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Examoogle • User • info@examoogle.com • EE-2024-8821
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Starting torque of squirrel cage induction motor is
Low
Medium
High
Zero
Low
The starting torque of a standard squirrel cage induction motor is low because it has a fixed, low rotor resistance and low reactance during start, which leads to a poor power factor and high starting current but low torque production. Because the rotor bars are permanently short-circuited by end rings, the rotor resistance cannot be externally adjusted to improve the starting torque.
The starting torque of a standard squirrel cage induction motor is low because it has a fixed, low rotor resistance and low reactance during start, which leads to a poor power factor and high starting current but low torque production. Because the rotor bars are permanently short-circuited by end rings, the rotor resistance cannot be externally adjusted to improve the starting torque.
Tst=2πNs3×R2°2+X2°2V1°2R2 — Starting torque formula
T∝R2°2+(sX2)2sR2 — General torque-slip relationship
The torque of an induction motor is proportional to the slip and the square of the applied voltage, specifically T∝R2°2+(sX2)2sE2°2R2. At standstill (s=1), the rotor resistance R2 is very small compared to the leakage reactance X2. Consequently, the phase angle of the rotor current is high, resulting in a low torque angle and low starting torque.
The squirrel cage rotor has a constant, non-adjustable resistance.
Low starting torque restricts the use of these motors to loads requiring low starting effort.
For high starting torque requirements, Wound Rotor (Slip Ring) induction motors are preferred.
Simple and rugged construction
Low maintenance cost
Low initial cost
Low starting torque
High starting current (5-7 times rated current)
Poor speed control capability
Centrifugal pumps
Fans and blowers
Small machine tools
| Feature | Squirrel Cage | Slip Ring |
|---|---|---|
Starting Torque | Low | High |
The starting torque can be improved by using a Double Squirrel Cage rotor, which increases effective resistance during starting.
Option B (Medium) and C (High) are incorrect as they characterize specialized designs or slip ring motors, not the standard squirrel cage motor.
A is correct — The squirrel cage induction motor is characterized by low starting torque due to the inherently low rotor resistance that cannot be varied.
Always remember that for a squirrel cage motor, starting torque is low, while for a slip ring induction motor, you can add external resistance to achieve high starting torque.