Which of the following factors is directly proportional to the torque developed by a three-phase induction motor?
C
Rotor current and rotor EMF
Analysis & Theory
Torque is proportional to the product of rotor current and rotor EMF (electromagnetic interaction in the rotor).
Which of the following is the correct torque equation for a three-phase induction motor?
B
T ∝ E² × R₂ / (s² × X₂²)
C
T ∝ (s × E₂² × R₂) / (R₂² + (sX₂)²)
Analysis & Theory
The torque developed is given by T ∝ (s × E₂² × R₂) / (R₂² + (sX₂)²), where E₂ is rotor EMF, R₂ rotor resistance, X₂ rotor reactance, and s is slip.
At low slip values (i.e., near synchronous speed), the torque in an induction motor is approximately:
A
Directly proportional to slip (s)
B
Inversely proportional to slip (s)
Analysis & Theory
At small slip values, torque T is approximately proportional to s (linear region of torque–speed curve).
At high slip values (starting condition), the torque is approximately:
B
Inversely proportional to s
Analysis & Theory
At high slip (near s = 1), torque is approximately inversely proportional to slip, T ∝ 1/s.
What is the condition for maximum torque in an induction motor?
Analysis & Theory
Maximum torque occurs when slip s = R₂ / X₂, assuming stator impedance is negligible.
Which parameter primarily controls the starting torque of a squirrel‑cage induction motor?
A
Rotor current frequency
Analysis & Theory
Rotor resistance plays a key role in determining the starting torque, especially in slip‑ring motors.
How does increasing rotor resistance affect the torque-speed characteristic of an induction motor?
A
Shifts maximum torque to lower slip
B
Increases synchronous speed
C
Shifts maximum torque to higher slip
D
Reduces starting torque
Analysis & Theory
Increasing rotor resistance shifts the maximum torque point to higher slip, improving starting performance.
Which of the following remains constant regardless of rotor resistance in an induction motor?
Analysis & Theory
The value of maximum torque remains constant; only the slip at which it occurs changes with rotor resistance.
What happens to torque if supply voltage is doubled (assuming frequency constant)?
C
Torque increases four times
Analysis & Theory
Torque is proportional to the square of the supply voltage, so doubling voltage increases torque by 4 times.
Why is it not efficient to increase rotor resistance in squirrel‑cage motors for better starting torque?
C
Causes excessive rotor heating
Analysis & Theory
In squirrel‑cage motors, increasing rotor resistance results in higher losses and heating, reducing efficiency.