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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!!!