The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum (Q s = 1).
Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action.
This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low.
Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.
However, this large starting current will produce large linevoltage drop.
This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum (Q s = 1).
Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action.
This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low.
Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.
However, this large starting current will produce large linevoltage drop.
This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.
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