Motor Inductions. Energi Smt 5 2011

8/10/2019 Motor Inductions. Energi Smt 5 2011

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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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Motor Inductions. Energi Smt 5 2011