deep bar & double cage rotors

8/3/2019 Deep Bar & Double Cage Rotors

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DEEP BAR & DOUBLE CAGEDEEP BAR & DOUBLE CAGE

ROTORSROTORS


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What is the Need??What is the Need??

y Conventional Squirrel Cage motors suffer from thedisadvantages of low starting torque because of lowrotor resistance.

y The starting torque can be increased by using the bar

material of higher resistivity.y However high rotor resistance reduces the full-load

speed, increases rotor ohmic loss and lowerefficiency.

y Therefore in order to achieve high starting torquewithout effecting the efficiency, the rotor resistanceis made higher at the time of starting & low undernormal operating conditions.


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Continued..Continued..y In wound rotor induction motors these conditions

are met by connecting external resistance in rotorcircuit at the time of starting & resistances are cutout in steps as the motor attains its normal speed.

y In squirrel cage motor this is not feasible as theconductors are short-circuited by end rings.

y In order to attain the above desired conditionsfollowing two types of rotors are used:-

a) Deep bar rotor.

b) Double cage rotor.


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DeepDeep--Bar cagerotorBar cagerotor

y Figure below shows a cage rotor with deep andnarrow bars. A bar may be assumed to be made up ofnumber of narrow layers (as shown in fig.) connectedin parallel.

y It is seen that the top most layer element A is linkedwith minimum leakage flux and therefore its leakageinductance is minimum.

y

On the other hand, the bottomlayer C links maximum flux,

therefore its leakage inductance

is maximum.


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y At starting the rotor frequency is equal to the supplyfrequency. The bottom layer element C offers more

impedance to the current flow then upper layer A.

y Therefore the maximum current flows through thetop layer and minimum through bottom layer. This

unequal current distribution causes the effectiverotor resistance to increase & leakage reactance todecrease.

y With a high rotor resistance at starting condition,the starting torque is relatively high & startingcurrent is low as desired.


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y Now under normal operating conditions, the slipand the rotor frequency are very small. thereactance of all the layers are small compared totheir resistances.

y The impedances of all the layers are nearly equal, so

current flows through all the parts of the barequally.

y The resulting large cross-section

area makes rotor resistance small,resulting in good efficiency at low

slips.


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y The slip-torque curve for deep-bar rotor is shownbelow along with the deep-bar stator.

y

Torque/Slip curve Deep-Bar rotor


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DoubleDouble--Cage RotorsCage Rotorsy

An induction motor with two rotor windings orcages is used for obtaining high starting torque atlow current.

y The stator of a double cage induction motor issimilar to that of ordinary induction motor. Indouble cage rotor there are two layers of bars asshown:

y The outer cage bars have a

smaller cross-sectional areathan the inner bars and are

made of high resistivity

materials like brass etc.


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y The inner cage bars are made up of low resistancecopper. Thus the resistance of outer cage is greater

than that of inner cage.

y There is a slit between the top and bottom slot.This increases premeance for leakage flux around

the inner cage bars.

y Thus the leakage flux linking the inner cagewinding is much larger than that of outer cage. The

inner cage winding, therefore has a greater selfinductance.


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y At starting voltage induced in the rotor is same asthe supply frequency. Hence the leakage reactance

of the inner cage winding is much larger than ofthe outer cage.

y Therefore most of the starting current is flowing inthe outer cage winding which offer low impedance

to current flow.y The high resistance outer cage winding therefore

develops high starting torque.

y As the rotor speed increases, the

frequency of rotor emf decreases.


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y At normal speed, the leakage reactance of both thewinding becomes negligibly small.

y

The rotor current division between the two cages isgoverned mainly by their resistances.

y Since the resistance of the outer cage is about 5 to6 times that of inner cage, most of the rotor

current flows through the inner cage.y Hence under normal operating speed, torque

developed mainly by low resistance inner cage.

y Double cage rotor


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ApplicationsApplications

y For high torque requirements a deep-bar cagemotor is used.

y A double-cage motor is used for still higher

torques.

y For large size motors with very large startingtorques and exceptionally long starting periods,

slip-ring construction is used.


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THANKS!!!THANKS!!!

deep bar & double cage rotors

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