates due to the applied AC,assume that the top is temporarily north, the bottom south. Pole pieces route the south flux fromthe bottom and outside of the coil to the top. These 6-souths are interleaved with 6-north tabs

bent up from the top of the steel pole piece of the coil. Thus, a permanent magnet rotor bar will

encounter 6-pole pairs corresponding to 6-cycles of AC in one physical rotation of the barmagnet. The rotation speed will be 1/6 of the electrical speed of the AC. Rotor speed will be 1/6of that experienced with a 2-pole synchronous motor. Example: 60 Hz would rotate a 2-pole

motor at 3600 rpm, or 600 rpm for a 12-pole motor.


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Shown below for the multi-horsepower (multi-kilowatt) motors used in industry, but an

electromagnet. Large industrial synchronous motors are more efficient than induction motors.

They are used when constant speed is required. Having a leading power factor, they can

correct the AC line for a lagging power factor.

The three phases of stator excitation add vectorially to produce a single resultant magnetic field

which rotates f/2n times per second, where f is the power line frequency, 50 or 60 Hz for

industrial power line operated motors. The number of poles is n. For rotor speed in rpm, multiplyby 60.

S = f120/n

Where: S = rotor speed in rpmf = AC line frequency

n = number of poles per phase

The 3-phase 4-pole (per phase) synchronous motor (Figure 13.12) will rotate at 1800 rpm

with 60 Hz power or 1500 rpm with 50 Hz power. If the coils are energized one at a time in the

sequence --1, --2, --3, the rotor should point to the corresponding poles in turn. Since the sine

waves actually overlap, the resultant field will rotate, not in steps, but smoothly. For example,when the --1 and --2 sine waves coincide, the field will be at a peak pointing between these

poles. The bar magnet rotor shown is only appropriate for small motors. The rotor with multiple

magnet poles (below right) is used in any efficient motor driving a substantial load. These will be

slip ring fed electromagnets in large industrial motors. Large industrial synchronous motors areself-started by embedded squirrel cage conductors in the armature, acting like an induction

motor. The electromagnetic armature is only energized after the rotor is brought up to near

synchronous speed.

Fig: 2.3Three phase four pole synchronous motor


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Small multi-phase synchronous motors may be started by ramping the drive frequency from zero

to the final running frequency. The multi-phase drive signals are generated by electronic circuits,and will be square waves in all but the most demanding applications. Such motors are known as

brushless DC motors. True synchronous motors are driven by sine waveforms. Two or three

phase drive may be used by supplying the appropriate number of windings in the stator. Only 3-phase is shown above.

Fig: 2.4Electronic synchronous motor

The block diagram shows the drive electronics associated with a low voltage (12 VDC)

synchronous motor. These motors have a position sensor integrated within the motor, which

provides a low level signal with a frequency proportional to the speed of rotation of the motor.

The position sensor could be as simple as solid state magnetic field sensors such as Hall effectdevices providing commutation (armature current direction) timing to the drive electronics The

position sensor could be a high resolution angular sensor such as a resolver, inductosyn

(magnetic encoder), or an optical encoder.

If constant and accurate speed of rotation is required, (as for a disk drive) a tachometer

and phase locked loop may be included. This tachometer signal, a pulse train proportional to

motor speed, is fed back to a phase locked loop, which compares the tachometer frequency andphase to a stable reference frequency source such as a crystal oscillator.

Fig: 2.5Phase locked loop controls in synchronous motor speed


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A motor driven by square waves of current, as provided by simple Hall Effect sensors, is

known as a brushless DC motor. This type of motor has higher ripple torque torque variationthrough a shaft revolution than a sine wave driven motor. This is not a problem for many

applications. Though, we are primarily interested in synchronous motors in this section. Ripple

torque, or cogging is caused by magnetic attraction of the rotor poles to the stator pole pieces.Note that there are no stator coils, not even a motor. The PM rotor may be rotated by hand but

will encounter attraction to the pole pieces when near them. This is analogous to the mechanical

situation. Would ripple torque be a problem for a motor used in a tape player? Yes, we do not

want the motor to alternately speed and slow as it moves audio tape past a tape playback head.Would ripple torque be a problem for a fan motor?

