MCQs on Electrical Machines

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01․ A frog leg winding is used in a DC machine to eliminate
the use of compensating winding.
the use equalizer in lap winding.
the use drum winding.
reactance voltage produced due to communication.

Frog leg winding composed of a multiplex wave winding in the same slots with a simplex or multiplex lap winding. Both have the same no. of parallel path and are connected to commutator. Thus the current distribution is equal at brush and thereby eliminate the equalizer in lap winding.

02․ A 4 pole DC shunt generator having a wave winding supplies 50 lamps, each of 60 W at 120 V. The armature and field resistance are 0.2 Ω and 60 Ω respectively. The current in each armature conductor is
11.5 A.
13.5 A.
23 A.
27 A.

Total load current IL = 0.5 X 50 = 25 A. Armature current Ia = IL + Ish = 25 + 2 = 27 A. A wave winding has two parallel paths. So current in each armature is equal to 27 / 2 = 13.5 A.

03․ A 6 pole lap connected DC generator with 480 conductors has armature resistance of 0.06 Ω. If the conductors are reconnected to from a wave winding other things remaining unchanged, the value of armature resistance will be
0.01 Ω.
0.06 Ω.
0.54 Ω.
0.54 Ω.

Where Z = number of conductor, P = number of pole, L = length of conductor and A = Cross section area of conductor. ⇒ Rwave = Rlap X 9 = 0.06 X 9 = 0.54 Ω.

04․ The sparking at the the brushes, in a DC machines is due to
reactance voltage.
presence of commutator.
high resistance of carbon brushes.
armature reaction.

The self induced emf in the coil under going commutation is called reactance voltage. Sparking at brushes is due to inability of the current in the short circuit coil to reverse completely by the end of short circuit period.

05․ In a DC machine, for the same value of Ãâ€ , Z and N; which one of the following statements is correct?
Armature emf is same as long as the flux density in the air gap remains the same.
Armature emf is less with wave winding than with lap winding.
Armature emf is more with wave winding than with lap winding.
Armature emf depends on weather the machine is running as a motor or generator.

In case of lap winding A = P and wave winding A = 3. So the emf generated in wave wound armature is greater than lap wound armature.

06․ An ideal DC generated is one that has _____________ voltage regulation.
positive.
negative.
zero.
low.

An ideal DC generator has no armature resistance and no field winding resistance. Thus there are no voltage drop due to armature winding and field winding resistance. Hence the generated emf is equal to terminal voltage. Thereby an ideal DC generated has zero voltage regulation.

07․ Series booster is a
shunt wound DC generator.
series wound DC generator.
separately excited DC generator.
permanent magnet DC generator.

Series booster is a series wound DC generator, which is inserted in a circuit to add a certain voltage. So that the excessive voltage drop in the feeder is compensated for.

08․ If residual magnetism of a shunt generator is destroyed accidentally it may be restored by connecting its shunt field
to a DC source.
to an AC source.
to earth.
in reverse.

In case the generator residual magnetism is destroyed accidentally it may be restored by connecting the field winding to a DC source for short while to magnetize the poles.

09․ A commutator with a diameter of 50 cm rotating at 1000 rpm. For a brush width of 1.5 cm, find the time of commutation in ms.
0.443.
0.573.
0.512.
0.649.

10․ The coil spam
must be exactly equal to people pitch.
may or may not be exactly equal to people pitch.
can never be equal to pole pitch.
none of above.

The coil span may or may not be exactly equal to pole pitch. Pole pitch is the number of conductor per pole.

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