1. Which test is sufficient for efficiency of two identical transformers under load conditions?
a) Short-circuit test
b) Back-to-back test
c) Open circuit test
d) Any of the above
Explanation: Open circuit test and short circuit test collectively gives the value of all parameters of an equivalent circuit of a transformer. While Sumpner’s back-to-back test gives the heat run effect of machine by considering rise in temperature.
2. In Sumpner’s test _____________
a) Two secondaries are connected in phase opposition
b) Two secondaries are connected in phase addition
c) Can be connected in either way
d) Never connected with each other
Explanation: In conducting Sumpner’s test two primaries are connected in parallel to the rated voltage supply and secondaries are connected in phase opposition. For the secondaries to be in phase opposition rated secondary voltage across the terminals to be zero.
3. When secondaries are connected in phase opposition, power drawn by the circuit is equal to ___________
a) 2*Pi
b) Pi2
c) Pi
d) 2*Pc
Explanation: If V2 source is assumed shorted, the two transformers appear in open circuit to source V1 as their secondaries are in phase opposition and therefore no current can flow in them. The current drawn from source V1 is thus 2I0 and power is 2P0 = 2Pi, twice the core-loss of each transformer.
4. When the AC supply at the primary side of a transformer are shorted, power drawn by the circuit is equal to ___________
a) 2*PC
b) 2*Pi
c) 2*PC + 2*Pi
d) Can’t be determined
Explanation: When the ac supply (10) terminals are shorted, the transformers are series-connected across V2 supply and are short-circuited on the side of primaries. Therefore, the impedance seen at V2 is 2Z and when V2 is adjusted to circulate full-load current (Ifl), the power fed in is 2PC (twice the full-load copper-loss of each transformer).
5. Total power required for Sumpner’s test is given by ________
a) PC + Pi
b) PC + 2Pi
c) 2PC + Pi
d) 2(PC + Pi)
Explanation: In the Sumpner’s test while the transformers are not supplying any load, full iron-loss occurs in their cores and full copper-loss occurs in their windings; net power input to the transformers being (2P0 + 2Pc). The heat run test could, therefore, be conducted on the two transformers, while only losses are supplied.
6. When will be the efficiency of a transformer maximum?
a) Copper losses = hysteresis losses
b) Hysteresis losses = eddy current losses
c) Eddy current losses = copper losses
d) Copper losses = iron losses
Explanation: When the variable copper losses of a transformer becomes equal to the fixed iron losses of a transformer then we will get maximum efficiency. From these losses we’ll get the value of current required.
7. Efficiency of a power transformer is near to the ___________
a) 100 per cent
b) 98 per cent
c) 50 per cent
d) 25 per cent
Explanation: The efficiency of the transformer obtained from various experiments conducted on various loads showed the efficiency greater than 90% always. Transformer thus, can be said highly efficient device.
8. On which factors transformer routine efficiency depends upon?
a) Supply frequency
b) Load current
c) Power factor of load
d) Load current and power factor of load
Explanation: Efficiency of the transformer can be calculated by the output power divided by input power. Both of these powers include power factor in their calculations while load current and load voltage is also required in calculations.
9. Normal transformers are designed to have maximum efficiency at ___________
a) Nearly full load
b) 70% full load
c) 50% full load
d) No load
Explanation: Every device is manufactured to get maximum efficiency at the rated loads, i.e. full load. Thus, transformer will give the maximum efficiency at nearly full load. Internal losses are so adjusted to get maximum efficiency.
10.At which load condition maximum efficiency of a distribution transformer will be achieved?
a) At no load
b) At 60% full load
c) At 80% full load
d) At full load
Explanation: The main difference between power transformer and distribution transformer is distribution transformer is designed for maximum efficiency at 60% to 70% load as these transformers normally doesn’t operate at full load all the time.