MCQs on Electrical Machines

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01․ Iron loss in a transformer mainly occurs in
iron made tank of transformer.
steel core in transformer.
winding of transformer.
transformer insulating oil.

Iron loss is also called core loss in transformer which consists of hysteresis loss and eddy current loss. Both of these losses associated with transformer core.

02․ Short circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short circuit test is now performed with the same magnitude of impressed voltage, but at frequency higher than the rated frequency then
The magnitude of circuit as well as the power factor will decrease.
The magnetude of current will increase, but power factor will decrease.
The magnitude of current will decrease but the power factor will increase.
The magnitude will both increase.

Since frequency has been increased, the leakage reactance and hence impedance will increase. Thus current will be less and power factor will be poorer.

03․ In a transformer the magnetic coupling between the primary and secondary winding can be increased by
increasing the number of turns.
using soft material for winding.
using the magnetic core of low reluctance.
using transformer oil of better quality.

Magnetic coupling between two windings depends upon flux linkage between them. Flux linkage can be maximized between two coils by placing magnetic core of low reluctance in between them.

04․ The short circuit test on a transformer is conducted to obtain
copper losses.
core loses only.
eddy current loss.
hysteresis loss.

The watt meter reading in short circuit test of a transformer, gives the copper losses and negligible amount of core losses of the transformer. Since, the core losses depend upon applied voltage and voltage applied in short circuit test, is very small compared to rated voltage of transformer. Therefore, from the watt-meter reading in short circuit test, we get mainly copper losses of transformer.

05․ An auto transformer having a transformation ratio of 0.8 supplies a load pf 10 KW. The power transfered inductively from the primary to secondary is
2 KW.
8 KW.
10 KW.

If transformation ratio were, whole power would have been transfered conductively only. Power transferred inductively \ [math] = Total \; power imes \left(1 - rac{N_2}{N_1} ight) = 10 imes (1 - 0.8) = 2 \; KW. [/math]

06․ In one 132 / 33 kV transformer the LV resistance is 0.02 per unit. The resistance referred to HV side is
0.08 pu.
0.02 pu.
0.16 pu.
0.01 pu.

The per unit value of any parameter of a transformer remain same irrespective of whether it is referred to primary or secondary.

07․ Lamination of transformer core is made of
cast iron.
silicon steel.
wrought iron.
cast steel.

The lamination of transformer core is made of cold rolled grain oriented silicon steel (CRGO silicon steel) to reduced its hysteresis loss.

08․ Oil is provided in an oil filled transformer for
both cooling and insulation.

Transformer insulating oil is used in transformer for both cooling and insulating purpose.

09․ Air core transformer as compared to iron core transformer, has
more copper loss.
less magnetic core loss.
more magnetic core loss.
no magnetic core loss.

Magnetic core losses consist of hysteresis loss and eddy current loss. Hysteresis loss is associated with ferromagnetic property of iron and eddy current loss is associated with conducting property of iron. As the air is not a ferromagnetic substance as well as it is not a conductor so there will not be any core loss in the air core transformer.

10․ CRGO steel is used in the core of transformer to reduce
eddy current loss.
ohmic loss.
both eddy current and hysteresis loss.
hysteresis loss.

CRGO means cold rolled grain orientation. Hysteresis loss mainly occurs in ferromagnetic material due to its random grain (small magnetic region) orientation. In CRGO steel these grain are properly oriented so that hysteresis loss can be minimized.