## Friday, December 30, 2011

### Electrical Interview questions Part 5

Electrical interview question:Give two basic speed control scheme of DC shunt motor?
Answer:1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method:in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa)can be varied,and since speed is directly proportional to Eb-IaRa the speed can be controlled.

Electrical interview question:what is the principle of motor?
Answer:Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

Electrical interview question:what is meant by armature reaction?
Answer:The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Electrical interview question:Give two basic speed control scheme of DC shunt motor?
Answer:1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled 2.armature control method:in this method a rheostat is connected across armature wdg.by varying the resistance the value of resistive drop(IaRa)can be varied,and since speed is directly proportional to Eb-IaRa the speed can be controlled.

Electrical interview question:what is the difference between synchronous generator & asynchronous generator?
Answer:In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing.This type of generators are used in windmills.

Electrical interview question:What is the Polarization index value ? (pi value)and simple definition of polarization index ?
Answer:Its ratio between insulation resistance(IR)i.e meggar value
for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .