2013|Online electrical engineering & Electrical Interview Questions

Monday, November 11, 2013


Here is the first video tutorial for electrical engineering.We thought that video would be more easier to understand the concept and started this video tutorial for electrical engineering.The video is regarding the types of Brushless DC motor.Start learning and enjoy Electrical Engineering !! Cheers..!!

Saturday, October 26, 2013


Here is the answer to one of the follower of Online Electrical Engineering.Her question is "I need a auto transformer for 2hp motor 380 volts 3phase.Now my problem is my line is 220volts 1phase.What should be the auto transformer capacity?"

Autotransformer is not similar to the power transformer.Power transformer works on the principle of mutual induction whereas autotransformer works on the principle of self induction.Autotransformer has only one winding and the same winding acts as primary winding and secondary winding.Also it is not electrically isolated where as in power transformer windings are electrically isolated.

Autotransformer will have one tapping common to both source and load side winding.The other tapping will be connected to the source on primary side winding.Multiple tappings are provided on secondary side winding to have multiple voltage outputs. Autotransformer is usually a step down transformer and can be used to provide the supply voltage to the  electrical appliances/electrical instruments.

There are two types of autotransformers based on the phase.Single phase autotransformers and three phase autotransformers are the types of autotransformers.Single phase autotransformer can be used to provide supply to the single phase devices.Three phase autotransformers can be used to supply single/three phase electrical devices but the single phase autotransformer cannot be used to supply the three phase electrical devices.

Thursday, October 24, 2013


Hi all.Please take up the Electrical Engineering Test-3.The test contains the questions which were asked in some competitive exams.Though the questions are theoretical it would be helpful in understanding the basic concepts.We have been preparing questions ourselves, so that it is useful for all Electrical Engineers.Online Electrical Engineering team has been taking efforts to satisfy the readers with the best they can...Your valuable comments are welcome to improve the quality of the tests.
Cheers !!

Tuesday, August 20, 2013


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 !!!

Thursday, August 15, 2013


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.

Tuesday, August 13, 2013


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.

Saturday, July 27, 2013


Here is an Electrical Engineering question from one of our friend.Really this is a good question and sometimes some small things would cause us a big question.The doubt of our friend is "let's say you have an electric motor that operated on 110vac at, say, 3 amps. What would be the outcome if you put a 1:1 isolation transformer (110 in - 110 out) between the source and the load (motor). Would the power consumption at the source be the same or would it be measured only as the consumption of the transformer?"

Let the motor have a load of 3A and it runs with the input voltage of 110V.The power drawn by the motor would be 330 Watts(P=V*I). So if you connect a 1:1 isolation transformer between the source and the motor for the same amount of load,will the power consumption at the source be same? The answer is a big NO.

The source has to supply the power to the isolation transformer and also to the motor load via the isolation transformer.So the transformer losses such as eddy current loss,heat loss etc would also consume power form the source.Therefore the power consumption wont be the same at the source.

If you are not convinced I will give you a simple analogy.Say you have a 100 litre water tank(100 V battery).There are two locations pointA and pointB.PointA is at 10 meters from the tank and the pointB is at 110 meters away from the tank.

SCENARIO 1 : Tank is at full capacity and you fill a bucket of 20 litres at pointA.Remaining capacity in the tank would be 100-20-the amount of water in the 10 meter pipeline.
SCENARIO 2 : Tank is at full capacity and you fill the bucket 0f 20 litres at pointB.Remaining capacity in the tank would be 100-20- the amount of water in the 110 meter pipeline.

Will the capacity of tank after the two scenarios will be the same?Not at all.This also applies to the electrical circuit.Hope the answer is clear.Please post you valuable comments and views.Cheers !!!

Sunday, February 17, 2013


Here is the new question, can we monitor the customer loads from substation ? To answer the question first we need to know the purpose of Substation.After generating power,we transfer using Transmission & Distribution networks at very high voltages in order to reduce the power loss .

The transmitted power needs to be delivered to the customers at 230 V/400 V based on whether the consumer is domestic or industry. So the distribution substation steps down the transmitted power from 33 KV to 11 KV and is further stepped down by the transformers to supply the customers.So basically the substations steps down/steps up the power for distribution and it records the readings of power flow between the substations in the transmission networks.

Why do we install meters at the consumers place? To know the actual power consumption with minor tolerances level we install meters at the consumer place.If we measure it at the substation, Who is going to account for the losses made during transmission and how will you know which consumer has consumed how much power? So the government takes all these losses and generation cost and other factors into consideration and fixes the rate for unit consumed for different consumers.

Wednesday, January 9, 2013


What is load shedding? and what is meant by load shedding? and what are its causes? These are the questions many of the electrical engineers and common people will have.

As the name indicates shedding means "removing something". Here we are shedding the load which means we are removing the load from the grid. Usually load shedding is done, when there is huge demand for electricity and the generation of power is much lesser when compared to the load.Generally in summer load increases as many of the consumers use air conditioners.Therefore in summer there are possibilities of power cut to a greater extent.

To make it very simple let us take this example.Say there are 12 customers who wants to use taxis in morning,afternoon and evening session.But we have only 8 taxis.So what we do is ,make the 8 taxis available to 8 persons at a time and send the taxis to the next 4 customers after 1 hour. Similarly we make the other set of 4 customers to wait in the afternoon session and next set in the evening session.All the three set of customers get late by 1 hour.All of them have same waiting period but in different sessions.

Presently the situation is same in Tamil Nadu.In major cities the power cut is for 2 hours and in town areas it is more than 8 hours but at different timings.This is done because the generation is much lesser than the demand.The government needs to build the new generation plants.Nuclear power plant has been built  in Kudankulam and the state government has demanded the central government to donate all the power generated from Kudankulam power plant to Tamil Nadu.

How can we reduce the load shedding from our side?It is as simple as that,reduce all the unwanted loads.Switch off your computers when you are not using.You may think it just a small unit which is going to be consumed,but behind that there is generation,transmission and distribution and it involves huge cost which can be avoided.Switch off lights,fans,televisions when you don't need it.Reduce the usage of air conditioners in winter.Avoid using lifts and escalators at times.