Polyphase Induction Motors

Initial cost is the main consideration ✘ Maintenance cost is to be kept low ✘ Higher starting torque is the main consideration ✘ All above considerations are involved ✔

Speed control is required ✘ Frequent starting, stopping and reversing is required ✘ High starting torque is needed ✘ All above features are required ✔

In the opposite direction as the direction of the rotating field ✘ In the same direction as the direction of the field ✘ In any direction depending upon phase sequence of supply ✘ None of these ✔

Graphite ✘ Grease ✘ Mineral oil ✘ Molasses ✔

Squirrel-cage induction motor ✘ Slip-ring induction motor ✘ Both (A) and (B) ✘ None of the above ✔

730 r.p.m. ✘ 960 r.p.m. ✘ 1440 r.p.m. ✘ 2880 r.p.m. ✔

To reduce the magnetic hum and locking tendency of the rotor ✘ To increase the tensile strength of the rotor bars ✘ To ensure easy fabrication ✘ None of the above ✔

Resistance rise method ✘ Thermometer method ✘ Embedded temperature method ✘ All above methods ✔

Inversely proportional to (V-slip) ✘ Directly proportional to (slip)2 ✘ Inversely proportional to slip ✘ Directly proportional to slip ✔

Self-starting with zero torque ✘ Self-starting with high torque ✘ Self-starting with low torque ✘ Non-self starting ✔