Induction Motor

INDUCTION MOTORINDUCTION MOTOR

(Construction and Operation)

What is an Electric Machine?What is an Electric Machine?

“An electric machine is a device that can convert either mechanical energy to electrical energy (Generator) or electrical energy to mechanical energy (Motor).”

A.C Machine (Motor)A.C Machine (Motor)

“An electric machine (Motor) that converts A.C electric energy to mechanical energy.”

Types of MotorsTypes of Motors

• D.C Motors

• A.C Motors

A) Synchronous Motors

B) Asynchronous Motors

a) Induction Motors

b) Commutator Motors

Types of Induction MotorTypes of Induction Motor

• There are mainly Two types of Induction Motor:-

Squirrel Cage Induction MotorSlip-Ring Induction Motor

Induction MotorInduction Motor

It is defined as ,“A rotating Transformer.”

An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.

This definition of Induction Motor is due to the fact that it works on the principle of a Transformer.

ConstructionConstruction

The figure shows the internal structure of Induction Motor.

Construction (contd.)Construction (contd.)



Main Parts of Induction Motor

Stator Rotor

StatorStator

The stationary part of the motor is called stator.

The stator is also called armature although it does not rotate.

In Induction Motor A.C power is applied to the stator directly.

The stator of an Induction Motor is shown in figure below.

StatorStator

StatorStator

StatorStator

StatorStator

StatorStator

Stator has three parts:

a) Frame

b) Core

c) Winding

Stator (Frame)Stator (Frame)

It is used for supporting and protecting the armature core stampings and winding coil ends.

It is made of cast iron. The frame takes various forms depending

on the operating conditions.


The open type provides better natural ventilation and cooling.

The drip proof type incorporates a cast-iron frame completely enclosing the top half of the motor but with opening below the centre line for the admission and discharge of air impelled by fan blades on the rotor.


The totally enclosed type prevents the exchange of air between the inside and outside of the enclosing case.

Enclosed self: fan cooled motors provide cooling by external fans which are integral part of the machine.

Enclosed separately: the air is circulated by fans or blowers separate from the motor itself.

Stator (core)Stator (core)

The core of an Induction Motor is built up ofsilicon steel laminations provided with slots in its inner cylindrical surface.There are three types of core of an Induction

Motor:-1) Open slot core2) Semi-closed slot core3) Segmental laminations

Stator (windings)Stator (windings)

The windings may be 1-phase or poly-phase according to the type of Induction Motor.

Each phase of the windings is made of multi-turn coils distributed in several slots per pole instead of placing all the coils of a phase in one slot.

The winding coils of each phase are placed in the slots and connected with each other in such a way as to obtain a required number of poles.


There are different types of windings:

1) Concentrated winding

2) Distributed winding

Distributed windingDistributed winding

(a) Lap winding(b) Wave winding(c) Spiral winding(d) Single-layer winding(e) Double-layer winding(f) Full-pitch winding(g) Fractional-pitch winding(h) Half-coil winding(i) Whole-coil winding


Concentrated winding

The coils of each phase are grouped together in a single slot under each pole, uniformly placed around the periphery of armature.

This winding is rarely used for armatures.


Distributed winding The coils of each phase are placed in several

slots (more than one) under each pole instead of placing a single slot per pole.

The conductors in adjacent slots are slots are acted upon successively and generate voltages that are not in phase.

All practical a.c. armature windings are of this type.


Distributed winding

The end connections of a distributed winding may be arranged in several ways, all electrically identical.

RotorRotor

The Rotor also has a core and a winding. It rotates inside the stator. The core is made of silicon steel

laminations having slots in its outer cylindrical surface.

The laminations are assembled and packed together with the help of two end plates made of cast-iron, on a hub which is keyed to the shaft.

