1. In a given transformer for given applied voltage, which of the following losses remain constant irrespective of load changes?
a) Friction and windage losses
b) Copper losses
c) Hysteresis and eddy current losses
d) Cannot be determined
Explanation: Hysteresis and eddy current losses together called as core-loss in a transformer. These losses remain constant for constant voltage and frequency applied to a transformer, these components remain same irrespective of load.
2. On which of the following degree of mechanical vibrations produced by the laminations of a transformer depends?
a) Tightness of clamping
b) Gauge of laminations
c) Size of laminations
d) Tightness of clamping, gauge and size of laminations
Explanation: Mechanical vibrations produced in a transformer are directly effective due to the tightness of the clamping, gauge og laminations, size of laminations as well. There are various methods in order to reduce their effects.
3. Variations in a hysteresis loss in a transformer (Bmax = maximum flux density) ____________
a) Bmax
b) Bmax1.6
c) Bmax3.83
d) Bmax/2
Explanation: According to Steinmetz’s formula, the heat energy dissipated due to hysteresis is given by Wh=ηβmax1.6, and hysteresis loss is thus given by Ph≈ Whf ≈ηfβmax1.6. That exponetital term varies fraom 1.4 -1.8 and is equal to 1.6 for iron.
4. Leakage flux in the transformer depends on _____________________
a) Load current
b) Load current and voltage
c) Load current, voltage and frequency
d) Load current, voltage, frequency and power factor
Explanation: Leakage flux is directly proportional to the current, as if is current increased net value of flux increases thus, flux leakage also increases which further contribute to the losses as it is then not able to link with secondary windings
5. The full-load copper loss of a transformer is 1600 W. At half-load, the copper loss will be _______
a) 6400 W
b) 1600 W
c) 800 W
d) 400 W
Explanation: Copper losses are defined as I2*R losses many times, as they are directly proportional to the square of current flowing through them. Thus, copper losses will reduce if load is reduced that too in square proportion.
6. Silicon steel used in laminations, because it reduces ________________
a) Hysteresis loss
b) Eddy current losses
c) Copper losses
d) Cannot be determined
Explanation: Electrical steels are also known as lamination steel or silicon steel. The main special thing related to the silicon steel is, its magnetic properties such as small hysteresis area and hence, small energy dissipation per cycle, thus low core loss.
7. If the supply frequency to the transformer is increased, the iron loss will ___________
a) Not change
b) Decrease
c) Increase
d) Cannot be determined
Explanation: As frequency increases, the flux density in the core decreases but as the iron loss is directly proportional to the frequency hence effect of increased frequency will be reflected in increase of the iron losses
8. Which of the following can measure iron loss of a transformer?
a) Low power factor wattmeter
b) Unity power factor wattmeter
c) Frequency meter
d) Any type of wattmeter
Explanation: As the secondary side is open in OC, the entire coil will be purely inductive in nature. So, the power will be lagging due to inductive property of the circuit. So LPF (Low power factor) wattmeter is used in open circuit test of transformer
9. How reduction in core losses and increase in permeability can be obtained simultaneously in a transformer?
a) Core built-up of laminations of cold rolled grain oriented steel
b) Core built-up of laminations of hot rolled sheet
c) Cannot be determined
d) Frequency Meter
Explanation: CRGO is supplied by the producing mills in coil form and has to be cut into laminations, which are then used in transformer core, which is an integral part of any transformer. Grain-oriented steel is used in large power, distribution transformers and in certain audio output transformers also
10. Losses which occur in rotating electric machines and do not occur in transformer are ______
a) Friction and windage losses
b) Magnetic losses
c) Hysteresis and eddy current losses
d) Copper losses
Explanation: Windage and friction losses occur in rotating parts of a machine generally in rotor of the machine, thus they will never occur in transformer, as transformer does not contain any rotating part at its secondary unlike induction motor