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Examoogle • User • info@examoogle.com • EE-2024-8821
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Which is true for slip ring induction motor ?
Low cost
Less size
Less starting torque
High starting torque
High starting torque
A slip ring induction motor, or wound rotor induction motor, allows the insertion of external resistance into the rotor circuit through slip rings. This ability to increase total rotor resistance significantly boosts the starting torque, making it ideal for high-inertia loads.
A slip ring induction motor, or wound rotor induction motor, allows the insertion of external resistance into the rotor circuit through slip rings. This ability to increase total rotor resistance significantly boosts the starting torque, making it ideal for high-inertia loads.
Tst∝R2°2+X2°2R2 — Starting torque is a function of rotor resistance
Tmax∝2X21 — Maximum torque is independent of rotor resistance
The starting torque of an induction motor is given by Tst∝R2°2+(sX2)2sE2°2R2. By connecting an external variable resistance to the rotor via slip rings, the total rotor resistance R2 increases. At the moment of starting (where slip s=1), increasing R2 brings the motor closer to the condition where starting torque is maximized, i.e., R2=X2.
Slip ring motors are also known as Wound Rotor Induction Motors (WRIM).
External resistance is only used during the starting phase to limit current and increase torque.
Once the motor reaches near-rated speed, the external resistance is shorted out to improve efficiency.
High starting torque capability
Low starting current draw
Adjustable speed control via rotor resistance
Higher maintenance due to brushes and slip rings
Higher cost compared to squirrel cage motors
Lower efficiency during operation with external resistance
Cranes and hoists
Elevators
Compressors and large conveyors
| Feature | Squirrel Cage | Slip Ring |
|---|---|---|
Starting Torque | Low | High |
Maintenance | Very Low | High |
Option A and B are incorrect because slip ring motors are more expensive and bulkier than squirrel cage induction motors.
Option C is the opposite of the motor's actual characteristic.
D is correct — The slip ring induction motor provides high starting torque by allowing the addition of external resistance to the rotor circuit.
Remember: Adding resistance to the rotor circuit of a slip ring motor improves starting torque but decreases full-load efficiency if left in the circuit, so it is always removed after acceleration.