RotorRotor

Types of Rotors in Induction Motor

1) Squirrel-Cage Rotor

2) Wound Rotor or Slip-ring Rotor

RotorRotor

The figure shows different types of Rotors:

RotorRotor

Squirrel-Cage Rotor

RotorRotor

Squirrel-Cage Rotor The Squirrel-Cage rotor (I.M) is the simplest type

of Induction Motor and the most generally used. The core of the rotor is usually , built up of

slotted steel punchings. The slots are mostly semi-closed or totally-

closed. The totally enclosed slot is particularly well

adopted to the casting of aluminium rotor windings.

Rotor (Squirrel-cage)Rotor (Squirrel-cage)

With motors upto 30 h.p and even greater the entire rotor winding is made of die-cast aluminium.

No insulation is required on the conductors of a cage rotor because the rotor induced voltages are too small to force an appreciable current through the laminated core having high electrical resistance parallel to the rotor conductors.

Rotor (wound)Rotor (wound)

The figures show wound Rotors:



This type of Rotor is provided with a proper winding consisting of coils of insulated thin copper wire similar to that of stator winding.

The Rotor winding is wound for same number of poles and phases as the stator but preferably it should be 3-phase for both a 2-phase and a 3-phase stator.

The 3-phases are star-connected internally. The other three winding terminals are brought

out through three slip rings made of high quality steel mounted rigidly on the shaft of the rotor and bearing on them brushes.


It is therefore possible to connect additional external resistance in the rotor circuit particularly at starting to develop high starting torque necessary to move the stationary rotor against its inertia and the inertia of the heavy load connected with the shaft of the rotor.

Principle of Operation Of I.MPrinciple of Operation Of I.M

How does an induction motor work?

There is no electrical connections to the rotor to supply power to, therefore how is a rotor mmf to be established that will interact with the rotating field and produce torque???


“The answer is given in the name - by a process of inducing currents by transformer action in the rotor.”

Consider an induction motor if the rotor is locked and prevented from moving;– At this time AC power is supplied to the stator

which establishes a rotating magnetic field. This field will rotate past stationary coils which will experience a changing level of magnetic flux.

– From Faraday’s Law a voltage will be induced in the coils, causing rotor currents to circulate.



The stator is supplied by three-phase voltages that drive three- phase balanced current through the windings.

The three-phase currents generate a rotating magnetic field.

The field rotates at synchronous speed. Synchronous speed is determined by the frequency of the supply voltage and the number of poles: ns = f / p/2 = 2 f / p. The unit is rpm.

The rotating field induces a voltage in the short-circuited rotor conductors.

The induced voltage generates current in the bars.


The interaction between the rotor current and the stator field produces a force that drives the motor: Force = B I L sin

The induced voltage magnitude is dependent upon the speed difference between the rotating stator field and the rotor.

The speed difference is maximum during starting when the motor draws large current. The frequency of the rotor current is 60 Hz when the rotor is stationary.

Principle of Operation Of I.MPrinciple of Operation Of I.M As the motor starts to rotate the speed difference is

reduced, which results in: reduction on the frequency of the induced voltage in

the rotor. reduced magnitude of rotor current and induced

voltage. If the rotor speed is equal to the angular speed of the

stator field, the induced voltage, current and torque become zero. Therefore the motor speed must be less than the synchronous speed.

Motor operation requires speed difference between the stator generated rotating field and the actual rotor speed. The speed difference is called slip (s).


Locked rotor: When the rotor is stationary, the field rotates at a

frequency (relative to the rotor) equal to the supply frequency. This induces a large voltage – hence large currents flow within the rotor, producing a strong torque.

Acceleration: When released, the rotor accelerates rapidly. As speed

increases, the relative frequency of the magnetic field decreases.

Therefore, the induced voltages and currents fall rapidly as the motor accelerates.


Synchronous speed: The relative frequency of the rotating field is zero,

so the induced currents and voltages are also zero. Therefore, the torque is zero too. It follows, that induction motors are unable to reach synchronous speed due to losses such as friction.

Motor under load: The motor speed decreases until the relative frequency is large enough to generate sufficient torque

to balance the load torque.

Comparison with Synchronous Comparison with Synchronous motorsmotors

The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

Fitting Arrangements of I.MFitting Arrangements of I.M

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Induction Motor