Electrical Machine Interview Question And Answer

[1] Define the term synchronous speed [Dec-2003]
For synchronous machines there exists a fixed relationship between number of poles P, frequency (f) and the speed of the machine. The speed of the synchronous machine for the given number of poles and the rated frequency is called the synchronous speed mentioned as NS.

[2] Write an expression for synchronous speed (Dec-2004)
An expression for synchronous speed is NS = 120f/P.
Where
f = frequency
P = No of poles of the machine.

[3] What does speed voltage mean? (Dec-2007)
When the magnetic flux is constant as well as stationary and the coil rotates to cut the flux then EMF gets induced due to relative speed between flux and coil. This EMF is called speed EMF, rotational EMF or dynamically induced EMF.

[4] Mention the factors on which hysteresis loss depends (Dec-2008)
The hysteresis loss is directly proportional to the area under the hysteresis curve ie area of the hysteresis loop.
It is directly proportional to frequency ie number of cycles of magnetization per second.
It is directly proportional to volume of the material.

[5] Distinguish between statically induced and dynamically induced EMF (Dec - 2010, 2011, 2009)
How is EMF induced dynamically ( May-2010)
An induced EMF which is due to physical movement of coil, conductor with respect to flux or movement of magnet with respect to stationary coil, conductor is called dynamically induced EMF or motional induced EMF.
The change in flux lines with respect to coil can be achieved without physically moving the coil or the magnet. Such induced emf in a coil which is without physical movement of coil or a magnet is called statically induced EMF.

[6] Define torque (May-2010)
A turning or a twisting force about an axis is called as torque.

[8] What are the three types of basic rotating electric machines? (May-2011)
DC machines
Induction Machines
Synchronous Machines

[9] What are the causes of core loss? what are the components of core loss?
When a core is subjected to an alternating flux then it undergoes the cycles of magnetisation and demagnetisation. This produces hysteresis effect which causes hysteresis loss in the core.
Similarly core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, EMF gets induced in the core. Such currents in the core which are due to induced emf in the core are called as eddy current loss. Thus eddy current and hysteresis are the two components of the core loss.

Transformer Interview Questions With Answer

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  We shared a lot of interview questions on electrical machines, power system and power electronics. Today we are sharing interview questions on transformer. These transformer interview questions is collected from various sources. lets start transformer interview questions with answer.

Q.1. How is magnetic leakage reduced to a minimum in commerical transformers ?
Ans.By interleaving the primary and secondary windings.

Q.2. Mention the factors on which hysteresis loss depends ?
Ans.(i) Quality and amount of iron in the core
       (ii) Flux density and
       (iii) Frequency.
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Q.3. How can eddy current loss be minimised ?
Ans.By laminating the core.

Q.4. In practice, what determines the thickness of the laminae or stampings ?
Ans.Frequency.

Q.5. Does the transformer draw any current when its secondary is open ?
Ans.Yes, no-load primary current.

Q.6. Why ?
Ans.For supplying no-load iron and copper losses in primary.

Q.7. Is Cu loss affected by power factor ?
Ans.Yes, Cu loss varies inversely with power factor.

Q.8. Why ?
Ans.Cu loss depends on current in the primary and secondary windings. It is well-known that current required is higher when power factor is lower.

Q.9. What effects are produced by change in voltage ?
Ans.1. Iron loss.........varies approximately as V2.
       2. Cu loss.........it also varies as V2 but decreases with an increase in voltage if constant kVA output is assumed
       3. Efficiency.........for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and viceversa.

       4. Regulation.........it varies as V2  but decreases with increase in voltage if constant kVA output is assumed.
     5. Heating.........for constant kVA output, iron temperatures increase whereas Cu temperatures decrease with increase in voltages and vice-versa
.
Q. 10.How does change in frequency affect the operation of a given transformer ?
Ans. 1. Iron loss .........increases with a decrease in frequency. A 60-Hz transformer will have nearly 11%      higher losses when worked on 50Hz instead of 60 Hz. However, when a 25-Hz transformer is worked on 60 Hz, iron losses are reduced by 25%.

2. Cu loss.........in distribution transformers, it is independent of frequecy.
3. Efficiency.........since Cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.
4. Regulation.........regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factors decreases with a decrease in frequency and vice-versa.

For example, the regulation of a 25-Hz transformer when operated at 50-Hz and low power factor is much poorer.

5. Heating.........since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.

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