Page 724 - Basic Electrical Engineering

36. A four-pole, three-phase induction motor in connected to a 50 Hz supply. Calculate synchronous
speed; the rotor speed when slip is 4 per cent, and the rotor frequency when the rotor is running at
1425 rpm.

[Ans 1500 rpm, 1440 rpm, 2.5 Hz]

37. The input power to the rotor of a 400 V, 50 Hz, six-pole, three-phase induction motor is 75 kW.
The frequency of the rotor induced EMF is 2 Hz. Calculate slip; rotor speed, and power developed
2
by the rotor; rotor I R loss.

[Ans 0.04, 960 rpm, 72 kW; 1 kW]

38. A 400 V, 50 Hz, six-pole, three-phase induction motor running at 975 rpm draws 40 kW from the
2
mains. The stator loss is 1kW. The friction and windage loss is 2 kW. Calculate (i) slip; (ii) I R loss
in the rotor; (iii) the shaft output in hp, and (iv) the efficiency of the motor.

[Ans 2.5 per cent; 975 W; 49 hp; 90 per cent]

C. Multiple Choice Questions

1. For production of a rotating magnetic field using stationary windings we must connect

a. a single-phase supply to a single-phase winding
b. a three-phase supply to a three-phase winding
c. a single-phase supply to a two-phase winding
d. either a single-phase winding or a two-phase winding to a single-phase supply.

2. When a three-phase 50 Hz, 400 V supply is applied across a four-pole, three-phase winding, a
rotating magnetic field is produced which is rotating at

a. 200 rpm
b. 1600 rpm
c. 1500 rpm
d. 3000 rpm.

3. A four-pole, three-phase induction motor is rotating at 1440 rpm when a 400 V, 50 Hz supply
is applied to its stator terminals. The slip of the motor expressed in percentage is

a. 4 per cent
b. 2 per cent
c. 5 per cent
d. 6 per cent.