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Circuits of the strand windings at the terminal board U; V; W winding designations
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.
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Equivalent Circuit of 3-Phase Induction Motor at Any Slip
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But the rotor is revolving at a speed of N relative to the stator core. Therefore, the
speed of rotor field relative to stator core
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Equivalent Circuit of the Rotor
We shall now see how mechanical load of the motor is replaced by the equivalentelectrical load. shows the equivalent circuit per phase of the rotor at slip s. The
rotor phase current is given by;
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Transformer Equivalent Circuit of Induction Motor
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Power Relations
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Starting of 3-Phase Induction Motors
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This method also aims at connecting the induction motor to a reduced supply at
starting and then connecting it to the full voltage as the motor picks up sufficient
speed. Fig shows the circuit arrangement for autotransformer starting.
The tapping on the autotransformer is so set that when it is in the circuit, 65% to80% of line voltage is applied to the motor.
At the instant of starting, the change-over switch is thrown to start position.
This puts the autotransformer in the circuit and thus reduced voltage is applied
to the circuit. Consequently, starting current is limited to safe value. When the
motor attains about 80% of normal speed, the changeover switch is thrown torun
position. This takes out the autotransformer from the circuit and puts the
motor to full line voltage. Autotransformer starting has several advantages viz
low power loss, low starting current and less radiated heat. For large machines
(over 25 H.P.), this method of starting is often used. This method can be usedfor both star and delta connected motors.
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Figure - Slip-ring rotor with rotor starting resistance 1 Rotor starting resistance
K; L; M Connecting terminals
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outer winding has relatively open slots and a poorer flux path around itsbars, it has a low inductance. Thus the resistance of the outer squirrel-cage
winding is high and its inductance is low.
(ii) The inner winding consists of bars of greater cross-section short-circuited
by end rings. Therefore, the resistance of this winding is low. Since the
bars of the inner winding are thoroughly buried in iron, it has a high
inductance . Thus the resistance of the inner squirrelcage
winding is low and its inductance is high.
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Shapes of slots and rotor bars for (a) a double cage, (b) a deep slot, (c) a typical castaluminium rotor slot. The slot
opening is closed to ease the squirrel cage die-cast process (no separate mould is required)
Dual cage rotor induction motors for large starting torque (source: ABB)
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Figure 1.9 Dual stator winding induction generator for wind turbines (source: ABB)
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