Transformers Questions and Answers

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: maximum efficiency occurs where copper loss equals iron loss, and iron loss is nearly constant because the flux is fixed by the applied voltage. Statement 3 is wrong - copper loss varies as the square of the load current, which is precisely the variation that makes the efficiency curve peak where it does.

Answer: D

The three work together on the oil system, and each has one clear job. A transformer without a breather would draw moist air straight into its oil, and one without a conservator would need room for expansion inside the tank itself.

Answer: A

Correct answer: 1-Q, 2-P, 3-R

Each connection has its characteristic property. Delta-star introduces the 30 degree phase shift between line voltages, which is why vector groups must match when transformers are paralleled. Delta-delta gives no shift and can continue in open delta on two units if one fails. Star-star without a tertiary has no path for triple harmonic currents, so its phase voltages are distorted.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. The primary current of a current transformer is set by the line, not by the transformer, so it continues regardless. With the secondary open there are no opposing ampere-turns, the whole primary magnetomotive force drives the core deep into saturation, and the flux changes so abruptly at each zero crossing that a dangerous voltage appears at the open terminals.

Answer: C

Thirty degrees between two nominally equal voltages leaves a substantial difference across the paralleled terminals, limited only by the winding impedances. The reason is exactly why the arrangement is a fault rather than a compromise.

Answer: D

Correct answer: False, since both windings link the same alternating flux

The frequency of the secondary emf is the frequency at which the common flux alternates, which is the frequency of the supply. A transformer changes voltage, current and apparent impedance, but nothing in it can change the rate at which the flux reverses. Changing frequency requires a rotating machine or a power electronic converter.

Answer: A

Ohmic heating in the windings and hysteresis and eddy currents in the core both dissipate power, which is why a large transformer needs radiators and sometimes fans. The absence of moving parts is what makes its efficiency so high, not perfect.

Answer: B

Correct answer: above 99 per cent

Large power transformers are among the most efficient machines built, commonly reaching 99.5 per cent or better at rated load, because they have no moving parts and hence no friction, windage or rotor loss. Even so, half a per cent of a 500 MVA unit is 2.5 MW of heat, which is why the cooling plant is substantial.

Answer: C

The core is a simple rectangle and the coils encircle its vertical limbs, which makes the winding accessible and the insulation straightforward. A shell type wraps the core around the coils instead and gives better mechanical bracing against short-circuit forces.

Answer: D

Correct answer: surrounding the windings on more than one side

In a shell type transformer the windings sit on a central limb and the core encloses them on both sides, so the magnetic circuit surrounds the coils. In a core type the coils surround the limbs instead. The shell arrangement gives better mechanical bracing against short circuit forces and a shorter mean flux path.