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D.C. Motors

Study 130 Electrical Engineering MCQs with correct answers and explanations.

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61Electrical EngineeringMedium

Speed control by Ward Leonard method gives uniform speed variation________?

Aabove normal speed only

Bbelow normal speed only

Cin both directions

Din one direction

Correct answer

C. in both directions

Explanation

The correct answer is in both directions.

62Electrical EngineeringMedium

A three point starter is considered suitable for________?

Ashunt as well as compound motors

Ball D.C. motors

Cshunt, compound and series motors

Dshunt motors

Correct answer

A. shunt as well as compound motors

Explanation

The correct answer is shunt as well as compound motors.

63Electrical EngineeringMedium

D.C. generators are normally designed for maximum efficiency around________?

Arated voltage

Ball of the above

Cfull-load

Drated r.p.m.

Correct answer

C. full-load

Explanation

The correct answer is full-load.

64Electrical EngineeringMedium

The plugging gives the________?

Ahighest torque braking

Bsmallest torque braking

Czero torque braking

Dnone of the above

Correct answer

A. highest torque braking

Explanation

The correct answer is highest torque braking.

65Electrical EngineeringMedium

The speed of a D.C. series motor is________?

Ainversely proportional to the armature current

Bproportional to field current

Cproportional to the armature current

Dproportional to the square of the armature current

Correct answer

A. inversely proportional to the armature current

Explanation

The correct answer is inversely proportional to the armature current.

66Electrical EngineeringMedium

In variable speed motor________?

Aa weaker commutating field is needed at low speed than at high speed

Bsame commutating field is needed at low speed than at high speed

Cnone of the above is correct

Da stronger commutating field is needed at low speed than at high speed

Correct answer

A. a weaker commutating field is needed at low speed than at high speed

Explanation

The correct answer is a weaker commutating field is needed at low speed than at high speed.

67Electrical EngineeringMedium

In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

A25% of the previous value

B50% of the previous value

C10% of the previous value

D100% of the previous value

Enone of the above

Correct answer

A. 25% of the previous value

Explanation

The correct answer is 25% of the previous value.

68Electrical EngineeringMedium

In Ward-Leonard control the lower limit of speed is imposed by________?

Aresidual magnetism of the generator

Bmechanical losses of motor and generator together

Ccore losses of motor

Dall of the above

Correct answer

A. residual magnetism of the generator

Explanation

The correct answer is residual magnetism of the generator.

69Electrical EngineeringMedium

If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor________?

Acan be used for 50°C ambient temperature also

Bof higher H.P. should be selected

Cof lower H.P. should be selected

Dis to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Correct answer

D. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Explanation

The correct answer is is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor.

70Electrical EngineeringMedium

Hopkinson’s test on D.C. machines is conducted at________?

Afull-load

Bpart load

Coverload

Dno-load

Correct answer

A. full-load

Explanation

The correct answer is full-load.

The speed of a D.C. series motor is________?

Ainversely proportional to the armature current

Bproportional to field current

Cproportional to the armature current

Dproportional to the square of the armature current

Correct answer

A. inversely proportional to the armature current

Explanation

The correct answer is inversely proportional to the armature current.

In variable speed motor________?

Aa weaker commutating field is needed at low speed than at high speed

Bsame commutating field is needed at low speed than at high speed

Cnone of the above is correct

Da stronger commutating field is needed at low speed than at high speed

Correct answer

A. a weaker commutating field is needed at low speed than at high speed

Explanation

The correct answer is a weaker commutating field is needed at low speed than at high speed.

If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor________?

Acan be used for 50°C ambient temperature also

Bof higher H.P. should be selected

Cof lower H.P. should be selected

Dis to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Correct answer

D. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

Explanation

The correct answer is is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor.