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Objective questions and answers or MCQ of Electrical Machines page-21


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326. Which of the material is used to insulate the segments of commutator?

a) Fibre glass
b) Plastic
c) Mica
d) PVC

Ans: (c)

327. The brushes of electrical machines are made of

a) Carbon
b) Copper
c) Cast iron
d) Steel

Ans: (a)

328. If the resistance of the field winding of d.c. generator is increased, then the output voltage

a) Increases
b) Decreases
c) Remains constant
d) Decreases proportional the resistance of field winding

Ans: (a)

329. Which of the following generation will be preferred if they are required to be run in parallel ?

a) Series generators
b) Shunt generators
c) Shunt and series generators
d) Compound generators

Ans: (b)

330. To separately excited motors have

a) Excitation which is independent of load current
b) The advantage over self excited motor that it can be utilised for zero volt to its maximum rated capacity
c) A and b both
d) None of the above

Ans: (c)

331. The armature voltage control of dc motor will provides

a) Constant current drive
b) Constant voltage drive
c) Constant torque drive
d) Constant torque drive

Ans: (d)

332. The field current control of d.c. shunt motor will provide

a) Constant current drive
b) Constant voltage drive
c) Constant HP drive
d) Constant torque drive

Ans: (a)

333. Which of the following method of speed control of d.c. machine will offer minimum efficiency?

a) Armature control method
b) Field control method
c) Voltage control method
d) All above methods

Ans: (a)

334. In which of the following methods of speed control computation is unsatisfactory?

a) Field control method
b) Armature current control method
c) Voltage control method
d) All above method

Ans: (a)

335. The dynamic braking can be used for

a) Series motors
b) Shunt motors
c) Compound motors
d) All above motors

Ans: (d)

336. The plugging gives the

a) Smallest torque breaker
b) Highest torque breaker
c) Zero torque breaker
d) None of them

Ans: (b)

337. Which of the method of breaking will be selected if the highest braking torque is required?

a) Plugging
b) Dynamic braking
c) Counter breaking
d) Regenerative braking
e) (a) or (c)

Ans: (e)

338. If the terminals of armature of d.c. machine are interchanged, this action will offer following kind of braking.

a) Plugging
b) Regenerative
c) Dynamic braking
d) Any of the above

Ans: (a)

339. If a d.c. motor for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor

a) Is to be derated by a factor recommended by manufacturer and select next higher HP motor
b) Can be used for 55°C ambient also
c) Of lower HP should be selected
d) Of high speed should be selected

Ans: (a)

340. If we have to control the speed of 150 HP d.c. motor from zero to 1000 rpm having rated speed of 1500 rpm. Then it will be preferred to

a) Select a motor of 150 HP, 1500 r.p.m.
b) Select a motor of 1.5 × 150 HP, 1500 r.p.m.
c) Select a motor of 150 HP, 750 r.p.m.
d) Select a motor of 75 HP, 1500 r.p.m.

Ans: (b)


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