Objective Questions or MCQs on DC Machine

Objective Questions or MCQs on DC Machine

01) If field current is decreased in shunt dc motor, the speed of the motor

remains same.

increases.

decreases.

none of the above.

As a shunt field current I_f decreases, φ also decreases and the speed rises as speed is inversely proportional to flux.

02) What is the mechanical power developed by a DC series motor is maximum?

Back emf is equal to half the applied voltage.

Back emf is equal to applied voltage.

Back emf is equal to zero.

None of above.

03) In Ward-Leonard system, the lower limit of the speed imposed by

Field resistance.

Armature resistance.

Residual magnetism of the generator.

None of above.

In Ward-Leonard method of speed control, the lower limit of speed is imposed by residual magnetism of the generator.

04) Ward-Leonard control is basically a ___________ control method.

Field control.

Armature resistance control.

Armature voltage control.

Field diverter control.

Ward-Leonard speed control method consists a motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. Hence it is armature voltage control method.

05) For very sensitive and wide speed control, the preferable control method is

Armature control.

Ward-Leonard control.

Multiple voltage control.

Field control.

In ward-Leonard method, very fine speed control over the whole range from zero to normal speed in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction.

06) Commutator pitches of duplex and simplex lap winding are respectively

4 and 2

2 and 1

4 and 1

2 and 2

Commutator pitches of duplex winding is 2 and Commutator pitches of simplex winding is 1. It is defined as the distance between two ends of same armature coil are connected. So, difference in duplex winding are 2 as there are two turns in the duplex winding.

07) In DC machine yoke offers

mechanical protection to the machine

flux path completion

produce working flux

both A and B

In case of DC machines yoke is used to

1. Mechanical protection to the machine
2. Flux path completion.

Poles will produce working flux.

08) In a dc machine 72 number of coils are used. Find the number of commutator segments required?

36

37

72

74

In DC machines, Number of coils = Number of commutator segments

09) which of the following type of brush and their application is/are correct

carbon brush → normal ratings

electro graphite → large ratings

copper graphite → low voltage high current density

all of the above

These are the different applications for different types of brushes in DC machines.

10) Which of the following windings are necessary in case of all dc machines?

closed winding

lap winding

wave winding

open type winding

In case of all DC machines we must use closed winding. Lap and wave windings are used based on the requirement of voltage and current levels, but winding of all DC machines is closed winding. It is never short pitched.

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