If a motor is driven by sine waves of current synchronous with the motor back emf, it is

classified as a synchronous AC motor, regardless of whether the drive waveforms are generatedby electronic means. A synchronous motor will generate a sinusoidal back emf if the stator

Fig: 2.6Motor ripple torque and mechanical analog

Fig:2.7Winding distributed in belt produce a more sinusoidal field


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Magnetic field has a sinusoidal distribution. It will be more sinusoidal if pole windings are

distributed in a belt in Figure across many slots instead of concentrated on one large pole (asdrawn in most of our simplified illustrations). This arrangement cancels many of the stator field

odd harmonics. Slots having fewer windings at the edge of the phase winding may share the

space with other phases. Winding belts may take on an alternate concentric form.

For a 2-phase motor, driven by a sine wave, the torque is constant throughout a

revolution by the trigonometric identity:

sin2tita + cos2tita = 1

The generation and synchronization of the drive waveform requires a more precise rotor

position indication than provided by the hall effect sensors used in brushless DC motors. Aresolver, or optical or magnetic encoder provides resolution of hundreds to thousands of parts

(pulses) per revolution. A resolver provides analog angular position signals in the form of signalsproportional to the sine and cosine of shaft angle. Encoders provide a digital angular positionindication in either serial or parallel format. The sine wave drive may actually be from a PWM,

Pulse Width Modulator, a high efficiency method of approximating a sine wave with a digital

waveform. Each phase requires drive electronics for this wave form phase-shifted by the

appropriate amount per phase.

Fig:2.8

PWM approximate sine wave

Synchronous motor efficiency is higher than that of induction motors. The synchronous

motor can also be smaller, especially if high energy permanent magnets are used in the rotor. Theadvent of modern solid state electronics makes it possible to drive these motors at variable speed.

Induction motors are mostly used in railway traction. However, a small synchronous motor,


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Advantages:

(i) It has simple and rugged construction.

(ii) It is relatively cheap.

(iii) It requires little maintenance.(iv) It has high efficiency and reasonably good power factor.

(v) It has self starting torque.

Disadvantages:(i) It is essentially a constant speed motor and its speed cannot be changed Easily.

(ii) Its starting torque is inferior to DC. shunt motor.

Construction:

A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated fromthe stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of themotor. (ii) Its starting torque is inferior to DC shunt motor.

Stator:

It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations

of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are

provided on the inner periphery of the laminations

When 3-phase supply is given to the stator winding, a rotating magnetic field (See Sec. 8.3) of

constant magnitude is produced. This rotating field induces currents in the rotor by

electromagnetic induction.


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Rotor:

The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. The

winding placed in these slots (called rotor winding) may be one of the following two types:

(i) Squirrel cage type (ii) Wound type

Principle of Operation:

Consider a portion of 3-phase induction motor as shown in Fig. The operation of the motor canbe explained as under:

(i) When 3-phase stator winding is energized from a 3-phase supply, a Rotating magnetic field is

set up which rotates round the stator at synchronous speed Ns (= 120 f/P).(ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are

stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s is

Induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in

the rotor conductors.(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the

stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanicalforces on all the rotor conductors produces a torque which tends to move the rotor in the samedirection as the rotating field.

(iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of

stator field) can be explained by Lenzs law. According to this law, the direction of rotor currentswill be such that they tend to oppose the cause producing them. Now, the cause producing the

rotor currents is the relative speed between the rotating field and the stationary rotor conductors.

Hence to reduce this relative speed, the rotor starts running in the same direction as that of statorfield and tries to catch.

