Transformers Questions and Answers

Answer: C

The transformation ratio of a transformer is the secondary turns divided by the primary turns, K = N2 / N1.

\[ K = \frac{80}{800} = 0.1 \]

So the ratio is 0.1, option (C), which means that the transformer steps the voltage down to a tenth. The value 100 would come from taking a wrong figure, 32 and 3.2 do not follow from the turns, and the rating of 25 kVA is not needed to find the turns ratio. The same ratio gives the voltages: a primary voltage of 1500 V would give 150 V on the secondary, and the currents would be in the inverse ratio, ten times larger on the secondary side.

Answer: D

The shunt branch of a transformer's equivalent circuit contains the core loss resistance and the magnetising reactance, and these parameters are found from the open circuit test, in which the rated voltage is applied to one winding with the other open, so that only the no-load current, which is small, flows. The short circuit test gives the series resistance and leakage reactance. The polarity test and the high voltage oil test do not give circuit parameters. The answer is open circuit test, option (D).

Answer: A

In the short circuit test of a transformer one winding is shorted and a low voltage is applied to the other, raised until the rated full-load current flows. The voltage required is only the voltage drop across the transformer's own impedance, which is a small fraction of the rated voltage, about 5 to 8 percent. Values of 20 to 33, 45 to 60 and 90 to 100 percent would drive a current far above the rated one through the short circuit. The answer is 5-8% of rated voltage, option (A).

Answer: B

A bushing in a transformer is the insulated passage by which a winding terminal comes out through the grounded metal tank. Its purpose is to provide electrical isolation between the winding terminals and the grounded tank, so the high voltage does not flash over to the tank. It does not isolate the winding from the core inside the tank, which is the job of the insulation, and it is not meant for magnetic isolation. The answer is option (B).

Answer: C

An autotransformer has a single winding, part of which is common to the primary and secondary. Its advantages are better voltage regulation, a smaller size and less copper for the same VA rating than a two winding transformer. Its disadvantage is that there is no electrical isolation between the primary and the secondary circuits, so a fault on the high-voltage side can reach the low-voltage side. So the disadvantage is no isolation between the secondary and primary circuit, option (C).

Answer: A

The number of turns per volt is 8, so the primary of 110 V needs 110 x 8 = 880 turns and the secondary of 25 V needs 25 x 8 = 200 turns. So the primary has 880 turns and the secondary has 200 turns, option (A). The options with 440 and 100 turns correspond to 4 turns per volt, and those with 100 and 440 or 200 and 880 put the numbers the wrong way round, as the primary has the higher voltage and needs more turns. The turns per volt is the same for the primary and secondary of an ideal transformer, which is why the number of turns is in proportion to the voltage.

Answer: A

The primary current of a single-phase transformer at full load is the rating in volt-amperes divided by the primary voltage.

\[ I_{1} = \frac{25000}{1500} = 16.66\ \mathrm{A} \]

So the primary current drawn is 16.66 A, option (A). The turns of 250 and 40 give a secondary voltage of 240 V, and the secondary current is 25000 / 240, about 104 A, which is not asked. The value 32.33 A is double the correct one, 10 A would need 2.5 kV and 0.06 A is a reciprocal.

Answer: D

Statement 1 is not true: the core of a transformer is made of a material of high permeability, such as silicon steel, so that the flux path has a low reluctance. Statement 2 is true, since in a shell type transformer both windings are wound on the central limb. Statement 3 is true, because the butt joints of the core are staggered so that there is no continuous air gap. Statement 4 is not true: the core of a 50 Hz transformer is laminated, with laminations less than 1 mm thick, not 5 mm. So statements 1 and 4 are not true, option (D).

Answer: A

The key gives zero as the power factor of an ideal transformer. An ideal transformer has no losses, and on no load it draws only the magnetising current, which lags the voltage by 90°, so its power factor on no load is cos 90° = 0 and no power is drawn.

Unity and the values 0.5 and 0.9 would imply real power drawn, and an ideal transformer takes none on no load. When loaded, the power factor is set by the load. The question is brief about this, so the answer follows the key. The first option is the one given.

Answer: D

The statement that is wrong for an ideal transformer is that the permeability of the core is zero. An ideal transformer has a core of infinite permeability, so that no magnetising current is needed and all the flux links both windings.

The other statements are right for an ideal transformer: the windings have no ohmic resistance, there is no magnetic leakage, and the efficiency is 100 %. So the fourth option is the answer.