Interview questions 2

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

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Interview Questions 1

[1] Why transformers rated in KVA instead of KW?
The copper loss (I2R) in the transformer depends on the current I through the winding while the iron or core loss depends on the voltage V as frequency of operation is constant. None of these losses depend on the power factor (cosφ) of the load. Hence losses decide the temperature rise and hence the rating of the transformer. As losses depends on V and I only, the rating of the transformer is specified as a product of these two parameters VxI. Thus the transformer rating is in kVA and not in kW.

[2] Why is the efficiency of transformers more than that of other rotating machines?
There are no moving parts in transformer hence the friction and mechanical losses are absent in transformer. Hence efficiency of the transformer is more than of other rotating machines.

[3] Define leakage inductance
Inductance offered by the winding due to the leakage flux associated with it is called leakage inductance. It is the ratio of leakage flux linkages with the winding to the current passing through the winding.

[4] What material is used in the transformer core?
High grade silicon steel laminations are used for the construction of the core.

[5] What is meant by leakage flux?
Part of the primary flux as well as the secondary flux completes the path through air and links with the respecting winding only. Such a flux is called leakage flux. Leakage fluxes link with the respective windings only and not to both the windings.

[6] Define voltage regulation of a transformer?
The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load ie, rating load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

[7] What is an ideal transformer?
A transformer is said to be ideal if it satisfies following properties:

1. It has no losses
2. Its windings have zero resistance
3. Leakage flux is zero ie, 100% flux produced by primary links with the secondary
4. Permeability of core is so high that negligible current is required to establish the flux in it.

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