MCQs on Electrical Machines

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01․ Scott connections are used for
single phase to three phase transformation.
three phase to single phase transformation.
three phase to two phase transformation.
any of the above.

Scott connections are used to convert three phase to two phase, particularly to start two phase motors and two phase furnaces. It requires two single phase transformers with adjustable tappings, one transformer is known as main transformer which is centre tapped through second one known as teaser transformer primary with a suitable number of turns to get a balanced two phase supply.

02․ In which transformer, the tertiary winding is used?
Star - delta
Star - star
Delta - delta
Delta - star

When tertiary winding is used in a transformer, then that is called three winding transformer. It is exclusively used in high voltage transformer windings which are connected in star. If both primary and secondary windings are in star, there will be third harmonic induced voltages, particularly if the magnetic circuits are separated. In order to eliminate them a third winding which is delta connected is placed on the same core known as tertiary winding.

03․ For successful parallel operation of two single phase transformers, the most essential condition is that their
percentage impedances are equal
polarities are properly connected
turn- ratios are exactly equal
KVA ratings are equal

The polarities should be exactly matched otherwise it is equivalent to a dead short circuit where huge current flows and damage the transformer. This is the essential condition. The other desirable conditions are 1. The voltage ratio should be same 2. X/R ratio should be same 3. The phase sequence should be same 4. Per unit impedance should be same.

04․ A delta/star transformer is connected in parallel to a star/delta transformer. The turn ratio former is x times latter, the x is

05․ Two transformers operating in parallel will share the load depending on their
leakage reactance
per unit impedance

In order to have perfect load sharing proportional to their KVA ratings the leakage impedance should be inversely proportional to the KVA ratings or the per unit impedance of the transformers in parallel should be same. If the per unit impedance are not same then the transformer which have smaller pu will share greater amount of load irrespective of their KVA ratings.

06․ A 100/10, 50 VA double winding transformer is converted to 100/110 V auto transformer. The rating of auto transformer is
550 VA
500 VA
110 VA
100 VA

07․ Tappings of a transformer are provided
at the phase end of lv side
at the phase end of hv side
at the neutral side end of hv side
at the middle of hv side

Tappings are provided in order to maintain the terminal voltage across the loads within the permissible specified limits with respect to load variations. Generally tappings are preferred on HV side, because the HV side is low current side, tap changer involves low current interruption and large variations or small steps are possible due to more no of turns. Tappings can be provided at phase end or neutral end. But to maintain magnetic symmetry tappings are preferred in the middle of the winding only.

08․ In an auto-transformer, power is transferred through
conduction process only.
induction process only.
both A and B.
none of the above.

In two winding transformer there is no electrical connection between primary and secondary. So the power is only transferred through induction or magnetically. But in auto-transformer there is a common electrical path between primary and secondary. So power is transferred through both conduction and induction process.

09․ It is an advisable to use auto transformer if the transformation ratio is
Greater than 1.
near to 1.

Auto transformer is mainly used for copper saving. Copper saving is maximum when the transformation ratio is near to unity.

10․ In auto transformer if the turn ratio is K, what is the power transferred inductively?
K × input power
(1-K) × input power
both A and B
none of the above

In auto transformer the power is transferred both conductively and inductively. If turn ratio K is near to 1, maximum power is transferred conductively and losses are less, efficiency is more.