1. In a given transformer for a given applied voltage, which losses remain constant irrespective of load changes?
a) Hysteresis and eddy current losses
b) Friction and windage losses
c) Copper losses
d) Cannot be determined
Explanation: Hysteresis and eddy current losses are voltage and frequency dependent losses that too from primary side thus, load change will not make any effect on these losses and they will remain constant as long as voltage and frequency is constant.
2. Which of the following loss in a transformer is zero even at full load?
a) Core loss
b) Friction loss
c) Eddy current loss
d) Hysteresis loss
Explanation:Friction losses are involved with rotating parts of a machine. Since in a transformer all parts are stationary, friction losses will always be equal to zero, irrespective of the loading condition
3. A shell-type transformer has __________
a) High eddy current losses
b) Reduced magnetic leakage
c) Negligible hysteresis losses
d) Cannot be determined
Explanation: Since windings are brought closer in shell type compare to core type transformer, leakage of flux is very less in shell type transformer. Most of the flux gets linked with both of the coils though there is some leakage which can’t be avoided
4. During open circuit test (OC) of a transformer _____________
a) primary is supplied rated kVA
b) primary is supplied full-load current
c) primary is supplied current at reduced voltage
d) primary is supplied rated voltage
Explanation:Open circuit test is normally conducted on rated voltage because any machine is constructed to give maximum efficiency near rated value. Hence, it is operated at rated voltage, and we have to perform the test on machine is to be used.
5. Open circuit test on transformers is conducted so as to get ______________
a) Hysteresis losses
b) Copper losses
c) Core losses
d) Eddy current losses
Explanation: Open circuit test gives the core losses also called as iron losses and shunt parameters of the equivalent circuit of transformer. Open circuit test and short circuit test both provide all the parameters of equivalent circuit
6. Why OC test is performed on LV side?
a) Simple construction
b) Less voltage is required and parameters can be transformed to HV side
c) It’ll not give losses ig conducted on HV side
d) HV side does not have connections for voltage
Explanation: Open circuit test can be performed on any side but for our convenience and supply voltage available we generally conduct the test on LV side, to get corresponding parameters on HV side we can use transformation ratio
7.In OC test all the power supplied is utilised for ______
a) Core losses
b) Iron losses
c) Windage losses
d) Cannot be determined
Explanation: In open circuit test all the power supplied is used to overcome iron losses and hence, by taking the reading of input power one can easily do the calculations to find shunt parameters of equivalent circuit of transformer.
8. How shunt branch component Gi is calculated?
a) Po/v12
b) V1/Io
c) Io/ V2
d) Any of the above
Explanation: Shunt branch resistance inverse is denoted by Gi. This Gi can be calculated by the power drop taking place in the resistance divided by square of the voltage applied across the resistor. Current by voltage will give net admittance.
9. Which of the following statements is/are correct statements?
a) EMF per turn in LV winding is more than EMF per turn in LV winding
b) EMF per turn in LV winding is less than EMF per turn in LV winding
c) EMF per turn in HV winding is equal to EMF per turn in LV winding
d) Can’t comment
Explanation: In a transformer, primary volt-ampere is equal to secondary volt-ampere and primary ampere turns are also equal to secondary ampere turns So, EMF per turn in both the winding are equal. Total induced emf on both sides depends on the number of turns, flux and frequency.
10. If the applied voltage of a transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will _____________
a) Changes to three times the original value
b) Changes to 1.5 times the original value
c) Changes to 0.5 times the original value
d) Remains the same as the original value
Explanation: Magnetic flux density α β/A. Magnetic flux φ α V/f. φ2/ φ1 = V2/V1 * f1/f2. Since voltage is increased by 50%, V2 thus becomes 1.5 times V1 and frequency becomes 0.5 times the original frequency. Thus, maximum flux density changes to 3 times the original value