Questions and Answers on Single Phase Transformers


Questions and Answers on Single Phase Transformers

  1. What is a transformer?

Ans: – A transformer is a static device which transfers electric power from one circuit to another circuit at same frequency but voltage level is usually changed.

  1. What are step-up and step-down transformers?

Ans: – When the voltage level is raised on the output side, the transformer is called step-up transformer, whereas, the transformer in which the voltages is lowered on the output side, is called a step-down transformer.

  1. Which are the two winding are present in a transformer?

Ans: – Primary and secondary windings

  1. Transformer is called static electrical machine. Why?

Ans: – There is no rotating part in the transformer, therefore, it is called static electrical machine.

  1. What are the functions of transformer oil?

Ans: – In provides additional insulation and protects the insulation from dirt and moisture and it carries away the heat generated in the cores and coils.

  1. Why an iron or steel core is provided in a transformer?

Ans: – To ensure a high permeability of the magnetic circuit. Because of high permeability, the magnitude of exciting current necessary to create the required flux in the core is small. The presence of steel core also causes hundred per cent of magnetic flux.

  1. What is the application of a isolation transformer?

Ans: – An isolation transformer has equal turns in primary and secondary windings. These transformers are employed for isolating the load from supply.

  1. What is the order of magnitude of no-load current?

Ans: – No-load current is transformers ranges from 2 to 5 per cent of full-load primary current.

  1. How is magnetic leakage reduced to a minimum in commercial transformers?

Ans: – By interleaving the primary and secondary windings.

  1. Define voltage transformation ratio.

Ans: –The ratio of secondary voltage to primary voltage is called a transformation ratio. It is denoted by capital letter K.

  1. From the construction point of view, name different types of transformers?

Ans: – According construction point of view, the transformers are divided into two types: (i) Core type and  (ii) Shell type.

  1. In practice, what determines the thickness of the stampings?

Ans: – Frequency.

  1. Why the core of a transformer is laminated?

Ans : – To reduce eddy current loss, the core of the transformer is laminated.

  1. What are the functions of transformer oil?

Ans: – Transformer oil’s primary functions are to insulate and cool a transformer.

  1. What is the cause of noise in a transformer?

Ans: – In a transformer, noise occurs mainly due to loosening of stampings and mechanical forces developed during operation.

  1. Which test gives the copper loss of a transformer ?

Ans: – Short circuit test.

  1. Tap changers are usually employed on H.V. side of a transformer, why?

Ans: – Tap changers are usually employed on H.V. side because, the high voltage side has a large number of turns which allows smoother variation of voltage. It is easily accessible physically and above all, it has to handle low currents.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 5Hz source of the same voltage?

Ans: – The power transformers are designed to operate at 50 Hz. On the other hand, if the primary is connected to a source of 5 Hz frequency, the primary winding will have an insufficient inductive reactance (XL = 2πfL). The result will be that, the primary will have excessive current producing considerable copper losses. There is every possibility that the transformer may start to smoke.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 500 Hz source of the same voltage?

Ans: – The power transformers are designed to operate at a particular frequency, generally at 50 Hz. If the frequency of the supply will be high, it will result greater inductive reactance. This high inductive reactance of the winding will prevent the primary from drawing sufficient power. Moreover, the iron losses i.e. hysteresis and eddy current losses will be excessive.

  1. Silica gel placed in the transformer breather. Why?

Ans: – Silica gel is placed in the transformer breather. The function of the silica gel is to absorb the moisture so that the life of the transformer can be increased.

  1. What is the thickness of laminations?

Ans: – 0.35mm to 0.5mm

  1. Why silicon content in electrical sheet steel is limited to 4.5% to 5%?

Ans: – Silicon content exceeding 5%   makes the sheet steel brittle and so causes in punching.

  1. Why LV winding is placed first on the core and then HV winding in the case of a core type transformer.

Ans: – Placing of LV winding near the core and HV winding around the LV winding in a core type transformer reduces the amount of insulation material required.

  1. Why circular coils are always preferred over rectangular coils for winding in a transformer?

Ans: – Circular coils are preferred for winding a transformer as they can easily wound on machines, conductors can easily be bent and winding does not bulge out due to radial forces developing during operation.

  1. Does the transformer draw any current when its secondary is open?

Ans: – Yes, no load primary current.

  1. Why are iron losses constant at all loads in a transformer?

Ans: – Since the induced primary ampere-turns and secondary ampere-turns always neutralize one another, the flux in the core on load is the same as the flux on no-load. Hence, the iron losses are constant and are independent of load.

  1. Where is shell-type construction suitable for a transformer?

Ans: –In shell type core, both the windings are wound on the central limb. This type of core is used for those transformers which work on poor power factor.

  1. What are the advantages of open and short-circuit tests on a transformer?

Ans: – The efficiency of a transformer is always determined by open-circuit and short-circuit tests due to the following reasons: (i) The power required to carry out these tests is very small. (ii) The tests give the core loss and copper loss separately.

  1. What are the types of bushing used in transformer terminals?

Ans: – (i) Porcelein insulator bushing (ii) Oil filled bushing (iii)  Capacitor bushing

  1. Why is it advantageous to make short circuit test on the high voltage side rather than on low voltage side?

Ans: –  It is due to the fact that the ranges of the meter at the high voltage (HV) side at the short circuit condition of the low voltage (LV) side is much more suitable.s

  1. What is tapped transformer?

Ans: – A tapped transformer is one whose windings are fitted with special taps for changing in voltage or current ratio.

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