Transformers Questions and Answers

Answer: B

With a copper core, the Eddy current loss will be more.

At a given supply voltage and frequency the core flux is fixed by \(V = 4.44fN\Phi\). Eddy current loss is inversely proportional to the resistivity of the core material, and copper's resistivity is many times lower than that of silicon steel, so very large eddy currents would circulate.

Copper is not ferromagnetic, so it has no hysteresis loss to speak of, and the magnetizing current would be enormous. Copper loss in the windings would rise with that larger current, not fall, so 'Copper loss will be less' is wrong as well.

Answer: B

The result is that Copper loss in the winding will be more.

Steel has a much higher resistivity than copper, so windings of steel wire would have much higher resistance and the \(I^{2}R\) loss in them would be larger for the same current.

The other statements are false. A copper core is non-magnetic, so the magnetizing current would rise enormously, not fall. Copper is a much better conductor than silicon steel, so eddy current loss in the core would increase. With those two wrong, 'All of the above' is wrong too.

Answer: D

The loss that is zero even at full load is Friction loss.

A transformer is a static machine with no rotating or moving parts, so it has no friction or windage loss at any load. That is one reason its efficiency is higher than that of rotating machines.

Eddy current loss and hysteresis loss together form the core loss, and all three occur whenever the transformer is energised, at no load and at full load alike.

Answer: B

A tap changer is used for Adjustment in secondary voltage.

Taps change the number of turns in use, and so the turns ratio. The purpose is to keep the secondary (output) voltage at its desired value as the supply voltage or the load changes. Taps are usually placed on the high-voltage winding, where the current is smaller, but the quantity being regulated is the secondary voltage.

The primary voltage is fixed by the supply and cannot be adjusted by the transformer. Changing turns does not change the power factor, which is set by the load.

Answer: D

Over currents affect All of the above.

Copper loss rises with the square of the current, so an over current raises the temperature of the windings and oil. Sustained high temperature degrades the paper and oil insulation and shortens insulation life. Large fault currents also produce electromagnetic forces between the windings, proportional to the square of the current, which cause mechanical stress and can deform them.

Each single option names a real effect, so none alone is complete.

Answer: D

A current transformer can be used with Any of the above.

A current transformer steps a large line current down to a standard value, usually 5 A or 1 A, and isolates the instrument from the high-voltage circuit. Any instrument that has a current coil can be fed from it: an ammeter, the current coil of a wattmeter, and the current coil of a watt hour (energy) meter.

Each of the first three options is a valid use, so naming only one of them is incomplete.

Answer: D

The item that is not a transformer fitting is the Commutator.

A commutator is the segmented rotating switch on a DC machine that reverses coil connections as the armature turns. A transformer has no moving parts and no commutator.

The others are standard fittings on oil-filled transformers. The breather dries the air drawn in as the oil contracts, using silica gel. The conservator is the expansion tank above the main tank. The Buchholz relay sits in the pipe between them and protects against internal faults.

Answer: C

The reactance of the transformer depends on Leakage flux.

The leakage reactance of each winding represents the flux it produces that does not link the other winding. The more leakage flux per ampere, the larger the reactance, so it depends on the winding arrangement and spacing that govern leakage.

The size of the core affects the magnetizing reactance, not the series (leakage) reactance quoted as the transformer's reactance, and the size of the tank has no direct bearing on it. So 'All of the above' is also wrong.

Answer: D

The transformer is the Autotransformer.

An autotransformer has a single continuous winding with a tapping. Part of the winding is common to both circuits, so it serves as part of the primary and as the secondary at the same time; energy is transferred partly by conduction and partly by induction.

Step up transformers, current transformers and potential transformers normally have two separate windings, electrically isolated from each other and linked only magnetically.

Answer: D

Maximum efficiency is obtained when copper loss becomes equal to core loss.

Efficiency is output divided by output plus losses. Core loss is constant at all loads, while copper loss varies as the square of the load. Differentiating the efficiency with respect to load shows that it is greatest at the load where the variable copper loss equals the constant core loss.

Core loss does not change with load, so it never becomes minimum or negligible as the load varies. Copper loss is least at no load, where the efficiency is zero.