MCQs on Electrical Machines

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01․ The material used for commutator brushes is mostly
cast iron.

The life of the carbon brush is high. The frictional loss between the brush and the commutator is very less. It has the capability to withstand the heat generation due to the friction during running time.That is why the carbon brush is mostly preferred for any kind of machines.

02․ A 4 pole DC generator rotates at 1500 rpm. How many times in a second, will the emf induced in each armature conductor changes its direction?

We know that the frequency \ Here, Hz is nothing but the cycles per second. The cycle is the combination of the positive half cycle and the negative half cycle. So, 100 times it changes the EMF generation in the armature in a second.

03․ Magnetic field of a DC generator is produced by what?
Permanent magnets.
Both B and C.
None of the options.

In a DC generator, the magnetic field is generated by using the electromagnets. If we use permanent magnet, then after some days it reduces its magnetic strength. Then the generator cannot generate the rated voltage at rated speed. That is why the permanent magnets are not preferred for the DC generators.

04․ In a DC generator the emf generated by the armature is maximum when
rate of change of flux is maximum.
rate of change of flux is minimum.
induced voltage is high.
shaft rotation is high.

As per Faraday's law of electromagnetic induction, the EMF induced in a conductor is directly proportional to the rate of change of flux linkage with it. Hence, the EMF generated by the armature is maximum when the rate of change of flux linkage in the armature is maximum.

05․ A simple method of increasing the voltage of a DC generator is to
increase the number of turns in field coil.
increase the number of turns in armature coil.
increase the speed of shaft rotation.
increase the source frequency.

Increasing the rate change of flux linkage is little bit complicated one because we need the strongest electro magnet for that. Hence, the initial cost is high. Instead of we can increase the speed of the shaft to generate the voltage. That means the time taken to cut the flux by the armature conductor is less. So, the rate of change of flux in the conductor is more and hence the higher EMF can be generated.

06․ What is the actual arrangement of series wound generator?
Armature and field both are parallel.
Armature and field both are connected.
Armature winding is in series with load.
Armature and field both are in series with the load.

Basically, in a DC series wound generator, the field winding is connected with armature winding in series connection. The series combination of field winding and armature winding is connected with the load.

07․ In DC generator, armature reaction refers to what?
Effect of armature flux on the main field flux.
Effect of field flux on the armature flux.
Effect of armature flux on the load.
None of the above.

When a DC machine is loaded, then the current flows through the armature winding conductors. We know that if current flows through any conductor, it creates the magnetic flux line. Hence the armature conductor also creates the magnetic flux. So this armature flux may oppose the main flux which is generated from the field winding of the machine. That effect depends on the nature of the armature flux. This phenomenon is called as armature reaction.

08․ What is the effect of armature reaction ?
Armature flux weakens the main field flux.
Armature flux distorts the main field flux.
Both option B and C.
None of the options.

The armature reaction is basically classified into two different types. One is de-magnetization effect and another one is cross magnetization effect. During the de-magnetization effects, the armature flux weakens the main field flux. During the cross magnetization effects, the armature flux distorts the main field flux. Hence, both are the different effects of the armature reaction.

09․ A single-phase transformer has its maximum efficiency at 60 percent of full load. At full load copper loss will be
equal to core loss.
less than core loss.
more than core loss.

Whenever if the efficiency of the transformer is maximum, then the copper loss is equal to the core loss. So, still if we increase the load, then the current taken by the transformer is more. So, the copper loss due to the I2R loss is getting increased. In transformer, after achieving the maximum efficiency the core loss is increased but not that much. So, according to this problem, at full rated load the copper is more than the core loss. This to be gone to transformer section.

10․ In DC generator, brushes are always placed along
magnetic neutral axis.
geometrical neutral axis.
armature axis.
load axis.

In order to achieve the sparkless commutation, the brushes on the commutator should be placed at the points known as neutral point where no voltage exists between adjacent segments. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis (MNA).