lect. rakesh kumar deptt. of ee bhsbiet 1bhsbiet lehragaga

BHSBIET LEHRAGAGA 1

LECT. RAKESH KUMAR DEPTT. OF EE BHSBIET

LECTURE ON INDUCTION MACHINE

BHSBIET LEHRAGAGA 2

Induction Motor Construction Basic Induction Motor Concepts The Equivalent Circuit of an Induction Motor. Power and Torque in Induction Motor. Induction Motor Torque-Speed

Characteristics Variations in Induction Motor Toque-Speed

Characteristics Starting of Induction Motors Speed Control of Induction Motor Some important features of IM

The following points are to be explained in this lecture:-

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PART-I

3 PHASE INDUCTION MACHINE

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Asynchronous machines are

those machines which do not run at synchronous

speed.

Most AC motors are induction

motors. Induction motors are favored

due to their ruggedness and

simplicity. In fact, 90% of industrial

motors are induction motors.

By induction motor, we mean that the stator

windings induces a current which flow

in the rotor conductors, like a

transformer.

INTRODUCTION

5

Overview of Three-Phase Induction Motor

Induction motors are used worldwide in many residential, commercial, industrial, and utility applications.

Induction Motors transform electrical energy into mechanical energy.

It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer.

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Cutaway view of a modern induction motor

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With rotary or linear motion

Three phase supply or single-

phase supply

With wound or cage rotor

The induction machines may be classified in many ways.

8

Construction The three basic parts of an AC motor are the rotor,

stator, and enclosure. The stator and the rotor are electrical circuits that

perform as electromagnets.

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An induction motor is composed of a rotor, known as an armature, and a stator containing windings connected to a poly-phase energy source.

Construction

10

Construction (Rotor construction)

The rotor is the rotating part of the electromagnetic circuit.

It can be found in two types:◦ Squirrel cage◦ Wound rotor

However, the most common type of rotor is the “squirrel cage” rotor.

11

Induction motor types:

Squirrel cage type:Rotor winding is composed of copper bars embedded

in the rotor slots and shorted at both end by end ringsSimple, low cost, robust, low maintenance

Wound rotor type:Rotor winding is wound by wires. The winding

terminals can be connected to external circuits through slip rings and brushes.

Easy to control speed, more expensive.

Construction (Rotor construction)

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The stator slotted magnetic core

The stator electric winding

The rotor slotted magnetic core

The rotor electric winding

The rotor shaft with bearings

The stator frame with bearings

The cooling system

The terminal box

The main parts of any IM are:-


STATOR OF IM


COILS IN THE ROTOR


SQUIRREL CAGE ROTOR


A WOUND ROTOR WITH SLIP RINGS


SLIP RINGS


Stator: The stator consists of stator frame, core and stator winding.

o Stator frame:- It is the outer body of the motor used to support stator core and windings and also to protect the inner parts of the machine. The frame may be die-cast or fabricated.

Constructional features of squirrel cage and wound type motor


Stator core :- The stator core is assembled of high grade, low electrical loss, silicon steel punching. oThe thickness varies from 0.35 mm to 0.7

mm. oThe laminations are used to reduce eddy

current loss. oThe laminations are slotted on the inner

periphery and are insulated from each other. The insulated stator conductors are placed in these slots.


Stator or field winding:- The stators conductors are connected to

form a three phase winding. The three phases of the winding can be connected in either star or delta.


ROTOR:- oThe rotor comprises of a cylindrical

laminated iron core with slots on outer periphery.

o The rotor conductor are placed in these slots.

oThe laminated cylindrical core is mounted directly on the shaft or a spider carried by the shaft.


There are basically 2 types of rotor construction.

Squirrel Cage – No windings and no slip rings

Wound rotor - It has 3 phase windings, usually Y connected, and the winding

ends are connected via slip rings


In cage construction, rotor conductors, in the form of bars made of copper, or aluminium are placed in rotor slots parallel to the rotor shaft.

The rotor bars are short circuited by end rings of same material at each end.

The rotor slots are not parallel to the motor shaft but are skewed to reduce magnetic locking of stator and rotor and also to reduce humming noise while running.

