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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Starting torque of induction motor is proportional to
V
V2
V/2
2V
V2
The starting torque (Ts) of an induction motor is directly proportional to the square of the applied stator voltage (V2). This relationship is derived from the interaction between the stator magnetic field and the rotor current induced at standstill.
The starting torque (Ts) of an induction motor is directly proportional to the square of the applied stator voltage (V2). This relationship is derived from the interaction between the stator magnetic field and the rotor current induced at standstill.
Ts=ωs3⋅R2°2+X2°2V2R2 — Starting torque expression for induction motor
Ts∝V2 — Simplified proportionality relationship
At the moment of starting, the induction motor acts like a transformer with a short-circuited secondary. The stator flux Φ is proportional to the applied voltage V. Since the rotor current I2 is induced by this flux, I2 is also proportional to V. Because torque is produced by the interaction of flux and rotor current (T∝ΦI2cosϕ2), and both flux and rotor current are functions of V, the resulting torque becomes proportional to V2.
The starting torque sensitivity to voltage is high, making voltage dips (e.g., during startup) very detrimental to motor performance.
This relationship holds true for both cage and slip-ring induction motors.
Reducing starting voltage (e.g., using a Star-Delta starter) reduces starting torque significantly (Tst=31TDOL).
Voltage control is a simple method for soft starting induction motors.
Predictable torque behavior allows for easy design of starters.
Low starting torque at reduced voltage may prevent the motor from starting under heavy mechanical load.
Starting current is also high, which can cause significant voltage drops in the supply system.
Star-Delta Starters
Auto-transformer Starters
Soft Starters
The squared relationship (V2) means that if the voltage drops by 10%, the torque drops by approximately 19% (0.9°2=0.81).
Options C and D are incorrect as they do not reflect the physics of electromagnetic torque generation where both flux and induced current scale linearly with voltage.
B is correct — The starting torque of an induction motor is directly proportional to the square of the applied voltage.
Always remember that torque in induction motors is more sensitive to voltage changes than power; a slight voltage drop can lead to failure to start under load.