What does a star–delta starter used with an induction motor do

1. Atwo-pole 60 Hz induction motor has a slip of2%on full load. The full-load


speed is

(a) 58.8 r/s (b) 3528 r/min

(c) 60 r/s (d) 2880 r/min

2.The slip of an induction motor can never be zero in a stable state because

(a) the rotor would be at standstill

(b) induced EMF and rotor current would be excessive

(c) it would then be a synchronous motor

(d) there would be no rotor EMF, rotor current or torque

3. The torque provided by the rotor of an induction motor is at a maximum when

(a) speed is maximum

(b) rotor resistance equals rotor reactance

(c) the connected mechanical load is at a minimum

(d) starting

4. If an electronic speed controller feeds an induction motor, the machine speed depends on (a) applied voltage (b) rotor current

(c) rotor power factor (d) applied frequency

5. A double-cage induction motor is sometimes used because

(a) it gives twice the torque of a single-cage machine

(b) the power factor of the machine is better

(c) starting torque is much higher than for the single-cage motor

(d) it gives twice the speed of a single-cage machine

6. A starter is usually needed for a three-phase induction motor

(a) because it is not self-starting

(b) to give greater safety for the operator

(c) because otherwise it will run up to speed too quickly

(d) to limit the high starting current

7. A star–delta starter used with an induction motor

(a) connects the stator in star for starting, then in delta for running

(b) reduces the supply voltage to the machine during starting

(c) connects the stator in delta for starting, then in star for running

(d) connects the rotor in star for starting, then in delta for running


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