SQUIRREL CAGE ROTOR:-


SQUIRREL CAGE ROTOR OF IM


Squirrel cage rotor — a schematic


The rotor is wound with an insulated winding similar to that of the stator.

The rotor winding is always 3 phase winding. The winding may be star or delta connected, but star connections are usually preferred.

The three terminals of star connections are brought outside the rotor and connected to three slip rings.

The carbon brushes are pressed on the slip rings. External resistors can be inserted in series with the rotor winding for speed and starting torque control.

Wound rotor:-


WOUND ROTOR OF IM


SQUIRREL CAGE IM

SIMPLE IN CONSTRUCTION

RELIABLE AND CHEAP

MAINTENANCE FREE

ITS WINDING CAN ADJUST ITSELF TO ONLY NUMBER OF STATOR POLES

STARTING TORQUE CANNOT BE CONTROLLED

Differences between squirrel cage IM and slip ring IM


PRINCIPLE OF OPERATION

•Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor.


If a short circuited conductor is placed within a rotating magnetic field, an emf is induced in the conductor due to EMI.

Due to this emf, current starts flowing in the conductor and sets up its own magnetic field.

Due to the interaction of these two field, a

torque is produced and conductor tends to move


A three phase IM stator have a three phase distributed winding.

When we give supply to stator then a rotating magnetic field produces which rotates at synchronous speed.

The rotating flux cuts the rotor conductors and emf produced in them.

Because these conductors are short circuited so current is produced in conductors so rotor m.m.f is produced which produces synchronously rotating rotor poles.

How rotor rotates?


PRODUCTION OF TORQUE


Stator equivalent circuit


Final equivalent circuit of IM


The difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slip

s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm)

SLIP


SLIP RINGS AND BRUSHES IN IM


• At no-load, the slip is nearly zero (<0.1%).

• At full load, the slip for large motors rarely exceeds 0.5%. For small motors at full load, it rarely exceeds 5%.

• The slip is 100% for locked rotor.


The frequency induced in the rotor depends on the slip:

Frequency induced in the rotor

fR= frequency of voltage and current in the rotorf = frequency of the supply and stator fields = slip


Torque- slip curve


Time Harmonics: o These affects torque and cause

considerable heating in the machine and are hence a cause for concern.

o These harmonics are called time harmonics since they are generated by a source that varies non sinusoidally in time.

Harmonics in Induction Machines


Space Harmonics: o The space harmonics, are a result of

non-sinusoidal distribution of the coils in the machine and slotting.

o These have their effects on the speed, torque and current of the machine.


Effects of harmonics on loaded machine


Active Power Flow


AIR GAP POWER:- Air gap power is the power transferred from

stator to rotor across the air gap. Pg = 3I22 (r2 / s)

Shaft power :- Shaft power is the output power i.e.

available at the shaft. Psh = Pg – rotor ohmic loss – friction and windage loss


OPERATING TORQUE:- Torque from light load to full load conditions.

STARTING TORQUE:- Torque at start when slip = 1

BREAKDOWN TORQUE:- Maximum torque that motor can develop. If

motor is loaded beyond this torque, the motor will decelerate and come to stand still.


SPEED-TORQUE CHARACTERISTIC


Induced Torque is zero at synchronous speed.

The graph is nearly linear between no load and full load (at near synchronous speeds).

Max torque is known as pull out torque or breakdown torque

Starting torque is very large. Torque for a given slip value would change

to the square of the applied voltage. If the rotor were driven faster than

synchronous speed, the motor would then become a generator.

Comments on the IM Torque Speed Curve


Plugging is a braking action to bring the rotor to a quick stop. Plugging is obtained by interchanging any two stator leads.

With this the phase sequence is reversed and the direction of rotating magnetic field becomes opposite to that of the rotor rotation.

The electromagnetic torque now acts opposite to rotor rotation and produces braking action

Braking an Induction Motor(Plugging)


Variations in Induction Motor Torque-Speed Characterictics


D.O.L Starter Star- delta starter Auto-transformer starter

Types of starters used in IM


Star-delta starter


Auto-transformer starter


Pole changing Stator voltage control Supply frequency control Rotor resistance control Slip energy recovery

Speed control methods:-


We know that ns=120f/p . By changing the value of p speed can be changed.

