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MCQs on Electrical Machines

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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 If 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․ What is the shunt resistance component equivalent circuit obtained by no load test of an induction motor representative of ?
windage and friction loss
core loss only
both 1 and 2
Copper loss

No load loss consists of core loss, friction and windage loss and no load copper loss. But shunt branch resistance Rc in the induction motor equivalent circuit does not represent the no load losses, it represents only the core loss of the induction motor.
04․ 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.
05․ 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.
06․ 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.
07․ Eddy current loss will depends on
Frequency
Flux density
Thickness
All of the above

. So eddy current loss will depend upon frequency, flux density and the area of the eddy current loop.
08․ Hysteresis loss will depends on
f
f1.6

. So, hysteresis loss will depends on frequency.
09․ Thin laminations are used in a machine in order to reduce
Eddy current losses
Hysteresis losses
Both 1 and 2
Copper losses

Thin laminations are used in order to reduce the eddy current losses only. Due to laminations the area of the eddy currents loops are minimized and the losses due to eddy current losses are minimized.
10․ Hysteresis loop represents the area of
copper loss
eddy current loss
hysteresis loss
total iron losses

Hysteresis loop will represents only hysteresis losses. It is found out by area of B-H loop curve of a magnetic material.
11․ Commutator pitches of duplex and simplex lap windings are respectively
4 and 2
2 and 1
4 ang 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.
12․ The emf induced in the dc generator armature winding is
AC
DC
AC and DC
None of the above

The emf induced in the dc generator armature winding is AC, but we need DC current from DC generator, so to convert this AC current to DC current mechanical rectifier called as commutator is used.
13․ Commutator in DC generator is used for
collecting of current
reduce losses
increase efficiency
convert AC armature current in to DC

Commutator is used as mechanical rectifier in DC machines. It converts AC armature current in to DC current.
14․ A DC generator without commutator is a
AC generator
DC motor
DC generator
induction motor

Commutator is used to convert AC current into DC current. If commutator is not present, then it will acts as an AC generator with huge losses.
15․ 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.
16․ 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
17․ In DC generators brushes are used for
collecting of current without any sparkings
collecting of voltage
reduce eddy current loss
convert ac armature current in to dc

In case of DC generator the brushes need to collect current with minimum sparking, which is known as successful commutation.
18․ 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.
19․ Which of the following bearings and their uses are correct
ball bearings → small machines
roller bearings → large machines
neither 1 nor 2
both A and B

These are different applications of different bearings. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, as contact area is very small, so if the ball bearing is overloaded, the balls can deform or squish the running bearing. So it has to be used for small rating machines. Since the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load over more area, allowing the bearing to handle much greater loads than ball bearings. Hence it is used for large machines.
20․ 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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