# MCQs on Electrical Machines

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01․ Consider the following statements : Interpoles in dc machines 1. reducearmature reaction effects in the interpolar region 2. have the same dimension as main polls 3. have their windings connected in series with the armature 4. have same number of turns as the armature 5. have the polarity same as that of the main pole ahead in a motor From these, the correct statements are
1, 2 and 3
1, 3 and 5
1, 2, 3 and 5
1 and 3

Inter poles in dc machines help to reduce armature reaction effects in the inter polar regions and their windings are connected in series with the armature so that its effect can vary with the armature current.

02․ Consider the following statements : 1. interpole windings are connected in series with armature windings 2. polarity of interpoles must be the same as that of the main poles in advance 3. distortion of the main field under the pole shoes is not affected by the use of interpoles From above, the correct answer is
1 and 2
2 and 3
1 and 3
1 alone

Inter pole does not affect the distortion of the main field under the pole shoes rather it affects the distortion under the inter polar region. Its windings are connected in series with the armature winding.

03․ The introduction of interpoles in between the main poles improves the performance of a dc machine, because
the interpoles produce additional flux to augment the developed torque
the flux waveform is improved with reduction in harmonics
the inequality of air-gap flux on the top and bottom halves of the armature is removed
a counter-emf is induced in the coil undergoing commutation

Inter pole produces a counter emf to nullify reactance voltage.

04․ In dc machines, 1. reactance voltage is proportional to armature current 2.reactance voltage is proportional to core length 3.reactance voltage is due to self-flux of the coil undergoing commutation 4. armature-cross flux is neutralized by interpolar flux 5. interpolarmmf is more than armature mmf in the interpolar zone From these, the correct answer is
1, 2, 3, 4
1, 2, 5
2, 3, 4
1, 3, 5

Reactance voltage is proportional to armature current. Reactance voltage is proportional to core length as inductance effect increases with core length. Inter polar mmf is more than armature mmf in the inter polar zone.

05․ The interpoles in dc machines have a tapering shape in order to
reduce the overall weight
reduce the saturation in the interpole
economise on the material required for interpoles and their windings
increase the acceleration of commutation

The interpoles in dc machines have a tapering shape in order to reduce the saturation in the interpole.

06․ If two 8-pole dc machines of identical armature sare wound, one with lap winding and the other with wave winding, then
wave-wound machine will have more rated current and more voltage
lap-wound machine will have more rated voltage and more current
lap-wound machine will have more rated voltage and less current
wave-wound machine will have more rated voltage and less current

Wave winding has two parallel paths where lap winding has parallel paths equal to the number of poles. As the both machines have identical armature wound so wave winding will have more rated voltage and lesser current than lap winding.

07․ If the applied voltage to a dc machine is 230 V, then the back emf, for maximum power developed, is
115 V
200 V
230 V
460 V

For maximum power transfer load resistance should be same as series field resistance. So applied voltage 230 V will be divided equally 115 V on each section i.e. series field resistance and load resistance. Hence for maximum power development back emf should be 115 V.

08․ A dc shunt generator has full-load voltage regulation of 10% at rated speed of 1000 rpm. If it is now driven at 1250 rpm, then its voltage regulation at full load would
be more than 10%
be less than 10%
remain unchanged
be 12.5%

When the speed of generator is 1250 rpm then no load voltage is more than when the speed of the generator is 1000 rpm. So voltage regulation will be more in case of generator speed 1250 rpm.

09․ Two dc machines A and B, using the same conductor material, have armature circuit resistances of 0.4 Ω and 1.2 Ω respectively. Of the two machines,
A is bigger than B for the same current rating
A is bigger than B for the same voltage rating
B is bigger than A for the same current rating
both are of the same size for the same current rating

From the given resistances we can determine for same voltage rating machine A will have more parallel path than machine B.

10․ A 10 kW, 230 V, dc shunt machine has four terminals brought out through four leads. For this machine
both the windings have thin leads
both the windings have thick leads