Electrical Transformer MCQ FOr GETCO JE Exam Part-02



1. The transformer oil should have _____ Volatility and _____ Viscosity.

(A) Low & High
(B) High & High
(C) Low & Low
(D) High & Low

Ans (C) Low & Low

 

Explanation:

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties.

Volatility is quantified by the tendency of a substance to vaporize. Volatility is directly related to a substance’s vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with lower vapor pressure.

Therefore the transformer should have low volatility.

Viscosity is a measure of a fluid’s resistance to flow.

Low viscosity substance moves quickly.

Therefore transformer Oil having low viscosity i.e greater fluidity will cool transformers at a much better rate.

 

2. During the open circuit test of a transformer

(A) Primary is supplied rated voltage
(B) Primary is supplied current at reduce the voltage
(C). Primary is supplied rated KVA
(D) Primary is supplied full load current

Ans: (A) Primary is supplied rated voltage

 

Explanation: 

An open circuit test is performed to determine the iron loss in the transformer.

In this method, the secondary of the transformer is left open-circuited.

A wattmeter is connected to the primary.

An ammeter is connected in series with the primary winding. A voltmeter is optional since the applied voltage is the same as the voltmeter reading.

Rated voltage is applied at primary.

If the applied voltage is normal voltage then normal flux will be set up. Since iron loss is a function of applied voltage, the normal iron loss will occur. Hence the iron loss is maximum at rated voltage. This maximum iron loss is measured using the wattmeter.

 

3. Which type of winding is used in a 3-phase shell type transformer?

(A) Rectangular Type
(B) Cylindrical Type
(C) Sandwich Type
(D) Circular type

Ans: (C) Sandwich Type

 

Explanation:

Sandwich Type Winding

LV winding is placed close to the core which is at ground potential.

HV section lies between two LV sections.

Sandwich winding provide control over the short circuit impedance of the transformer.

In sandwich coils, leakage can be controlled easily by bringing HV and LV coils close on the same magnetic axis.

Reactance can be reduced by increasing the number of sandwich coil hence mutual flux is increased. 

 

4. The transformer laminations are insulated from each other by

 

(A) Mica strip
(B) Paper
(C) Thin coating of Varnish
(D) Any of the above

Ans: (C)  Thin coating of Varnish

 

Explanation: The purpose of providing a coating of varnish in windings are

To keep moisture out of the windings to preserve insulation.

To protect the windings from humming or vibrating when magnetized.

To increase the electrical insulation and dielectric strength.

Provide heat dissipation from coil.

 

 

5. Two transformers are connected in parallel. These transformers have different percentage impedance. It is likely to result in

(A) Loading in the transformer is not proportional to their KVA rating.
(B) Short circuit in secondary
(C) Higher copper loss
(D) Power factor of one of the transformer is leading while that of the other is lagging.

Ans: (A) Loading in the transformer is not proportional to their KVA rating.

 

Explanation:

If percentage impedance is unequal means one of the transformers has high impedance as compared to other transformers.

In this case, the transformer having high impedance will have a smaller amount of current flowing through the windings.

While Low impedance transformer offers a large amount of current to flow in the windings.

Therefore high percentage information will be lightly loaded while subjected to heavy load.

Whereas low percentage impedance will be overloaded for the same load.

 

6. Which of the following is the main advantage of autotransformer over a two winding transformer?

 

(A) Reduces hysteresis losses
(B) Reduce eddy current losses
(C) Copper losses are negligible
(D) Saving of copper material

Ans: (D) Saving of copper material 

 

Explanation: 

An autotransformer is built with only a single conductor, which serves both as the primary and secondary coil. Thus, it saves the cost of extra wire for the secondary winding.

  

7. Which winding of the transformer has less cross-section area?

 

(A) Primary winding
(B) Secondary Winding
(C) High Voltage Winding
(D) Low Voltage Winding

Ans: (C) High Voltage winding

 

Explanation:

We know that current density is defined as the ratio of current to perpendicular cross-section area through which current is crossing.

Value of current density of HV winding is more in comparison to LV winding because of better cooling of HV winding

Since HV winding is placed far from the core in Comparison to LV winding which is placed near to the core.

Current density is inversely proportional to the area of the core so the thin wire is used for HV winding.

Therefore HV winding has Low cross-sectional area.

 

8. The core used in the high-frequency transformer is usually

 

(A) Copper Core
(B) Iron Core
(C) Mild Steel Core
(D) Air core

Ans: (D) Air Core

 

Explanation: 

An air-core transformer is designed to be used at a high frequency, used in radio circuits.

The currents are usually small but the voltages can vary.

They can be used to change voltages, match one stage to other,  for matching antennas to the radio circuit.

They have many uses within a radio.

 

 

9. Harmonics in transformer result in

(A) Increase core losses

(B) Increases I2R Losses

(C) Interference with communication circuits

(D) All of the above

Ans: (D) All of the Above

 

Explanation: We know that

 Inductive reactance XL = ωL = 2Ï€fL.

So as the frequency changes it also changes XL ( Inductive Reactance).

And I= V/XL 

So if XL changes current also gets affected.

Whenever high current flows and if the frequency is higher than normal than the transformer gets Overheated.

Therefore Eddy’s current and Hysteresis Loss will increase in the transformer.

 





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