Induction Motor

THREE PHASE INDUCTION MOTOR

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction.

CONSTRUCTION

Two major parts:

1. Stator

2. Rotor

An induction motor differs from a dc machine in :

1. Having a shorter air gap between stator and rotor.

2. Absence of commutator

3. Having laminated stator

4. Having speed limitation.

Stator

Stationary part

Eddy current and hysteresis losses occur because of changing flux.

Laminated by dynamo grade sheet steel , with varnish or oxide insulations between layers

Circular laminations for small motors and segmented ones for large motors.

3-phase windings are connected in star/delta in the slots on the inner periphery of laminations.

Radial ventilating ducts along the length of stator core improves cooling.

Minimum thickness of laminations prevent eddy current loss.

ROTOR

Rotating part

Made of dynamo grade sheet steel laminations of greater thickness.

Frequency of flux change is less, so iron loss minimized

No. of slots in rotor laminations = no. of slots in stator laminations.

Rotor slots skewed at an angle with the shaft to :– Reduce magnetic noise– Prevent magnetic locking of rotor and stator.– Produce uniform torque

TYPES OF ROTOR

• SQUIRREL CAGE ROTOR

• WOUND OR SLIP RING ROTOR

SQUIRREL CAGE ROTOR

Has copper bars placed on the

rotor slots

Rotor bars short-circuited at ends

by brass, aluminium rings.

Limitation : low starting torque

Torque can be increased by double

cage rotor or deep bar rotor.

Suited for medium starting torque

requirements.

WOUND ROTOR

3-phase windings provided on

rotor slots.

Windings connected in star.

Ends of windings connected to slip

rings on shaft.

Rotor slots < stator slots.

External resistances reduce

starting current and thus increase

starting torque.

SLIP

The slip is defined as the speed of rotor relative to the rotating magnetic field produced in the stator.

S = Ns – Nr

Ns – speed of rotating magnetic field

Nr - speed of the rotor

condition Percentage slip

standstill 1

No load 1 to 1.5

Full load 4-5 in medium size

2-2.5 in large size

WORKING

3-phase ac-supply applied magnetic field of constant

magnitude and rotating at synchronous speed is produced

lines of force cut across rotor alternating emf

produced current circulates as it is a short-circuited winding

flux density on right strengths flux density on left

weakens flux in air distorts torque acts on

motor

and rotates it .

Working contd.

• At first , frequency of induced emf = frequency of ac supply

• With increase in rotor speed, relative motion between rotor and magnetic field becomes less , and frequency falls.

• Rotor induced emf • Rotor current depend on relative motion• Torque

• No load : rotor speed > synchronous speed, so as to produce torque

• Loaded : speed falls, relative motion increases, torque increases.

TORQUE-SLIP CHARACTERISTICS

• For small values of slip up to 0.05, expression of torque becomes

T = KT x s/α

• Torque is directly proportional to slip. The graph is a straight line.

• In low speed region, slip is high and eqn. becomes:

T = KTα/s

• Torque is inversely proportional to slip, rectangular hyperbola

• Torque is maximum when s = α = R2/X2

• R2 – rotor winding resistance per phase

• X2 – Rotor winding leakage reactance per phase at standstill

STATOR INDUCED EMF

• Induced emf per phase ES = 4.44f T1 ø Kw

• ø – air gap flux per pole• T1 – no. of turns per phase• Kw – winding factor• f -- frequency of ac supply

• Kw = Kc x KdKc – coil span factor : it reduces the resultant emf due to short-pitching of

stator coils

Kd – distribution factor : ratio of resultant emf to sum of separate emfs

ROTOR INDUCED EMF

• Er = 4.44 Fr T2 Kwr

CONDITION CHARACTERISTICS OF FREQUENCY

FORMULA

standstill Fr = f Es x T2 / T1

At the instant of starting High frequency Large magnitude

loaded Small, Fr = s x f Slip x Es T2 / T1

APPLICATIONS

They are mainly used for heavy industrial applications and for

machine tools.

