MCQs on Electrical Machines

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01․ Consider the following statements regarding dc machines : 1. possible resistance of shunt-field winding is 1 Ω 2. possible resistance of series-field winding is 1 Ω 3. electrographite brushes are used for 230 V, 10 A machines 4. Copper-graphite brushes are used for 230 V, 10 A machines 5. Possible armature-circuit resistance is 1 Ω From these, the correct statements are
1, 4
2, 3 , 5
2, 4 ,5
1, 3, 5

Resistance of series field is kept small and shunt field is kept high. Armature resistance is also kept least to reduce loss. Electrographite brushes are used for 230 V,10 A machines.

02․ A 4-pole dc generator runs at 1500 rpm. The frequencyof current in the armature winding is
25 Hz
50 Hz
0 Hz
100 Hz

We know

03․ In normal dc machines operating at full-load conditions, the most powerful electromagnet is
field winding
armature winding
interpole winding
interpole and compensating windings together

At full load condition field winding is most powerful electromagnet as it contains maximum ampereturn.

04․ A self-excited shunt generator fails to build up, because 1. the field circuit is open 2. The armature circuit is open 3. the residual magnetism is absent 4. The direction of rotation is wrong From these, the correct answer is
3 only
1, 3
1, 2, 3 and 4
1, 2 and 3

If field circuit is open then there is no field flux i.e. no voltage build up. If the armature circuit is open there is no generated emf. If residual magnetism is absence there is no initial field flux i.e. no voltage build up. If the direction of rotation is wrong then residual magnetism will be cancelled out i.e. no voltage build up.

05․ A DC shunt generator builds up 230 V when driven in clockwise direction. In case it is driven in anti clockwise direction, other things remaining unchanged, then the voltage build up is
230 V
some what less than 230 V
some what more than 230 V
not possible

In case of DC shunt generator if we reverse the rotation there is no voltage build up.When we rotate the generator in reverse direction, flux due to residual magnetism will be cancelled out and hence generator will not get sufficient initial flux for emf build up.

06․ A separately-excited DC generator, running at rated speed, has 6 volts across its armature terminals with no field current. When field winding is excited with one ampere, the voltage builds up to 250 V at no load. In case field current is reduced to zero, with the speed remaining unaltered, the armature terminal voltage would be
6 V
less than 6 V
more than 6 V

When field current is reduced from 1 amp to 0 amp due to residual magnetism terminal voltage will be slightly more than 6 volt. When 1 amp current is flowing through the field some flux would build up. Now, if we reduce the field current to zero then due to residual magnetism property of the field core it would hold some flux which leads to slightly increase the voltage above 6 Volt.

07․ A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because
field resistance was higher than critical resistance
there was no residual magnetism in the field circuit
field winding was wrongly connected
there was a break in the armature circuit

If field winding is connected wrongly then residual flux will be eliminated and voltage will drop to zero.

08․ A dc shunt generator, when commissioned after a major repair of the field system, failed to build Up voltage. It was because
field resistance was higher than critical resistance
field system lost residual magnetism
field winding was wrongly connected
there was a break in the armature circuit

After repair of field system it will lose residual magnetism. To get residual magnetism field winding must be connected to suitable voltage source for a while.

09․ A 230 V dc series generator is driven at its rated speed. The no-load voltage across its armature terminals would be
230 V
somewhat more than 230 V
somewhat less than 230 V
6 V

If there is no load connected in series generator then there is no armature current. Hence voltage only due to residual flux will be appeared across the terminal.

10․ A short-shunt cumulatively-compounded dc generator builds up a voltage of 220 V at no load at rated speed. If it is operated as a differentially compounded generator, other things remaining the same, it would build up
6 V
220 V
more than 220 V
somewhat less than 220 V

At no load there is no current flowing through the series field of cumulatively-compounded generator, so there is no contribution in increasing or decreasing the main field flux by series field winding. Hence in both cases terminal voltage will be same.