Transformers.—These, as a whole, are simple in construction, high in efficiency, and comparatively inexpensive. Their principles of operation are also readily understood.
The efficiency of a transformer, that is, the ratio between full load primary and full load secondary is greatest when the load on it is such that the sum of the constant losses equals the sum of the variable losses.
In general, transformers designed for high frequencies and large capacities are more efficient than those designed for low frequencies and small capacities. As a whole, however, a transformer leaves but little to be desired as regards efficiency, a modern 60 cycle transformer of 50 kilowatts capacity or more possesses an efficiency of approximately 98 per cent. at full load and an efficiency of about 97 per cent. at half load.
Ques. How should a transformer be selected, with respect to efficiency?
Ans. One should be chosen, whose parts are so proportioned that the point of maximum efficiency occurs at that load which the transformer usually carries in service.
In many alternating current installations, comparatively light loads are carried the greater part of the time, the rated full load or an overload being occurrences of short durations. For such purposes special attention should be given to the designing or selecting of transformers having low core losses rather than low resistance losses, because the latter are then of relatively small importance.
Ques. What kind of efficiency is the station manager interested in?
Ans. The "all day efficiency."
This expression, as commonly met with in practice, denotes the percentage that the amount of energy actually used by the consumer is of the total energy supplied to his transformer during 24 hours. The formula for calculating the all day efficiency of a transformer is based upon the supposition that the amount of energy used by the consumer during 24 hours is equivalent to full load on his transformer during five hours and is as follows:
| 5w | |
| E = | —————— |
| 24c + 5r + 5w | |
| where | |
| E = | the all day efficiency of the transformer, |
| w = | the full load in watts on the primary, |
| c = | the core loss in watts, |
| r = | the resistance loss in watts. |