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Examoogle • User • info@examoogle.com • EE-2024-8821
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Which of the following method used to limit heavy starting current in 3-Phase squirrel Cage Induction Motor?
By Increasing Rotor Resistance
By Reducing Stator Frequency
By reducing Stator Voltage
All of these
By reducing Stator Voltage
To limit the heavy starting current in a 3-phase squirrel cage induction motor, the stator voltage applied during starting is reduced using methods such as Star-Delta starter, Auto-transformer starter, or Stator resistance/reactance starter. Since the rotor resistance of a squirrel cage induction motor is fixed and cannot be easily varied from the outside, rotor resistance control is not applicable.
To limit the heavy starting current in a 3-phase squirrel cage induction motor, the stator voltage applied during starting is reduced using methods such as Star-Delta starter, Auto-transformer starter, or Stator resistance/reactance starter. Since the rotor resistance of a squirrel cage induction motor is fixed and cannot be easily varied from the outside, rotor resistance control is not applicable.
Ist=ZscVst — Starting current per phase in terms of starting voltage and standstill impedance
Tst∝V2 — Starting torque is proportional to the square of the applied stator voltage
The starting current of an induction motor is directly proportional to the applied stator voltage per phase (Ist∝V). By reducing the stator voltage at the time of starting, the starting current drawn from the mains is reduced proportionally. Once the motor reaches near rated speed, full rated voltage is applied across the stator windings.
A 3-phase squirrel cage rotor has short-circuited end rings, making external rotor resistance insertion impossible.
Starting current of a squirrel cage motor with direct-on-line (DOL) starting can be 5 to 8 times the full-load current.
Reducing stator voltage reduces starting current linearly, but starting torque reduces quadratically (Tst∝V2).
Protects supply mains from severe voltage dips during motor startup.
Reduces thermal stress and mechanical shocks on the motor windings and shaft.
Significantly reduces starting torque due to the square law relationship (Tst∝V2).
Requires additional starting equipment (Star-Delta, Auto-transformer, or Soft starters).
Star-Delta starting for medium-capacity 3-phase squirrel cage induction motors.
Auto-transformer starting for large squirrel cage induction motors where high starting torque is required.
Option A is incorrect because rotor resistance can only be increased in Slip Ring (Wound Rotor) Induction Motors, not in Squirrel Cage Motors.
Option B is incorrect because frequency variation is primarily used for speed control (V/f control), not standard reduced-voltage starting method for limiting starting current.
C is correct — Reducing stator voltage limits the high starting current since starting current is directly proportional to the applied voltage.
Remember that Ist∝V while Tst∝V2; reducing voltage by half reduces starting current to 50%, but starting torque drops to 25% of its full-voltage value.