They are now finding use in automotive applications for electric and hybrid electric vehicles.

Induction motors are seen as more rugged for these applications than permanent magnet motors which are vulnerable to possible degradation or demagnetization of the magnets due to over-temperature or accidental over-current at power levels over about 5kW.

SINGLE PHASE INDUCTION MOTOR

• Works on single phase ac supply

• Does not have inherent self-starting torque

• Reduced efficiency, reduced power factor

PRINCIPLE OF OPERATION

• Parts : 1. single phase distributed winding on stator.

2.Squirrel cage short circuited winding on rotor

• CROSS-FIELD THEORY

• DOUBLE REVOLVING FIELD THEORY

DOUBLE REVOLVING FIELD THEORY

• Ferrari’s principle: The alternating magnetic field produced by stator

can be split into two rotating magnetic fields each having half the

magnitude and rotating at synchronous speed in opposite directions.

• Øs, flux in stator is resolved as øf rotating in clockwise direction and

øb rotating in anti-clockwise direction.

• Emf induced by øf is taken +ve and that by øb is taken –ve

• Starting torque developed by both fields is same but directed

opposite, so net torque is zero at standstill.

THEORY CONTD.

• While rotating, the slip due to two

rotating fields are different, so

emfs induced in rotor is also

different.

• Torque developed by forward

field > torque developed by

backward field.

• Resultant torque acts in forward

direction

• Once rotation is initiated, the

motor continues to rotate in that

direction as long as,

max. net torque > load torque

STARTING THE INDUCTION MOTOR

• Simplest method:

1. An auxiliary winding is provided on stator

2. Axes of 2 windings are displaced by 90°

3. Currents flowing in the 2 windings are phase shifted

4. The motor resembles an unbalanced 2-phase motor now.

5. Stator field rotates and torque is produced

6. When the motor attains 75% of the synchronous speed, auxiliary

winding is removed

CLASSIFICATION• Based on starting arrangement provided:

single phase induction motor

Split phase motor

Capacitor start motor

Capacitor start-and-run motor

shaded pole motor

SPLIT-PHASE MOTOR

• Auxiliary winding is present in addition to main winding. It is

connected in parallel to single phase ac supply.

• Main winding has high reactance, auxiliary winding has high

resistance.

• The currents drawn from the windings have a phase difference. they

produce revolving flux and the motor self-starts.

• Im lags the voltage by a greater angle than Ia

• The centrifugal switch disconnects the auxiliary winding from supply

• used in fridges, grinders, washing machines.

Torque-speed characteristics

CAPACITOR START INDUCTION RUN MOTOR

• Capacitor included in series

with auxiliary winding

• Phase angle between the two

currents is increased

• For maximum starting torque,

phase angle should be 90

degrees.

• Electrolytic ac capacitor is used

• Centrifugal switch removes

added arrangements

• Starting torque is high

CAPACITOR-START-AND-RUN MOTORS

• Auxiliary winding and capacitor

connected in circuit at all times

• Generally two capacitors are used

• The start capacitor is of electrolytic

ac type

• Run capacitor is of paper oil type

• Advantages of having a permanent

capacitor:

1. improved overload capacity

2. high power factor and

efficiency

3. no noise

• Used in stokers, blowers

SHADED-POLE MOTOR

• Has salient poles on stator and rotor

• Slots are cut across laminations in each pole

• Short-circuited Cu coils are in smaller part of

pole [shaded pole]

• Supply voltage alternating flux in pole

flux links with shading coil

voltage induced flux in shaded

portion lags flux in unshaded portion

magnetic axis of pole shifts [ equivalent to

motion of pole ] torque produced

• Less efficient

APPLICATIONS

TYPE APPLICATION

Split phase Fans, blowers, centrifugal pumps, office equipments

Capacitor - start Compressors, fridge and AC

Capacitor-start - and -run Direct connected fans, loads requiring low starting torque

Shaded pole Toys, hair-drier, deskfans