Strictly speaking, on no load (that is when the secondary circuit is open) a slight copper loss takes place in the primary coil but because of its smallness is not mentioned. It is, to be exact, included in the expression "iron losses," as the precise meaning of this term signifies not only the hysteresis and eddy current losses but the copper loss in the primary coil when the secondary is open.
The importance of the iron losses is apparent in noting that in electric lighting the lights are in use only a small fraction of the 24 hours, but the iron losses continue all the time, thus the greater part of each day energy must be supplied to each transformer by the power company to meet the losses, during which time no money is received from the customers.
Some companies make a minimum charge per month whether any current is used or not to offset the no load transformer losses and rent of meter.
Figs. 1,946 to 1,948.—Methods of connecting the low tension sides of Westinghouse transformers using the connectors illustrated in figs. 1,949 to 1,953.
Ques. How may the iron losses be reduced to a minimum?
Ans. By having short magnetic paths of large area and using iron or steel of high permeability. The design and construction must keep the eddy currents as low as possible.
As before stated the iron losses take place continually, and since most transformers are loaded only a small fraction of a day it is very important that the iron losses should be reduced to a minimum.
With a large number of transformers on a line, the magnetizing current that is wasted, is considerable.