# MCQs on Electrical Machines

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01․ Demagnetization effects which of the following?
commutation
reduction in main field flux
reduces the terminal voltage
both B and C

Demagnetization is one of the effect of armature reaction. By vector addition it is found that it reduces or weakens the main field flux. So it reduces the induced emf or terminal voltage in case of generator because Eg is directly proportional to the flux (φ). In case of a motor it reduces the torque and increases the speed because Te is proportional to flux and speed(N) is inversely proportional to the flux.

02․ Brushes are always placed on--------------- , in order to achieve sparkles commutation?
GNA
MNA
either GNA or MNA
none of the above

Brushes should be placed where the direction of current are changes or production of zero e.m.f., under no load condition MNA and GNA coincides with each other. At this axis current direction is reversed or no emf is produced. But due to armature flux the main field flux gets distorted and MNA does not coincides with GNA under loaded condition. That means neutral zone is shifted. In order to achieve sparkless commutation brushes is placed on MNA. So, brushes are always placed in MNA in loaded or unloaded condition.

03․ Flux density under trailing pole tips in case of generator will
increase
decrease
either increase or decrease
none of the above

The pole tip towards which the armature conductor leaves the influence of pole is called trailing pole tips (symbol by dot sign). In a generator the main field flux and armature flux are aiding each other under trailing pole tips region. Therefore total flux density will increase under trailing pole tips.

04․ Flux density under leading pole tips in case of generator will
increase
decrease
either increase or decrease
none of the above

The pole tip towards which the armature conductor enters into the influence of pole is called leading pole tips (symbol by cross sign ). In a generator the main field flux and armature flux are opposing each other under leading pole tips region. Therefore total flux density will decrease under leading pole tips.

05․ Which of the following is/are effects of armature reaction?
increase the iron loss
increase the maintenance and repair
increases the design cost
all of the above

Due to armature reaction flux density is increased under one half of the pole, so iron loss increases. The commutation process deteriorate and severe sparking in brushes. To improve commutation inter-polar and compensating winding are placed. So the design and maintenance cost increases. These are all the effects of armature reaction.

06․ Which of the following is/are the methods to reduce the armature reaction and its effects?
pole chamfering
pole stacking
compensating winding
all of the above

Methods to reduce armature reaction in DC machines are 1. Pole chamfering 2. Pole stacking 3. Pole core slotting 4. Compensating winding 5. Interpolar winding

07․ In DC machine torque depends on which of the following?
flux (φ)
armature current (I)
both A and B
speed

08․ Compensating winding is placed in the
pole shoe
armature core
main field
all of the above

Compensating winding is placed in the pole shoe or pole face, cut in to slots embedded in the pole. They are connected in series with the armature having same axis as armature but opposite polarity of the pole.

09․ Direction of the current in the compensating winding is________ to the direction of current in the armature conductors?
same
exactly opposite
either of these
none of the above

The direction of the current in the compensating winding is exactly opposite to the direction of current in the armature conductors under respective pole, this produces an extra flux which neutralize armature flux and thus armature reaction.

10․ Find the reactance voltage when current is changed from -2A to 2A in 4 sec and self inductance is 1H?
0 V
4 V
1 V
2 V

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