Monday, June 23, 2014

Alternators related Interview Questions with Answers

Frequently Asked Interview Questions on Alternators
In this post we had given few Alternators related Interview questions with solutions. We believe that it will be helpful for your interview preparation. Please leave your comments in comments section.

[Q] What is meant by synchronising the alternators?
The process of connecting two or more alternators in parallel for supplying a common load is called synchronising.

[Q] What are the conditions to be satisfied for proper synchronising of alternators?
(i) The terminal voltage of the incoming alternator must be the same as bus-bar voltage.
(ii) The speed of the incoming alternator must be such that its frequency equals to the bus-bar frequency
(iii) The phase sequence of the incoming alternator must be the same as that of the other alternators or busbars.

[Q] What are the advantages of Parallel operation of alternators?
(i) It ensures continuity of power supply to consumers in case of breakdown of an alternator in an generation station.
(ii) This is economical and improves the efficiency of the generating station.
(iii) When the demand of power increases, new alternators can be installed to operate in parallel.
(iv) It is not possible to built single large size alternator to meet the requirement.

[Q] What are types of rotors used in alternators?
There are two types of rotors used in alternators namely
(i) Salient pole rotor
(ii) Smooth cylindrical type rotor

[Q] What are the advantages of providing damper winding?
The damper winding is useful in preventing the hunting ( momentary speed fluctuations) in generators.
The damper winding also used to maintain balanced 3 phase voltage under unbalanced load conditions.

[Q] What are the various methods to determine the voltage regulation of the large alternators?
In case of small machines, the regulation can be found by direct loading.
For large alternators, to find the voltage regulation indirect methods are used. They are
(i)Synchronous Impedance Method.
(ii) The Ampere-turn method.
(iii) Zero power factor or Pointer Method.

[Q] What is the basic principle of alternators?
Alternators is noting but a AC generators. They operate on the fundamental principle of electromagnetic induction as dc generators.
ie, when the rotor rotates, the emf is induced in the stator.

[Q] Which type of rotor is suitable for low speed alternators? salient pole type or cylindrical type rotor?
Salient pole type alternators are suitable for low and medium speed alternators. It has large number of projecting poles. It has large diameters and short axial lengths.

[Q] What is meant by turbo alternators?
High speed alternators are called as Turbo alternators.
As it runs at very high speed, salient pole rotors are not used. Smooth cylindrical type rotor is suitable for turbo alternators.

Please leave your comments below...That is highly appreciated...

Sunday, June 15, 2014

Basic Electrical Engineering Interview Questions Answers 1

Basic Electrical Engineering Interview Questions Answers 1

[Q] Define Power and Energy?
The rate at which work is done in an electric circuit is known as Electric power.
Power = Voltage X Current
The basic unit for electric power is Watts
The instrument used to measure the power is known as watt meter.

The total amount of work done in an electric circuit is called as electric energy. 

Energy = Power X Time
the basic unit is Joule or watt-sec
The practical unit is Kilo watt-hour
The instrument used to measure the energy is known as Energy meter.

[ Like this you have to explain elaborately in the interviews. The interviewer will expect detailed answer from the candidate.

If he feels that you are deviating too much, then he will stop you and ask another question. Please don't answer in one line. Add related points
where ever possible]

[Q] In our house are we monitoring/measuring power or energy?

Energy. It is done by energy meter.

[Q] what is meant by cycle in ac waveform? what is period? Define frequency?

One complete set of change in value is known as waveform.
If you any basic question on AC waveform, then first draw an ac waveform and explain with reference to the waveform.

[Q] Explain about various AC Values?
Instantaneous value:
Instantaneous values are the values of the alternating quantities at any instant of time. They are represented by small letters like i, e etc.
Peak value:
The largest value reached in a half cycle is called the peak value or the maximum value or the amplitude of the waveform. Such values are represented by Vm, Im etc.
Peak-to-peak value is the difference between the maximum and minimum values in a cycle.
Average value:
The average or mean value of a symmetrical alternating quantity (such as a sine wave), is the average value measured over a half cycle (since over a complete cycle the average value is zero). For a sine wave, average value = 0.637 times maximum value 
RMS Value: 
The effective value of an alternating current is that current which will produce the same heating effect as an equivalent direct current. The effective value is called the root mean square (rms) value and whenever an alternating quantity is given, it is assumed to be the rms value.

[Q] What is form factor and peak factor?

The ratio of rms value to average value of an ac quantity is called form factor.
Form Factor = RMS value/ Average value
The ratio of maximum value to the RMS value of an ac quantity is called as peak factor.
Peak Factor = Maximum Value/RMS value

Wednesday, June 4, 2014

Induction Motor Interview Questions: Part 1

Basic Interview Questions on Induction Motors:

[Q] How does the Induction motor work? (OR) Why does the Rotor rotate?
  • When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up. 
  • The flux passes through the air-gap and sweeps past the rotor surface  thus it cuts the rotor conductors. 
  • Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday's laws of electromagnetic induction.
  • The frequency of the induced emf is the same as the supply frequency.
  • Its magnitude is proportional to the relative velocity between the flux and the conductors.
  • Its direction will be as per Fleming's right hand rule.
  • Since the rotor conductors form a closed circuit the rotor current is produced.
  • This current's direction will oppose the very cause producing it ( as per Len's law)
  • Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.
  • Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.
[Q] What is the general working principle of Induction motor?
  • The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. 
  • In DC motors the electrical power is conducted directly to the armature through brushes and commutator. 
  • Thus the DC motor can be called as conduction motor. But in case of AC motors, the rotor receives electric power, not by conduction but by induction. 
  • This is exactly in the same way as the secondary of two winding transformer receives its power from the primary. 
  • That is why such motors are known as induction motors. 
  • Thus an induction motor is also known as rotating transformer ( ie, one in which primary winding is stationary and the secondary is free to rotate)
[Q] What is the advantage of skewed stator slots in the rotor of Induction motors?
In the induction motor design, the rotor slots are purposely made with a slight skew arrangement. It will not be parallel to the shaft. 
This is for the purpose of reducing magnetic locking or reducing magnetic attraction between stator and rotor teeth. 
In addition to that this arrangement will help to reduce the magnetic hum and noise.

[Q] What is meant by cogging in the Induction motor? How to prevent the cogging?
When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to each other, since this corresponds to minimum reluctance position. In such case the rotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging. 
This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

[Q] What are the various methods of measuring slip?
1. By actual measurement of rotor speed
2. By measurement of rotor frequency
3. Stroboscopic method

[Q] What are the various methods of speed control in three phase induction motors?
(i) Control from stator side
1. By changing the supply frequency
2. By changing the number of stator poles
3. By changing the supply voltage
(ii) Control from rotor side
1. By inserting resistance in rotor circuit
2. By various ways of cascade connection
3. By injecting EMFs in the rotor circuit.

[Q] What is meant by crawing in the induction motor?
In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.