Polyphase Induction Motors

41. In the circle diagram for induction motor, the diameter of the circle represents

1. slip
2. rotor current
3. running torque
4. line voltage

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42. In the squirrel-cage induction motor the rotor slots are usually given slight skew

1. to reduce the magnetic hum and locking tendency of the rotor
2. to increase the tensile strength of the rotor bars
3. to ensure easy fabrication
4. none of the above

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43. In three-phase induction motors sometimes copper bars are placed deep in the rotor to

1. improve starting torque
2. reduce copper losses
3. improve efficiency
4. improve power factor

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44. In three-phase squirrel-cage induction motors

1. rotor conductor ends are short-circuited through slip rings
2. rotor conductors are short-circuited through end rings
3. rotor conductors are kept open
4. rotor conductors are connected to insulation

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45. Insertion of reactance in the rotor circuit

1. reduces starting torque as well as maximum torque
2. increases starting torque as well as maximum torque
3. increases starting torque but maxi-mum torque remains unchanged
4. increases starting torque but maxi-mum torque decreases

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46. Insertion of resistance in the rotcir of an induction motor to develop a given torque

1. decreases the rotor current
2. increases the rotor current
3. rotor current becomes zero
4. rotor current rernains same

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47. It is advisable to avoid line starting of induction motor and use starter because

1. it will run in reverse direction
2. it will pick up very high speed and may go out of step
3. motor takes five to seven times its full load current
4. starting torque is very high

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48. It is advisable to avoid line-starting of induction motor and use starter because

1. motor takes five to seven times its full load current
2. it will pick-up very high speed and may go out of step
3. it will run in reverse direction
4. starting torque is very high

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49. Less maintenance troubles are experienced in case of

1. slip ring induction motor
2. squirrel cage induction motor
3. both (a) and (b)
4. none of the above

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50. Low voltage at motor terminals is due to

1. inadequate motor wiring
2. poorely regulated power supply
3. any one of the above
4. none of the above

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