MCQs on Electrical Machines

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01․ Saving of copper in auto transformer is increased as compared to two winding transformer, when transformation ratio K approaches to

Saving of copper weight in auto transformer = Transformation ratio (K) X ( weight of copper in equivalent two winding transformer ). Thus it can be visualized that saving will increase as K approaches unity.

02․ Tertiary winding in a transformer is mostly
star connected.
delta connected.
zig - zag star connected.
zig - zag delta connected.

When faults occur on the primary or secondary sides, considerable unbalanced of phase voltage may be produced which is compensated by large circulating current through this closed delta connected tertiary winding.

03․ The main purpose of providing tertiary winding in a three phase transformer is
to suppress harmonic and unbalance in voltage.
to interconnect three voltage systems.
to carry extra load.
none of the above.

The purposes of providing tertiary winding in a three phase transformer is to carry extra load added to the transformer which for some reason must be kept isolated, to inter connect three supply systems operating at different voltages but most important purpose is to suppress harmonic voltages and to limit voltage unbalance when the main load is asymmetrical.

04․ When operating, two or more transformers in parallel, % impedance of these transformers should be
inversely proportional to MVA rating of transformers.
proportionate to MVA rating of transformers.
such that actual impedance of all transformers would be same.

Load sharing by transformers operating in parallel are as per their MVA rating if % impedance of the transformers are same.

05․ Tap changer is generally provided in
current transformer.
power transformer.
voltage transformer.
earthing transformer.

Tap changer is used to control the output voltage of a transformer. As power transformer is connected to the load whose voltage level to be maintained at desired level to serve the consumer a quality power, tap changer is generally used in power transformer. Current, voltage and earthing transformers have no direct dealing with consumer load hence there may not be any tap changer.

06․ The heat generated in the transformer is dissipated mainly by
all of the above.

Heat generated in transformer is mainly dissipated in atmosphere from radiator outer surface mainly due to convectional flow of air surrounds the radiators.

07․ Transformer cooling and insulating oil must be of
low viscosity.
high viscosity.
low BDV.
low resistivity.

The transformer oil subjects to convectional flow for cooling the transformer. As low viscous liquid flow easily than high viscous one, transformer insulating oil should be of low viscosity.

08․ Normally for the design of transformer the ambient temperature is taken as

Normally for the design of transformer, the ambient temperature is taken as 45°C.

09․ Which of the following machine will have maximum efficiency?
Electrical power transformer.
Electrical power generator.
DC motor.
Induction motor.

Transformer has maximum efficiency as it is static device means it does not have any moving parts in the main body of a transformer hence no mechanical loss is counted in transformer.

10․ The heat transfer by convection in a transformer least depends on
temperature difference.
air velocity.
surface area.
color of the paint.

Heat dissipated in air from surface of transformer radiator depends upon obviously the temperature difference between oil and atmospheric temperature, velocity of flowing air through the surface of radiators, and surface area of radiator. It also depend upon color of paint of radiator surface since many colors obstruct radiation of heat to the atmosphere but the effect of color is minor compared to other conditions of heat dissipation.