By providing the stator with independent windings each wound for different member of poles. This results in two speed motor.

The stator is provided with one winding or with two independent windings, but coils of each winding can be re-connected to produce a different number of poles in the ratio of 2:1.

Two independent windings on the stator, each being designed to give different no. of poles in the ratio of 2:1 can give four different no. of poles in the ratio of 3:2:1.5:1 and thus, a four speed induction motor can be obtained.

Pole changing method:


POLE CHANGING METHOD:


Pole Changing: Various connections


1.


2.


3.


The speed can be controlled by varying the supply voltage until the torque required by the load is developed at the desired speed.

The torque developed is proportional to the square of the supply voltage and current is proportional to the voltage.

Therefore, voltage is reduced to reduce speed for the same current, the torque developed by the motor is reduced.

This method is suitable where load torque decreases with speed e.g. fan load.

Stator voltage control:-


Speed-torque curves: voltage variation


The variable frequency supply is obtained by the following converter.◦Voltage source inverter◦Current source inverter◦Cycloconverter

An inverter converts a fixed voltage d.c. to a fixed ( or variable ) voltage a.c. with variable frequency.

A Cycloconverter converts a fixed voltage and fixed frequency a.c. to a variable voltage and variable frequency a.c.

Variable frequency control:


Torque-speed curves with V/f constant


Torque-speed curves with E/f held constant


This method is applicable to slip ring induction motor only.

The speed of motor can be controlled by connecting external resistance in the rotor circuit.

The starting torque increases with increase in resistance, the pull out speed of the motor decreases but the maximum torque remains constant, the speed can be controlled from the rated speed to lower speed.

Rotor resistance control


Speed-torque curves : rotor resistance variation


Effect of changing rotor-circuit resistance(torque-slip curve)


The basic principle of slip power recovery is to connect an external source of emf of slip frequency of the rotor circuit.

This method is known as scheribus scheme.

Slip energy recovery a scheme


A portion of rotor a.c. power is converted into d.c by a diode bridge.

The output of the rectifier is connected to the d.c. terminals of the inverter, which inverts this d.c. power to a.c. power and feeds it back to the a.c source. It provides the speed control synchronous speed.


Some important features of IM


Leakage reactance is the impedance due to the leakage flux.

More the leakage reactance more magnetizing current is required to obtain working m.m.f .

It also decreases the power factor and gives more losses in motor.

Leakage reactance


COGGING:- When number of slots of rotor and stator

are equal, then a magnetic interlocking takes place and motor does not start. It occurs during starting of motor.

CRAWLING:- In this phenomenon motor starts to run

stably at speed lower than rated speed due to presence of harmonics. It occurs during running of motor.


FIXED LOSSES- These loses are composed of◦Core loss◦Bearing friction loss◦Brush friction loss in slip ring IM only◦Windage loss

VARIABLE LOSSES-These losses are composed of◦Stator ohmic loss◦Rotor ohmic loss◦Brush contact loss only for slip ring IM◦Stray load loss

LOSSES IN AN IM


The relationship betweenthe input electric power and the output

mechanical power of this motor is shown :

elow


The higher the speed of an induction motor, the higher the friction, windage, and stray losses.

On the other hand, the higher the speed of the motor (up to nSYNC), the lower its core losses.

Therefore, these three categories of losses are sometimes lumped together and called rotational losses.

ROTATIONAL LOSSES


Difference betweenSINGLE CAGE IM DOUBLE CAGE IM

Low starting torque High operating slip Low operating

efficiency

High starting torque Low operating slip High operating

efficiency


If a polyphase IM is given a voltage and frequency for supply mains and rotate at speed higher than a synchronous speed by a prime mover, then rotor overtakes rotating magnetic field as a result emf and currents in rotor reverse their direction. This is called induction generator.

Induction generator


3-phase IM has an air gap between stator and rotor winding due to which motor needs high magnetizing current for the production of working magnetic flux.

