Double Cage Rotors

8/12/2019 Double Cage Rotors

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Induction Motors


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Introduction

An induction motor carries alternating current in both the statorand the rotor windings.

An induction motor is a singly-fed motor. Therefore, it does notrequire a commutator, slip-rings, or brushes.In fact, there are no moving contacts between the stator and therotor.This results in a motor that is rugged, reliable, and almostmaintenance free.The absence of brushes eliminates the electrical loss due to thebrush voltage drop and the mechanical loss due to frictionbetween the brushes and commutator or the slip-rings.Thus, an induction motor has a relatively high efficiency.


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Introduction

Three-phase induction motors are the most common andfrequently encountered machines in industry

simple design, rugged, low-price, easy maintenance

wide range of power ratings: fractional horsepower to10 MWrun somehow at constant speed from no-load to fullloadIts speed depends on the frequency of the powersource


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Construction An induction motor has two main parts

a stationary statorThe outer (stationary) member of an induction motor iscalled the stator and is formed by stacking thin-slotted,highly permeable steel laminations inside a steel or cast-

iron frame.The frame provides mechanical support to the motor. Although the frame is made of a magnetic material, it isnot designed to carry magnetic flux.Identical coils are wound (or placed) into the slots andthen connected to forma balanced three-phase winding.

Stator of IM


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ConstructionA revolving rotor

The rotor is also composed of thin-slotted, highly permeable steellaminations that are pressed together onto a shaft.

Basic design types depending on the rotor designSquirrel-cage:

The squirrel-cage rotor is commonly used when the load requireslittle starting torque.Circular rings called the end-rings are also formed on both sidesof the stack.These end-rings short-circuit the bars on both ends of the rotor, .

Each pair of poles has as many rotor phases as there are barsbecause each bar behaves independently of the other.It is a common practice to skew the rotor laminations to reducecogging and electrical noise in the motor.


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Induction Machines_Introduction

Stator winding is essentially the same as that of a synchronousmachine.

Rotor may be one of two types: wound rotor or squirrel-cage rotor .

Wound rotor has a polyphase winding similar to the stator winding.The rotor terminals are brought outside with the help of slip rings.The rotor winding is usually short-circuited through externalresistances that can be varied.

Squirrel-cage rotor has a winding consisting of conducting bars ofcopper or aluminum, short-circuited at each end by conducting endrings.


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Construction

Squirrel cage rotor

Wound rotor

Notice theslip rings


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Operating Principle

When supplied from a balanced three-phase (polyphase) source, the

three-phase (polyphase) stator winding produces a rotating magneticfield. The magnetic field rotates at a synchronous speed given by:

This phenomenon applies to synchronous motors as well.Due to the stator magnetic field, an emf is induced in the rotorwinding. The emf sets up a current in the rotor.

Current-carrying rotor winding (conductors) under the statormagnetic field produces a torque (force) in the direction of therotating stator magnetic field.

An induction machine may be regarded as a transformer. The rotor(current) frequency is, however, different from the stator (current)frequency.

P

f n s

120


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Slip

Slip: The difference between the speed of the rotating flux produced

by the stator and the speed of the rotor is called slip speed, and theratio of slip speed to synchronous speed is called slip.

where:n

s = synchronous speedn

r = rotor speed (r/min)

s = slip

The slip depends on the mechanical load connected to the rotor shaft.Increasing the shaft load decreases the rotor speed, thus increasingthe slip.

s

r s

n

n n S


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Equivalent Circuit


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Modeling Induction Machines_1

Motor Mode of Operation: At no-load, the machine operates atnegligible slip. If a mechanical load is applied, the slip increasessuch that the induced voltage and current produce the torquerequired by the load. The machine thus operates as a motor.

Generator Mode of Operation: If the rotor is driven by a prime-mover at a speed greater than that of the stator field, the slip isnegative. The polarities of the induced voltages are reversed sothat the resulting torque is opposite in direction to that of rotation.The machine thus operates as a generator.


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Alternative Rotor Constructions

High efficiency at normal operating conditions requires a low rotorresistance.

On the other hand, a high rotor resistance is required to produce a highstarting torque and to keep the magnitude of the starting current low andthe power factor high.The wound rotor is one way of meeting the above mentioned need forvarying the rotor resistance at different operating conditions. Wound-

rotor motors are, however, moreexpensive than squirrel-cage motors.

Effect of the rotor resistancethe torque-slip curves.


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Double Squirrel- Cage Rotor Construction (contd)

At starting, rotor frequency is high and very little current flows throughthe lower bars; the effective resistance of the rotor is then the highresistance upper bars.(due to high leakage flux . Impedence is high)At normal low slip operation, leakage reactances are negligible, and therotor current flows largely through the low resistance lower bars; theeffective rotor resistance is equal to that of the two sets of bars in

parallel.

Double squirrel-cage rotor bars


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Deep-Bar Rotor Construction

The use of deep, narrow rotor bars produces torque-slip characteristicssimilar to those of a double-cage rotor.

Leakage inductance of the top cross-section of the rotor bar is relativelylow; the lower sections have progressively higher leakage inductance.At starting, due to the high rotor frequency, the current is concentratedtowards the top layers of the rotor bar.At full-load operation, the current distribution becomes uniform and theeffective resistance is low.

Deep-bar rotor construction


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Equivalent Circuit with a Double Cage or Deep Bar Rotor

Equivalent circuit of a single-cage induction motor (with onerotor winding).

Equivalent circuit of a double-cage induction motor (two rotorwindings).


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Equivalent Circuit_Single Rotor Circuit Representation

2

21

21

210

22

2

10

1

22

0122

0

,

X

R Rm

R R

R R R

where

sm

RmR

R

X s X

sm

R R

msm R s R

r

r

r

r

r r

For system studies, the rotor should berepresented by a single rotor circuit whose

parameters vary as a function of slip, s .

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