Q9
(a) List the factors that determine the starting torque of the three-phase Induction motor. How does this torque generally compare with the value of the rated torque?
(6)
(b) A 3 ph, 440 V, 60 Hz 8 pole induction motor runs at a power factor of 0.85 lag and drives a load of 8 kW at a speed of 14.4 rev/sec. The stator loss is 1 kW and the rotational losses (windage and friction) amount to 0.8 kW. Calculate EACH of the following:
(10)
(i) the synchronous speed;
(ii) the rotor copper loss;
(iii) the input power to the motor;
(iv) the motor current.
Reference Answer
### Part (a): Starting Torque of a Three-Phase Induction Motor #### Factors Determining Starting Torque The starting torque of a three-phase induction motor is the torque produced by the motor at the instant of starting, i.e., when the rotor speed is zero and the slip is unity (s=1). The magnitude of this torque is determined by several key factors derived from the fundamental torque equation: $$ T_{start} \propto \frac{E_2^2 \times R_2}{R_2^2 + X_2^2} $$
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