Power factor can be improved by following methods.◦ Reducing the air gap between stator and rotor

winding◦ By use of static capacitor across stator terminal◦ For wound motor, by use of auxiliary machines

Improving power factor of 3 phase IM


Increasing rotor circuit resistance

Improving starting torque of a 3-phase IM


Induction machine never reach synchronous speed. Why?


PART-II

SINGLE PHASE INDUCTION MACHINE

Single Phase InductionPermanent-split capacitor motor

One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split capacitor motor in Figure

Main and Auxiliary windings

1-Phase Induction Motor This type of motor suffers increased current

magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor.

1-Phase Induction Motor This motor configuration works well up to

1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter

Capacitor-start induction motor In Figure a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes.

Capacitor-run motor induction motor

A variation of the capacitor-start motor Figure is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

1-phase Capacitor start

Capacitor Start-Run Induction Motor

A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Resistance split-phase motor induction motor

If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.

Wound rotor induction motors

A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting.

This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart.

Wound Rotor Induction M/C

Why put resistance in series with the rotor? Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting. While this is not a severe problem for small motors, it is for large (10's of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT).


Figure shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting.


The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is started. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor.

Wound Rotor Induction This motor is suited for starting high inertial

loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its' synchronous speed

Speed Control Motor speed may be varied by putting variable

resistance back into the rotor circuit. This reduces rotor current and speed. The high starting torque available at zero speed, the down shifted break down torque, is not available at high speed. See R2 curve at 90% Ns, Resistors R0R1R2R3 increase in value from zero. A higher resistance at R3 reduces the speed further. Speed regulation is poor with respect to changing torque loads. This speed control technique is only useful over a range of 50% to 100% of full speed. Speed control works well with variable speed loads like elevators and printing presses.

Speed Control

Shaded Pole Induction Motor

Main windings and Shaded Pole winding at the Stator, while shaded pole is short circuited.

RELUCTANCE MOTOR

Same as: Split phaseorCapacitor Start

Motor

Stator

RELUCTANCE MOTOR

Same as: Squirrel cage motor

Rotor

Uneven slots cut into laminations to form Salient poles

BUT, with

Uneven slots assist in startingRotor Slots generally ≠ Stator Slots

RELUCTANCE MOTOR

Starting

Motor becomes Synchronous

As per induction motor with squirrel cage providing torque

Centrifugal switch operating as per normal(75%)

As motor is lightly loaded slip speed is small

Rotor salient poles become magnetised and lock with RMF

RELUCTANCE MOTOR

If rotor poles are a multiple of the stator poles

Motor will operate at sub-multiples of synchronous

speed

HYSTERESIS MOTOR

Rotor Outer section made up of hardened steel

HYSTERESIS MOTOR

Rotor Outer section made up of hardened steel

This outer section supported on the shaft by a NON-MAGNETIC “Arbour”

HYSTERESIS MOTOR Rotor has a very high Hysteresis loss

The rotor tends to become magnetised

A synchronous motor is born

PROBLEM

Synchronous motors have ZERO START TOURQUE!A Shaded pole stator is used

UNIVERSAL MOTORNot the same as a series DC Motor

Fields are laminated for AC current

UNIVERSAL MOTOR


Following are the limitation of the of single phase induction motor :

Single phase motor is not self starting . Single phase induction motor has low power factor

as compared to three phase induction motor. For same rating single phase motor has big frame

size. Single phase motor has lower efficiency. Single phase motor has higher core and copper

losses.

Limitation of single phase induction motor in reference to 3 phase induction motor.


Centrifugal switch automatically disconnected the starting winding at about to 80% of synchronous speed. If centrifugal switch is not used then starting winding remains in circuit which gives noisy performance.

Why in split phase motor ,starting winding is connected through centrifugal switch?


Single phase induction motor are different from three phase induction motor in following aspects:

Single phase motors are not self starting while three phase induction motors self starting.

For same load torque, 1-φinduction motor requires more stator current and operates at a higher slip.

For the same size ,1-φinduction motor output is less. It has higher temperature rise and lower efficiency as compared to three phase induction motor.

