1. Power transformers other than distribution transformers are generally designed to have maximum efficiency around ______
a) No-load
b) Half-load
c) Near full-load
d) 10% overload
Explanation: Similar to normal transformers power transformers are also designed to get maximum efficiency at load which is near to the full load of a transformer specified. Only in the case distribution transformer maximum efficiency is achieved at 60% of full load
2. For a transformer given, operating at constant load current, maximum efficiency will occur at ______
a) 0.8 leading power factor
b) 0.8 lagging power factor
c) Zero power factor
d) Unity power factor
Explanation: Maximum efficiency for a transformer will be achieved at full load. While in the case of power factor also every device is set to get maximum efficiency at unity power factor. Thus, one will have maximum efficiency if load is nearly equal to full load and at unity power factor.
3. Why efficiency of a transformer, under heavy loads, is comparatively low?
a) Copper loss becomes high in proportion to the output
b) Iron loss is increased considerably
c) Voltage drop both in primary and secondary becomes large
d) Secondary output is much less as compared to primary input
Explanation: At heavy loads current drawn by the transformer circuit increases, as we know, variable copper losses are proportional to the square of the current. Thus, we’ll get higher copper loss in proportion to the output.
4. The efficiencies of transformers compared to electric motors of the same power are ___________
a) About the same
b) Much smaller
c) Much higher
d) Can’t comment
Explanation: Transformer is a highly efficient device compare to all other electrical instruments. In motor we need to add windage and friction losses along with the copper losses and iron losses thus, we’ll get lee efficiency for motor compare to transformer
5. A transformer having maximum efficiency at 75% full load will have ratio of iron loss and full load copper loss equal to ___________
a) 4/3
b) 3/4
c) 9/16
d) 16/9
Explanation: Condition for maximum efficiency is, Copper loss= Iron loss, i.e. Pc= I2 R = Pi. transformer can be operated at any load but maximum efficiency occurs at a particular load condition only. Let x be that load factor corresponds to maximum efficiency. Given that, maximum efficiency will occur at 3/4 load. The load factor= (3/4)2.
6. What is the correct formula of efficiency of a device?
a) Input /output
b) Output/losses
c) 1- (losses/ (output + losses))
d) Cannot be determined
Explanation: Efficiency of any device is equal to the ratio of output power to the input power. Here, one can write input power is equal to the addition of output power with losses. Thus, expressing all these terms mathematically will give the answer
7. A 500 kVA transformer is having efficiency of 95% at full load and also at 60% of full load; both at unity power factor. Then Pi is ___________
a) 16.45 kW
b) 9.87 kW
c) 14.57 kW
d) Can’t be calculated
Explanation: Efficiency of a transformer is given by, [transformer capacity*loading/ (capacity*loading + Pi + k2*PC)]. Thus, η= 500*1/ (500 + Pi +PC) = 0.95. also from the second condition given η= 500*0.6/ (500*0.6 + Pi +0.6^2*PC) = 0.95. Thus, solving simultaneously we get 9.87 kW
8. A 500 kVA transformer is having efficiency of 95% at full load and also at 60% of full load; both at unity power factor. Then Pc is ___________
a) 16.45 kW
b) 9.87 kW
c) 14.57 kW
d) Can’t be calculated
Explanation: Efficiency of a transformer is given by, [transformer capacity*loading/ (capacity*loading + Pi + k2*PC)]. Thus, η= 500*1/ (500 + Pi +PC) = 0.95. also from the second condition given η= 500*0.6/ (500*0.6 + Pi +0.62*PC) = 0.95. Thus, solving simultaneously we get an answer 16.45 kW.
9. For a power transformer operating at full load it draws voltage and current equal to 200 V and 100 A respectively at 0.8 pf. Iron and copper losses are equal to 120 kW and 300kW. What is the efficiency?
a) 86.44%
b) 96.44%
c) 97.44%
d) 99.12%
Explanation: Power output= VI cosθ= 200*100*0.8 = 16000 W (Independent of lag and lead). While total losses are equal to iron loss+ k2*copper losses =120+ 300= 420 W. Efficiency is equal to 1- 420/(16000+420)= 97.44%.
10. The highest voltage for transmitting electrical power in India is _______
a) 33 kV
b) 66 kV
c) 132 kV
d) 00 kV
Explanation: Transmission voltage in power transfer in India (highest) is 750KV AC and these lines are erected by Power Grid Corporation for interstate connections throughout India. However, work on 800KV is in the progress. DC transmission voltage (highest) in India is 600KV