31. The direction of rotation of a 3-phase induction motor can be reversed by
- Transposing any two leads from supply
- Transposing any two leads from slip-rings
- Transposing all the three leads from the supply
- Disconnecting any one phase
32. The rotor of an induction motor never runs at synchronous speed, because then the relative speed between the rotating flux and rotor will be
- Maximum and hence, torque will be maximum
- Maximum and hence, torque will be zero
- Zero and hence, torque will be maximum
- Zero and hence, torque will be zero
33. The voltage actually used for setting up of the useful flux in the air gap of 3-phase induction motor is
- = applied voltage
- > applied voltage
- < applied voltage
- = rotor induced emf
34. Synchronous speed is defined as the speed at which the
- Stator magnetic field rotates
- Rotor rotates on no load
- Rotor rotates on full load
- None of the above
35. If Ns is the synchronous speed, N is the rotor speed and s is the slip, then the relation is
- Ns = (1 – s ) N
- N = sNs
- N = ( s – 1 ) Ns
- None of these
36. A three- phase 6 pole, 50 Hz, induction motor is running at 5% slip. What is the speed of the motor?
- 850 rpm
- 900 rpm
- 950 rpm
- 1000 rpm
37. At the instant of starting, the per unit slip of the 3-phase induction motor is
- 0.005
- 0.1
- 1.0
- 0.5
38. If the full-load speed of a 6-phase, 50 Hz induction motor is 950 rpm, what is its half-load speed nearly equal to?
- 1000 rpm
- 450 rpm
- 1900 rpm
- 975 rpm
39. The mmf produced by the current of a 3-phase induction motor
- Rotates at the speed of rotor in the air gap
- Is stand still with respect to stator mmf
- Rotates at the slip speed with respect to stator mmf
- Rotates at synchronous speed with respect to rotor
40. The speed of rotating field due to rotor currents related to rotor currents relative to rotor surface in an induction motor is
- Ns
- sNs
- N
- s