2010|Online electrical engineering & Electrical Interview Questions

Saturday, December 11, 2010


Here are some Electrical Engineering questions which i came across in internet.I will give the answers for the questions.Hope it would be useful for all Electrical Engineers.Visitors please give your feedback so that i can improve in helping other Electrical Engineers.

Sunday, December 5, 2010


    These questions were asked in the competitive exams.The test is for 10 minutes and you can submit the test if you have finished it early.Hope it would useful for all Electrical Engineers.More Tests would be conducted online related to the electrical subjects.Keep watching Online Electrical Engineering for more ....


Wednesday, December 1, 2010


Here i am answering another question asked by Amit.
What is the importance of Transformer Oil ?
Transformer Oil is a highly refined mineral oil that has excellent insulating properties and is very stable at high temperatures.It also serves as a coolant.

In a Transformer, the primary winding and secondary windings are placed inside a casing.To provide electrical insulation between the live parts inside the transformer casing we use Transformer Oil.Tranformer Oil should also withstand high temperatures for a long period.The cooling of transformer can be increased by providing cooling tubes and external fans outside the transformer casing.  

Buchholz Relay is a protective device used in transfomers.Also various testing is also done on the transformer to test the oil quality.Another important thing is we must have seen some tranformers bursting near our place.This occurs due to insulation failure inside the transfromer.Reason behind this might be the Transformer Oil used would have become old and lot of sludge would have formed which causes insulation failure. 

Saturday, November 20, 2010


What is the difference between Relay,Circuit Breaker,Isolator?

Relay is a protective device which senses the faulty conditions in a power system line and initiates the operation of circuit breaker to isolate the faulty part from the rest of the healthy power system.Examples of relays are Over Current relay,Directional relay,etc..

Circuit Breaker is a device which can
  • Make or break circuit under normal conditions either manually or through remote control.
  • Break circuit automatically under faulty conditions.
  • Make or break circuit under faulty conditions either manually or through remote control.
Some types of circuit breakers are Air blast circuit breakers,Vaccum Circuit Breakers ,SF6 Circuit Breakers,etc.You can notice the circuit breakers near to the transformers.Next time when you see a transformer search for the circuit breaker.Mostly it will be SF6 Circuit Breakers.

Isolator or Isolating Switch is a device which is used to disconnect a part of the system for maintanance purpose and repairs.Isolator can open a circuit under no load condition only.If two isolators are connected on either sides of circuit breaker,then the circuit breaker must be opened before opening the isolators.
After perfoming maitanancee work on the circuit breaker,isolators must be closed first and then the circuit breaker must be closed to bring the system into operation.

Wednesday, November 17, 2010


Hey Amit first of all thanks for asking your doubts.Let me apologize for not posting the answer for your questions for a long time.I couldn't post because of internet problems and other work issues.

1.When there is a sudden loss in the demand what happens to the transformer and generation? 
Consider a train running at 50 km/hr with 20 coaches.Assume that 5 coaches detaches when the train is running.What happens at this time is train will be experiencing an increase in speed and the engine driver has to reduce the speed to bring the train speed to 50 km/hr.

Similarly in a Power System,when there is a sudden loss in load or demand the speed of turbine(steam turbine, hydraulic turbine..etc)  driving the generator increases.So a SPEED GOVERNOR is employed to reduce the speed of the turbine by limiting the input to the turbine.

Consumers need a constant voltage profile for proper functioning of the electrical equipments.But when there is a sudden loss in load, generator terminal voltage is increased.We know that field coils are used instead of permanent magnets in generator.The field coils are excited using a exciter.If we adjust the input to the exciter, the magnetic field changes thereby changing the generator's terminal voltage.this is done by using AUTOMATIC VOLTAGE REGULATOR(AVR).

Speed Governor controls Real Power generation and the frequency of supply.
AVR controls Reactive Power generation and the voltage.

Tuesday, September 14, 2010


First of all what are Brushless DC motors?   In normal DC motors  carbon brushes are used for commutation.Whereas in Brushless DC motors power electronic devices are used for commutation.This type of commutation is called as Electronic commutation.

Based on construction of the motor ,BLDC motors are classified as In-Rrunner and Out-Runner.In In-Runner type, the stator encloses the rotor and in Out-Runner type,the rotor encloses the stator.

Where do you use the BLDC motors?
Almost everyone will be using computers.You must have seen the fans used for cooling your CPU.These fans are nothing but BLDC motors.They are all of Out-Runner type.Just take a PC fan which is not working,and break it and see.You will see stator windings with 4 poles or 2 poles around which a permanent magnet is placed which acts as rotor.The fan blades are attached to this rotor magnet.

Thursday, May 13, 2010


Speed control of DC Shunt motors are done by i) Flux Control method. ii) Armature Control method iii) Voltage Control method.
Speed control of DC Series motors are done by i)Flux Control method. ii) Variable Resistance in series with motor.


DC Shunt motor is a constant speed motor.
DC Series motor has high starting torque .Therefore it is started with a minimum in order to avoid dangerous speeds.


DC motors are classified based on the type of winding arrangement.
1.DC Series Motor
                          In this type of motor, the field winding is connected in series with the armature winding.Thus the same current flows through both the field and armature winding.
2.DC Shunt Motor
                         In this type of motor, the field winding is connected in parallel with the armature winding.So the current is divided between these windings and the applied voltage is same across both the field and armature winding.


Every motor consists of a stationary part called Stator and a rotating part called Rotor.In DC motor, field windings are placed in the stator and the armature windings are placed in the rotor.Therefore DC motor is said to have a rotating armature.

DC motor has the following parts
1. Magnetic Frame or Yoke
2. Pole Cores.
3. Field Coils.
4. Armature Core.
5. Armature Windings
6. Commutator.
7. Brushes.

Saturday, May 8, 2010


Motors are the electromagnetic devices which converts electrical energy into mechanical energy.Its working is based up on the principle that when a current-carrying conductor is placed in a magnetic field,it experiences a mechanical force whose direction is given by Fleming's Left hand rule

Basically motors are classified based on the input electrical supply.They DC motors and AC motors.
DC motors are further classified as 
    i)  Separately excited DC motor.
    ii) DC shunt motor.
    iii) DC series motor.
    iv) DC compound motor.
           a)Cumulative-compound motors.
           b)Differential-compound motors. 
Classification of AC motors are given as
    i)Synchronous motors.
    ii)Asynchronous motors.
           a)Squirrel cage induction motor.
           b)Slip-ring or wound rotor induction motor. 
Other motors which come under special machines are
     i) Stepper motor.
    ii) Brushless DC motor.
    iii) Switched Reluctance motor.
    iv) Variable Reluctance motor.


It states that "If a conductor of length 'l' is moved with a velocity 'v' in a magnetic field of constant flux density 'B', then the EMF induced is equal to the rate of change of flux."
E=Blv (Volts)
where,  E =  Induced EMF (Volts)
            B =  Flux density (Wb/m^2)
            l  =  length of the conductor (m)
            v =  velocity of the conductor (m/s)
It states that "If the fingers of the right hand are placed around the wire such that the thumb points in the direction of current flow, the fingers will be pointing in the direction of the magnetic field produced by the wire."