Chapter 1 - Introduction to AC Motor

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Chapter 1: Introduction to AC Motor ET601 DET 6D Jun 2013

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1. Synchronous MotorA synchronous machine is an AC rotating machine whose speed under steady state condition is

proportional to the frequency of the current in its armature. The magnetic field created by the armature

currents rotates at the same speed as that created by the field current on the rotor, which is rotating at

the synchronous speed, and a steady torque results.

Synchronous machines are commonly used as generators especially for large power systems,

such as turbine generators and hydroelectric generators in the grid power supply. Because the rotor

speed is proportional to the frequency of excitation, synchronous motors can be used in situations

where constant speed drive is required. Since the reactive power generated by a synchronous machine

can be adjusted by controlling the magnitude of the rotor field current, unloaded synchronous machines

are also often installed in power systems solely for power factor correction or for control of reactive

kVAflow.

Such machines, known as synchronous condensers, may be more economical in the large sizes

than static capacitors. With power electronic Variable Voltage Variable Frequency (VVVF) power

supplies, synchronous motors, especially those with permanent magnet rotors, are widely used for

variable speed drives. If the stator excitation of a permanent magnet motor is controlled by its rotor

position such that the stator field is always 90o (electrical) ahead of the rotor, the motor performance

can be very close to the conventional brushed dc motors, which is very much favored for variable speed

drives. The rotor position can be either detected by using rotor position sensors or deduced from the

induced EMF in the stator windings. Since this type of motors does not need brushes, they are known

as brushless DC motors.

Synchronous Machine Structures

Stator and Rotor

The armature winding of a conventional synchronous machine is almost invariably on the stator and is

usually a three phase winding. The field winding is usually on the rotor and excited by dc current, or

permanent magnets. The dc power supply required for excitation usually is supplied through a DC

generator known as exciter, which is often mounted on the same shaft as the synchronous machine.

Various excitation systems using AC exciter and solid state rectifiers are used with large turbine

generators. There are two types of rotor structures: round or cylindrical rotor and salient pole rotor as

illustrated schematically in the diagram below. Generally, round rotor structure is used for high speed

synchronous machines, such as steam turbine generators, while salient pole structure is used for low

speed applications, such as hydroelectric generators. The pictures below show the stator and rotor of ahydroelectric generator and the rotor of a turbine generator.


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2. Asynchronous Motor / Induction MotorInduction motors are simple and rugged and relatively cheap to construct. They consist of a wound

stator and a rotor assembly. They have fixed stator windings that are electrically connected to an AC

power source. Current is induced in the rotor circuit. The resulting magnetic field interacts with the

stator field for the induction to occur. No separate power source is required to provide the rotor field.

An induction motor can be started and accelerated to steady state running conditions simply by

applying AC power to the fixed stator windings of the motor. They do not rely on brushes like a DC

motor does. Induction motors have a longer life than synchronous motors and are common for

applications above 1 kW.

There are a couple of types of induction motors a squirrel-cage motor and a wound rotor

motor. A squirrel-cage motor is one where the secondary circuit consists of a number of conducting

bars that have their end pieces connected by metal rings or plates at each end. A wound-rotor motor in

one where the secondary circuit has a poly phase winding or coils whose terminals are either short

circuited or closed through suitable circuits.

The rotor assembly of an induction motor, when looked at from the end, resembles a squirrel

cage (or a hamster exerciser). Thus the name squirrel-cage motor refers to an induction motor. The

most common rotor type has cast aluminum conductors (bars) and short-circuiting end rings. The

position of the bars in relation to the surface of the rotor, the shape, cross sectional area and material of

the bars determine the rotor characteristics. A bar with a large cross sectional area will exhibit a low

resistance.

A copper bar will have a low resistance compared to a brass bar of equal proportions. The rotor

design will determine the starting characteristics of the motor. The rotor turns when the moving

magnetic field induces a current in the shorted conductors. The stator of an induction motor is the outer

body of the motor. This houses driven windings on an iron core. The standard stator has three windings

for a three-phase design. A single-phase motor typically has two windings. The core of the stator is


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made up of a stack of round pre-punched laminations pressed into a frame that is made of aluminum or

cast iron. Laminations are round with a round hole where the rotor is positioned.

The inner surface of the stator has slots or grooves where the windings are positioned. The

arrangement of the windings determines the number of poles that a motor has. A stator is like an

electromagnet and has poles (north and south) in multiples of two (2-pole, 4-pole, etc.). The voltage

rating of the motor is determined by the number of turns on the stator. The power rating of the motor is

determined by the losses. These include copper loss, iron loss and the ability of the motor to dissipate

the heat generated by the losses. The design of the stator determines the rated speed of the motor as

well as the full load/full speed characteristics. The synchronous speed of the motor is the speed where

the magnetic field rotates. It is determined by the number of poles in the stator and the frequency of the

power supply. It is the absolute upper limit of motor speed.

There is no difference between the rotor speed and rotating field speed. This means no voltage

is induced in the rotor bars and therefore no torque is developed. When running, the rotor must rotate

slower than the magnetic field, to cause the proper amount of rotor current to flow so that the torque

that develops is able to overcome the winding and friction losses and therefore drive the load. This

speed difference is called slip. Most motors use the squirrel cage design. An alternate design, wound

rotor, is used when variable speed is desired.

