
Problem 
Answer 
1 
The efficiency of a singlephase, 10kva, 2200/220volt, 60cycle transformer is to be measured by loading. Show by a diagram the proper locations of all necessary meters, and specify the required ranges of all instruments. 

2 
The singlephase transformer of Problem 1 has a primary resistance of 3.0 ohms, a secondary resistance of 0.03 ohm, and a noload power input of 100 watts. Calculate the energy efficiency in a period of 24 hours on the following load schedule: 4 hours full load at 90% power factor; 2 hours 1/2 rated load at 100% power factor; 18 hours no load. The exciting current may be neglected. 

3 
Show by a diagram a threephase group of singlephase transformers connected YA. Let the transformers be similar, except that one is of additivepolarity and two are of subtractive polarity. 

4 
Specify the voltage ratio for each of a group of three singlephase transformers to be used to "step down" YΔ from a threephase 2200volt line to 220 volts for factory service. 

5 
Specify the voltage ratios and kva capacities of two singlephase transformers to "step down" from 2200 volts to 220 volts, threephase, by opendelta connection to supply a 175kva load. 

6 
An autotransformer is to be used to "step down" a voltage of 220 volts to 120 volts. The load current is to be 50 amp. Neglecting the exciting current, find the current in each part of the transformer. 

7 
A 2300/230volt, 10kva, singlephase transformer has an effective resistance of 2% and a leakage reactance of 1%, Calculate the regulation at full load at 60% power factor leading. 

8 
Calculate the regulation of the transformer of Problem 7 with 80 per cent of full load at 60% power factor lagging. 

9 
A 50kva, 60cycle, 2300/230volt transformer is to be used on a 25cycle line as a temporary expedient. What would be the approximate maximum voltage permissible on the 2300volt winding? 

10 
An anthracite mine has the following 3phase, 60cycle motors: two 100hp induction motors with estimated efficiency of 90% and power factor of 91% lagging; five 50hp induction motors with estimated efficiency of 82% and power factor of 78% lagging; three 100hp synchronous motors with estimated efficiency of 88% and power factor of 80% leading. The available transmission line is 3100volt, 60cycle, 3phase. Write purchase specifications for three singlephase transformers to step down the voltage to 440 volts by a YA connection for the motors. Assume a load diversity factor of 69%. 

11 
A transformer has a fullload voltage of 108 volts and a noload voltage of 115. What is the regulation in per cent? Assume constant primary voltage. 

12 
A balanced threephase load of 1740 kva is to be supplied by two transformers connected open delta. What must be the singlephase kva rating of each transformer? 

13 
An autotransformer for starting an induction motor steps down the line voltage from 230 volts to 84 volts per phase linetoneutral for the motor. If the transformer furnishes 20 kva per phase at 50% power factor, calculate the primary and the secondary currents in the autotransformer. Neglect the exciting current. 

14 
What would be the maximum permissible continuous kw output at unity power factor, threephase, from two similar 100kva singlephase transformers connected in T to a twophase line? 

15 
Two 100kva, 2400/480volt transformers are connected in open delta for a threephase load of unity power factor. How many kilowatts of power can be delivered to the load without overloading the transformers? 
174 kw 
16 
A welding transformer operating from a 240volt line has 500 turns in its primary winding and 1 turn in its secondary winding. When the transformer during a welding cycle draws 20 amp from the line, approximately how many amperes are flowing in the weld? 
10000 amp 
17 
A 100kva, singlephase, 2400/240volt, 60cycle transformer has an iron loss of 1200 watts at rated voltage and a copper loss of 1500 watts at rated current. If it is assumed that 80 per cent of the iron loss is caused by eddy currents, what will be the approximate efficiency at ratedload current when supplying a resistance load from a 25cycle power line of 1000 watts? 
95.8% 
18 
Under the conditions of Problem 17, the iron losses are abnormally low but the transformer's heatdissipative capacity is as great as ever. What per cent of increase in ratedload current is permissible, if the operating temperature is not to exceed that for which the transformer was designed? 
24.8% 