MCQs on Electrical Machines

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01․ Dummy coils are used for
increasing efficiency.
reducing armature reaction.
mechanical balancing.
all of the above

Dummy coils are required under wave winding only to make the machine mechanically balanced. Consider a problem to understand the dummy coils.
Design wave winding for 60 conductors, 15 slots, 4 poles
For designing of wave winding we should find average pitch (Ya) and that should be integer. For some set of data Ya will not be integer. Therefore the nearest possible number of conductors are considered to make Ya integer. But this will make the armature mechanically unbalance. For example one of the 15 slots will contain only 2 conductors as 2 are eliminated in the design itself. To make the machine mechanically balanced those two conductors were cut shot, insulated and placed in the slots as dummy coils. In this way dummy coils will mechanically balance the machine.

02․ A 4 pole lap wound dc generator has 4 brushes, if one of the brush is damaged, what will be the change in V, I and P ratings
V, I and P
V/2, I/2 and P/4
V/2, I and P/2
V, I/2 and P/2

If one brush is damaged then two parallel paths will be damaged. So only two parallel paths will provide the I/2 current and voltage is same for parallel paths. As the current is halved, delivered power is also halved for same terminal voltage.

03․ For a dc machine shunt resistance and armature resistance values are
high and high
high and low
low and low
low and high

For a DC machine the values of armature resistance is very low and shunt resistance is high. The power delivered by the DC Machine depends upon the armature current Ia. Ia should be high to deliver maximum power. The Shunt field is parallel to the armature field, so its resistance must be high for minimum value of shunt field current. If its value will be low then armature current will be lower and power delivered will be less. The series field is connected in series. So if series resistance is more then there would considerable series drop and again armature current will be lower.

04․ Shunt field of DC generators consists of--------number of turns and ---------- conductors respectively
large and thick
large and thin
less and thick
less and thin

To deliver maximum power armature current must be high and the shunt field current is minimum. So, shunt field resistance of DC Machine is very high value around 50Ω to 500 Ω. , it can be concluded that for high resistance length of the coil is to be large and area to be small. So shunt field must have large no of turns and thinner wire than the series field.

05․ Series field consists of--------number of turns and ---------- conductors respectively
large and thick
large and thin
less and thick
less and thin

The power delivered by a DC machine depends upon armature current. The series field is in series with the armature so they are carrying same current through them. Series field are kept at low resistance for minimum drop. , we can conclude that area should be high and no of turns should be less.

06․ What is/are the necessary conditions for voltage build up in self excited generator?
Poles should contain residual flux.
Field winding should be properly connected to armature winding.
Field winding resistance should be less than critical resistance.
All of the above.

The necessary condition for voltage build-up process in a self-excited DC generator are
  1. The poles should retain some residual magnetism. If the poles lost its residual magnetism it can't start voltage build up process. It may be started by a separate DC source at shunt field when armature is at rest.
  2. The field winding should be properly connected to armature winding. If the field connection is reversed then the field flux would oppose residual flux.
  3. The field winding resistance should be less than critical resistance. Voltage will not build up if the field resistance is greater than critical resistance.
  4. The speed of the generator should be greater than the critical speed. This can be remedied by increasing prime mover speed above critical speed.

07․ No load saturation characteristics are plotted between
no load voltage and field current
no load voltage and armature current
short circuit current and field current
short circuit current and armature current

This operating characteristics is also known as saturation curve or open circuit characteristics or magnetization curve or no load magnetizing curve of DC Generator. This curve is drawn between no-load armature generated voltage with the field or exciting current, keeping the speed constant by prime mover. This curve starts slightly above the zero due to residual magnetism. It also determines the design of flux per pole under linear magnetization or saturation curve.

08․ The effect of ------------------ on main field flux is armature reaction?
armature mmf
armature current
armature flux
all of the above

On no load armature flux is negligible as armature current is small but with load the rotating armature produces a flux due to sufficient armature current or load current. The effect of this armature flux on the main field flux is known as armature reaction. Due to armature reaction the main field flux distribution is weakened and distorted.

09․ Armature flux is.............with respect to main field flux or main field poles?
rotates opposite direction
rotates same direction
none of the above

The rotating armature produce a rotating armature flux with respect to armature and there is a working flux which is also under the pole distributed uniformly. Therefore armature flux is stationary with respect to main filed flux. Armature mmf is stationary w.r.t. field poles but rotating w.r.t. the armature.

10․ Cross magnetization effects which of the following?
reduction in main field flux
reduces the terminal voltage
both 2 and 3

Cross magnetizing is one of the effect of armature reaction. By vector addition it is found that it distorts the main field flux. As a result it shifts the MNA (magnetic neutral axis). There would be sparking at the time of commutation if the brushes are not shifted to the MNA.