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 = 2πfL.
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.