Transformer Interview Questions Part 1

Q. What is a Transformer?

A. A transformer is a static electrical apparatus designed to convert alternating current from one voltage to another .It transforms electrical energy from one circuit to another without any direct electrical connection, with the help of mutual induction between to windings. It can be designed to “step up” or “step down” voltages.

Q. Explain the working principle of Transformer?

A. Working principle of Transformer is based on Faraday’s Laws of Electromagnetic Induction. If an alternating current is applied to an electric coil, there will be an alternating flux surrounding that coil. Now if we bring another coil near the first one, there will be an alternating flux linkage with that second coil. As the flux is alternating, there will be obviously a rate of change of flux linkage with respect to time in the second coil. Therefore emf will be induced in it as per Faraday’s laws electromagnetic induction.

Q. How do you classify transformers based on construction?

A. Based on construction transformers are of two types. They are distinguished from each other based on merely the manner by which primary and secondary windings are placed around the core.
1. Core type: In these windings surround considerable part of the core.
2. Shell type: In these core surrounds considerable part of windings.
Another recent development is wound core type or spiral core.

Q. What are taps and when are they used?

A. Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. The standard ASA and NEMA designation for taps are “ANFC” (above normal full capacity) and “BNFC” (below normal full capacity).

Q. In a Tap changing transformer on which side is the tap connected, primary side or secondary side?

A. Tapings are always connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Q. Explain about the losses in Transformer?

A.  As the Transformer is a static device mechanical losses do not come into picture. Transformer losses have two sources-copper loss and core loss.

Copper losses are caused by the resistance of the wire (I2R). In primary side it is I12R1 and in secondary side it is I22R2 loss, where I1 & I2 are primary & secondary currents of transformer and R1 & R2 are resistances of primary & secondary winding. As the both primary & secondary currents depend upon load of transformer, copper losses vary with load.

Core losses are caused by eddy currents and hysteresis in the core.

Eddy current losses: In transformer we supply alternating current in the primary, this alternating current produces alternating magnetizing flux in the core and as this flux links with secondary winding there will be induced voltage in secondary, resulting current to flow through the load connected with it. Some of the alternating fluxes of transformer may also link with other conducting parts like steel core or iron body of transformer etc. As alternating flux links with these parts of transformer, there would be an locally induced emf. Due to these emfs there would be currents which will circulate locally at that parts of the transformer. This type of energy loss is called eddy current loss of transformer.

Hysteresis losses: The magnetic core of transformer is made of Silicon Steel, Steel is very good ferromagnetic material. The domains are arranged inside the material structure in such a  manner, that net resultant magnetic field of the said material is zero. Whenever external magnetic field or mmf is is applied to that substance, these randomly directed domains arrange themselves in parallel to the axis of applied mmf. After removing this external mmf, maximum numbers of domains again come to random positions, but some few of them still remain in their changed position. Because of these unchanged domains the substance becomes slightly magnetized permanently. This magnetism is called “Spontaneous Magnetism”. To neutralize this magnetism some opposite mmf is required to be applied. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss of transformer.

We = Kef2Kf2Bm2     watts
Wh = KhfBm1.6     watts
Where, Kh = Hysteresis Constant.
Ke = Eddy Current Constant.
Kf = form Constant.

Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.