1. What voltage of On-load tap-changer is required for unit transformer?
A) 11 kV
B) 23 kV
C) 400 kV
D) Not required
Answer: D) Not required
Explanation: Paralleling of unit and station transformers during changeover of station and unit supplies can result in a large circulating current between station and unit switchboard. This generally adds to the unit transformer load current, and subtracts from that of the station transformer.
2. A transformer has comparatively much higher efficiency than a similar induction machine due to _________
A) small air gaps
B) no moving parts
C) strong coupling
D) all of the mentioned
Answer: D) all of the mentioned
Explanation: Transformer does not has any moving components so losses are anyway reduced and also the coupling is very strong between two sides.
3. Which is the most widely used material in the core of the transformer?
A) cold rolled grain oriented sheet steel
B) cold rolled grain steel
C) soft iron
D) steel
Answer: A) cold rolled grain oriented sheet steel
Explanation: CRGO has magnetization in the rolling direction and low core losses and very high permeability than present materials.
4. The leakage flux is the flux in side the transformer which?
A) links either of the windings
B) links both of the windings
C) yoke of the core
D) windows of the core
Answer: A) links either of the windings
Explanation: Leakage flux is meant to to be loss as it does not link two windings.
5. A coupling magnetic field inside a rotating machine or static machine-like transformers must involve with _________
A) mechanical parts
B) electrical parts
C) both electrical and mechanical
D) either of the electrical or mechanical parts
Answer: A) mechanical parts
Explanation: The coupling field should interact with both the electrical as well as mechanical parts in order to achieve electromechanical energy conversion.