Polyphase Induction Motors

51. Lubricant used for ball bearing is usually

1. graphite
2. grease
3. mineral oil
4. molasses

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52. Reduced voltage starter can be used with

1. slip ring motor only but not with squirrel cage induction motor
2. squirrel cage induction motor only but not with slip ring motor
3. squirrel cage as well as slip ring induction motor
4. none of the above

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53. Rotor rheostat control method of speed control is used for

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

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54. Short-circuit test on an induction motor cannot be used to determine

1. windage losses
2. copper losses
3. transformation ratio
4. power scale of circle diagram

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55. Size of a high speed motor as compared to low speed motor for the same H.P. will be

1. bigger
2. smaller
3. same
4. any of the above

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56. Slip of an induction motor is negative when

1. magnetic field and rotor rotate in opposite direction
2. rotor speed is less than the synchronous speed of the field and are in the same direction
3. rotor speed is more than the synchronous speed of the field and are in the same direction
4. none of the above

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57. Slip ring motor is preferred over squirrel cage induction motor where

1. high starting torque is required
2. load torque is heavy
3. heavy pull out torque is required
4. all of the above

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58. Slip ring motor is recommended where

1. speed control is required
2. frequent starting, stopping and reversing is required
3. high starting torque is needed
4. all above features are required

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59. Slip rings are usually made of

1. copper
2. carbon
3. phospor bronze
4. aluminium

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60. Star-delta starting of motors is not possible in case of

1. single phase motors
2. variable speed motors
3. low horse power motors
4. high speed motors

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