Compared to squirrel cage rotors, wound rotors are expensive and require more maintenance. A

wound rotor motor has controllable speed and torque. Single-phase AC induction motors are typicallyused in devices requiring low torque like fans and other household appliances. A split-phase induction

motor is used in larger household appliances.

Idealized three phase, two pole induction motor


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Wound Rotor / Slip Ring

A wound rotor has a complete set of three phase windings that are mirror images of the windings on the

stator. The three phases of the rotor windings are usually Y connected, and the ends of the three rotor

wires are tied to slip rings on the rotors shaft. The rotor windings are shorted through brushes riding on

the slip rings. Wound rotor induction motors therefore have their rotor currents accessible at the stator

brushes, where they can be examined and where extra resistance can be inserted into the rotor circuit.


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6. Comparison between Squirrel Cage Rotor and Wound (Slip Ring) Rotor


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7. Describe StatorStator is the stationary part of the Induction motor. It is the part to which supply is given. Stator of

Induction motor is built up of the low hysteresis loss or high permeability material such as CRGO

(Cold Rolled Grain Oriented) Silicon Steel.8. Stator ArrangementTechnically whenever flux is changed in a core, in accordance with Faradays Law of Electro-Magnetic

Induction, an EMF is induced in the core and as the core is short circuited, this results in the current

which is termed as Eddy Current.

Some energy is dissipated in form of heat due to this current and this is called eddy current loss.

It depends upon the thickness of the sheet of core because more thick the core is there more content or

material will be there and hence more EMF will be induced resulting in more eddy losses. So the point

is to reduce eddy current loss we will have to reduce current, for which induced voltage have to be

reduced and this is possible by limiting thickness of the sheet. Hence Eddy current losses are reduced

by using thin laminated sheets to construct the stator of the machine.

These stator laminations require a base to be mounted and this base is termed as stator frame.

This is usually made up of Cast Iron or Steel with slots in it. These slots are used to host stator winding.


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The other term used to describe the relative motion is slip, which is the relative speed expressed on a

per unit (or sometimes a percentage) basis. That is, slip is defined as:

This equation can also be expressed in terms of angular velocity (radians per second) as:

Notice that if the rotor turns at synchronous speed, s = 0, while if the rotor is stationary, s = 1. All

normal motor speeds fall somewhere between those two limits.

It is possible to express the mechanical speed of the rotor shaft in terms of synchronous speed and slip.

or

11. The Electrical Frequency on the RotorAn induction motor works by inducing voltages and currents in the rotor of the machine, and for that

reason it has sometimes been called a rotating transformer. Like a transformer, the primary (stator)

induces a voltage in the secondary (rotor), but unlike a transformer, the secondary frequency is not

necessarily the same as the primary frequency.

If the rotor of a motor is locked so that it cannot move, then the rotor will have the same

frequency as the stator. On the other hands, if the rotor turns at synchronous speed, the frequency on the

rotor will be zero.

At nm= 0 rpm, the rotor frequencyfr=f, and the slips=1. At nm= nsync, the rotor frequencyfr=

0 Hz, and the slip = 0. For any speed in between, the rotor frequency is directly proportional to the

difference between the speed of the magnetic field nsyncand the speed of the rotor nm.


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The rotor frequency can be expressed as:

or

or

( )

12. Diagram of the Power Transmission of an Induction MotorLosses and the Power-Flow Diagram

An induction motor can be basically described as a rotating transformer. Its input is a threephase set

of voltages and currents. For an ordinary transformer, the output is electric power from the secondary

windings. The secondary windings in an induction motor (the rotor) are shorted out, so no electrical

output exists from normal induction motors. Instead, the output is mechanical. The relationship

between the input electric power and the mechanical power of this motor is shown in the power flowdiagram below.

The input power to an induction motorPinis in the form of three phase electric voltages and

currents. The first losses encountered in the machine are I2R losses in the stator windings (the stator

copper loss PSCL).

Then some amount of power is lost as hysteresis and eddy currents in the stator (Pcore). The

power remaining at this point is transferred to the rotor of the machine across the air gap between the

stator and rotor. This power is called the air gap power,PAGof the machine.


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After the power is transferred to the rotor, some of it is lost as I2R losses (the rotor copper

losses PRCL), and the rest is converted from electrical to mechanical form (Pconv). Finally, friction and

winding losses, PF&Wand stray losses, Pmiscare subtracted. The remaining power is the output of the

motor, Pout.

13. Construct Equivalent Circuit of an Induction Motor Using Calculation and RelatedFormula

An induction motor relies for its operation on the induction of voltages and currents in its rotor circuit

from the stator circuit (transformer action). Because the induction of voltages and currents in the rotor

circuit of an induction motor is essentially a transformer operation, the equivalent circuit of an

induction motor will turn out to be very similar to the equivalent circuit of transformer. An induction

motor is called asingly excitedmachine (as opposed to a doubly excitedsynchronous machine), since

power is supplied to only the stator circuit. Because an induction motor does not have an independent

field circuit, its model will not contain an internal voltage source such as internal generated voltage EA

in a synchronous motor.

The rotor current can be found as:

Where:

s = slip

ER0 = induced rotor voltage at lockedrotor condition

Chapter 1 - Introduction to AC Motor

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