Methods of Starting 3-Phase Induction Motors:

The method to be employed in starting a given induction motor depends upon the size of the

motor and the type of the motor. The common methods used to start induction motors are:

(i) Direct-on-line starting (ii) Stator resistance starting (iii) Autotransformer starting (iv) Star-delta starting(v) Rotor resistance starting.


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Induction Motor Rating

The nameplate of a 3-phase induction motor provides the following information:

(i) Horsepower (ii) Line voltage (iii) Line current (iv) Speed (v) Frequency (vi) Temperature rise

2.7.3 Servo motors:

A closed-loop motion system using a current amplifier, servo controller, servomotoranda position feedbackdevice such as an encoder to precisely control speed and position of

a load.

The MOTORcan be electric, hydraulic, pneumatic or even internal combustion.

The FEEDBACKsensor can be mechanical or electronic, analog or digital and it can berotary or linear

A servo controller with external input

Motor connected to a feedback device is a closed loop system The Feedback device can be Relative or Absolute

Servo Motor Working:

The servo motor has some control circuits and a potentiometer (a variable resistor, aka pot) that

is connected to the output shaft. The potentiometer allows the control circuitry to monitor thecurrent angle of the servo motor. If the shaft is at the correct angle, then the motor shuts off. If

the circuit finds that the angle is not correct, it will turn the motor the correct direction until the

angle is correct. The output shaft of the servo is capable of travelling somewhere around 180degrees. Usually, its somewhere in the 210 degree range, but it varies by manufacturer. A normal

servo is used to control an angular motion of between 0 and 180 degrees. A normal servo is

mechanically not capable of turning any farther due to a mechanical stop built on to the mainoutput gear. The amount of power applied to the motor is proportional to the distance it needs to

travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to

turn only a small amount, the motor will run at a slower speed. This is called proportional

control. How do you communicate the angle at which the servo should turn? The control wire isused to communicate the angle. The angle is determined by the duration of a pulse that is applied

to the control wire. This is called Pulse Coded Modulation. The servo expects to see a pulse

every 20 milliseconds (.02 seconds). The length of the pulse will determine how far the motorturns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree position

(often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the

shaft to closer to 0 degrees. If the pulse is longer than 1.5ms, the shaft turns closer to 180

degrees.


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2.7.4 Stepper Motors:

A stepper motor is a digital version of the electric motor. The rotor moves in discrete

steps as commanded, rather than rotating continuously like a conventional motor. When stopped

but energized, a stepper (short for stepper motor) holds its load steady with a holding torque.Wide spread acceptance of the stepper motor within the last two decades was driven by the

ascendancy of digital electronics. Modern solid state driver electronics was a key to its success.

And, microprocessors readily interface to stepper motor driver circuits.

Application wise, the predecessor of the stepper motor was the servo motor. Today this is

a higher cost solution to high performance motion control applications. The expense and

complexity of a servomotor is due to the additional system components: position sensor and error

amplifier. (Figure 13.24) It is still the way to position heavy loads beyond the grasp of lowerpower steppers. High acceleration or unusually high accuracy still requires a servo motor.

Otherwise, the default is the stepper due to low cost, simple drive electronics, good accuracy,good torque, moderate speed, and low cost.

Fig: 2.9Stepper motor Vs Servo motor

A stepper motor positions the read-write heads in a floppy drive. They were once used for thesame purpose in hard drives. However, the high speed and accuracy required of modern hard


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drive head positioning dictates the use of a linear servomotor (voice coil). The servo amplifier is

a linear amplifier with some difficult to integrate discrete components.

A considerable design effort is required to optimize the servo amplifier gain vs phase

response to the mechanical components. The stepper motor drivers are less complex solid state

switches, being either on or off. Thus, a stepper motor controller is less complex and costly

than a servo motor controller.