Difference between single phase induction motor and three phase induction motor .


The advantage of capacitor start, capacitor run induction motor are◦Improvement of over load capacity of the motor ◦Higher efficiency ◦Higher power factor ◦Quieter operation of the motor

What are the advantage of capacitor start capacitor run induction motor ?


Single phase induction motor is not self starting . It provided with starting winding in addition to main winding to temporarily convert it into a two phase motor at the time of starting . The two currents produce a revolving flux and make the motor self starting.

Why starting winding is required in single phase induction motor?


Due to the presence of shading coil in shaded pole motor ,the shifting of magnetic axis takes place when an alternating current is passed through field winding .the rotor starts rotating in the direction of this shift i.e. from unshaded part to the shaded part.

What is the role of shading coil in shaded pole motor?


The rotor of a shaded pole motor will rotate in a direction from the unshaded part to the shaded part.

In what direction will the rotor of shaded pole motor rotate?


The direction of rotation can be reversed by reversing the line connections of either the main winding or the starting winding .

How is the direction of rotation of a single phase induction motor reversed?


The single phase capacitor type induction motors has relatively good starting torque, high power factor because the phase angle between the running winding current and the starting winding current is practically 90 electrical degree.

Why single phase capacitor type induction motor are superior in performance?


The main reason of lagging power factor is high magnetizing current which is lagging in nature .single phase motor carries magnetizing current for both forward and backward fields. Therefore ,this magnetizing current is higher than 3 phase induction motor.

Why power factor of single phase motor is lagging?


The increase in rotor resistance of a single phase induction motor reduces its breakdown torque , lower the efficiency and increases the slip at which maximum torque occurs.

What is the effect of increasing rotor resistance in a single induction motor ?


The auxiliary winding should be more resistance to give higher starting torque . As this winding stays in the circuit at the time of starting only , so does not affect the copper losses in the machine.

A single phase induction motor is provided with a main winding and an auxiliary winding . Which of the two winding

should be more resistance and why?


Stator of repulsion motor consist of single phase exciting winding and rotor have distributed d.c. winding.

The brushes are short circuited on themselves and not connected to the supply circuit.

Armature receives power from the stator by transformer action .

Repulsion motor


Fig.(a) , θ=α Fig.(b),α=90 [ Fig.(c), α=180 [

Schematic diagram shown in fig.(a). when angle α is 906 then there is no mutual induction between stator and rotor. So no torque will be produced as shown in fig(b) .When angle α=0 then magnetic axis of stator and rotor coincides so mutual induction between stator and rotor will be maximum as in fig(c).


The field axis and quadrature to field axis position of the brushes are such in which repulsion motor does not developed any starting torque.

What are the two position of brushes at starting in which the repulsion motor does not develop the starting torque?


The speed variation is affected either by changing the position of brushes or by varying the impressed voltage

How speed variation is affected in repulsion motor?


With the increases in torque on a repulsion motor stator current increases and power factor decreases.

What happens when the shaft torque of a repulsion motor with two stator windings increases?


Yoke , pole and armature cores of a.c series motors are laminated so as to reduce eddy current losses, and so efficiency is improved and heating is reduced.

Why laminated yoke is used in an a.c series motor?


The centrifugal switch is provided to short circuit all the commutator segments at about 75-80 percent of synchronous speed. It is also used to lift the brushes from the commutator in some motors.

Why centrifugal switch is provided in repulsion start induction motor?


The normal full load slip of a single phase induction motor is higher than that of a three phase motor, owing to development of backward rotating field . The power is to be delivered to the backward field from the power converted into mechanical power by forward field.

Why is the normal full load slip of a single phase induction motor higher than that of a three phase

induction motor?


The capacitor start motor develops higher starting torque than does an equally rated resistance start split phase with a lower in rush current.

What are the advantage of using a capacitor start motor over a resistance start split phase motor?


THANKS

lect. rakesh kumar deptt. of ee bhsbiet 1bhsbiet lehragaga

Documents

bhsbiet lehragaga slide

rotor winding

induction motor types

stator core

rotor slots

rotor conductors

stator windings

squirrel cage rotor