ELECTRICAL ENGINEERING INTERVIEW QUESTIONS ANSWERS

Here is a electrical question which i came across.A DC cummulative compound motor delivers rated load torque at rated speed. if the series field is short circuited then what happens to armature current and speed?

There are two types of compound motor.They are cummulative compound motor and Differential compound motor.In differential motor the series field flux and the shunt field flux are in opposite direction whereas in the cummulative compound motor, series field flux & shunt field flux are in same direction and they produce an aiding effect.Therefore the speed regulation is good in cummulative DC compound motor when compared to the Differential compound motor.

The cummulative compound motor running at rated speed and rated torque.If you cut the field resistance in series ,it would act as a shunt motor. Probably in the practical you might have done this.We won't cut the resistance abruptly,instead move the rheostat to the minimum resistance position and observe the readings.So if we cut the series field resistance,the armature current increases as the resistance is lesser when compared to the motor with series field winding.

From the formula ,speed is inversely proportional to the the flux.As a result of removing the series field winding resistance, the total flux is reduced.This reduced flux increases the speed.You might add some more load to reduce the speed but the motor would run beyond its rated load and the speed would get reduced.

Hope this answers the question.Please post your valuable comments in the comments section and post more questions to share the knowledge.Cheers !!!

DCMOTOR HIGH TORQUE STARTING ELECTRICAL

Here is another electrical question from one of our followers.Why DC Motor has high torque?

All motors have two main parts which is the stator and rotor.The field windings constitute the stator and the armature windings constitute the rotor part.Usually the DC Series motor have low armature resistance.when the voltage is supplied there is high armature current which flows in.It works on the basic Ohm's law principle.
I= V/R.

Torque is directly proportional to the armature current.So the high current gives a high starting torque in the DC motor.When the rotor starts rotating, the back EMF is produced and it opposes the applied voltage.As the speed picks up the back EMF also rises and hence the resultant EMF is reduced.As the resultant EMF reduces, the armature current reduces and in turn reduces the torque.

The torque reaches the steady state when the armature current reduces.Usually the DC motor uses the external resistance to reduce the armature current.Till late 1980s DC motor was used for electric traction and after the advancement in power electronics, AC Induction motors have replaced the DC motor in traction.

FUSE BREAKING CAPACITY ELECTRICAL QUESTION

Here is another question related to electrical fuse from one of our follower.The question goes like this.

I face problem it really makes me confused about section of MV fuse.We have a 6MW, 24 kv project we need a disconnect fuse type switch.I calculate the fusion rate: 200 Amps.

When I refer to fuse catalog I found the fuse rate as below:Rated voltage = 24 Kv,

Rated current= 200 A,  Breaking current= 840 A. I do not understand what is that 840 A, what this mean any body could help me about that?

Fuse is a device that is made of low resistance material which is used to protect the electrical devices from the high current surges.Rated current is the the current at which the fuse is rated for.Breaking current is the current at which the fuse breaks.When the fuse breaks it breaks by giving out spark.

In practical classes, while doing armature control or field control of DC motor you would have seen breaking of fuse by giving out spark and a big sound.Also in house, some times we would have seen the fuse breaks.Then we take out the fuse carrier and replace it with the new fuse wire and put it in place.

Imagine without these fuses the high current would have damaged the motors and other electrical appliances.Similarly it applies to the high voltage circuits too.