Slo-synsynchronous motors can run from AC line voltage like a single-phase permanentcapacitor induction motor. The capacitor generates a 90o second phase. With the direct line

voltage, we have a 2-phase drive. Drive waveforms of bipolar square waves of 2-24V are more

common these days. The bipolar magnetic fields may also be generated from unipolar (one

polarity) voltages applied to alternate ends of a center tapped winding. In other words, DC can beswitched to the motor so that it sees AC. As the windings are energized in sequence, the rotor

synchronizes with the consequent stator magnetic field. Thus, we treat stepper motors as a classof AC synchronous motor.

Characteristics:

Stepper motors are rugged and inexpensive because the rotor contains no winding slip rings,or commentator. The rotor is a cylindrical solid, which may also have either salient poles or

Fig: 3.0Unipolar drive of center tapped coil at (B) emulates AC current in single coil at(A)

Fine teeth. More often than not the rotor is a permanent magnet. Determine that the rotor is apermanent magnet by unpowered hand rotation showing detent torque, torque pulsations. Stepper

motor coils are wound within a laminated stator, except for can stack construction. There may be

as few as two winding phases or as many as five. These phases are frequently split into pairs.

Thus, a 4-pole stepper motor may have two phases composed of in-line pairs of poles spaced 90oapart. There may also be multiple pole pairs per phase. For example a 12-pole stepper has 6-pairs

of poles, three pairs per phase.

Since stepper motors do not necessarily rotate continuously, there is no horsepowerrating. If they do rotate continuously, they do not even approach a sub-fractional hp rated

capability. They are truly small low power devices compared to other motors. They have torque


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2.7.4.1 Variable Reluctance Stepper

A variable reluctance stepper motor relies upon magnetic flux seeking the lowest

reluctance path through a magnetic circuit. This means that an irregularly shaped soft magnetic

rotor will move to complete a magnetic circuit, minimizing the length of any high reluctance airgap. The stator typically has three windings distributed between pole pairs, the rotor four salient

poles, yielding a 30o step angle.(Figure 13.27) A de-energized stepper with no detent torque

when hand rotated is identifiable as a variable reluctance type stepper.

Fig 3.2

Three and Four phase variable reluctance steeper motors

The drive waveforms for the 3-- stepper can be seen in the Reluctance motor section.

The drive for a 4-- stepper is shown in Figure. Sequentially switching the stator phases producesa rotating magnetic field which the rotor follows. However, due to the lesser number of rotor

poles, the rotor moves less than the stator angle for each step. For a variable reluctance stepper

motor, the step angle is given by:


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Fig: 3.3

Stepping sequence for variable reluctance stepper

In Figure moving from -1 to -2, etc., the stator magnetic field rotates clockwise. The rotor moves

counter clockwise (CCW). Note what does not happen! The dotted rotor tooth does not move to

the next stator tooth. Instead, the -2 stator field attracts a different tooth in moving the rotor

CCW, which is a smaller angle (15o) than the stator angle of 30o. The rotor tooth angle of 45oenters into the calculation by the above equation. The rotor moved CCW to the next rotor tooth

at 45o, but it aligns with a CW by 30o stator tooth. Thus, the actual step angle is the differencebetween a stator angle of 45o and a rotor angle of 30o . How far would the stepper rotate if therotor and stator had the same number of teeth? Zero notation. Starting at rest with phase -1

energized, three pulses are required (-2, -3, -4) to align the dotted rotor tooth to the next CCW

stator Tooth, which is 45o. With 3-pulses per stator tooth, and 8-stator teeth, 24-pulses or stepsmove the rotor through 360o.

By reversing the sequence of pulses, the direction of rotation is reversed above right. The

direction, step rate, and number of steps are controlled by a stepper motor controller feeding adriver or amplifier. This could be combined into a single circuit board. The controller could be a

microprocessor or a specialized integrated circuit. The driver is not a linear amplifier, but a

simple on-off switch capable of high enough current to energize the stepper. In principle, thedriver could be

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