The output of a generator or motor is always less than the input because some of the energy supplied is lost as heat. These losses raise the temperatures of the machine parts above that of surrounding air until such temperatures are reached that the heat losses are radiated as fast as they are generated. Certain of the losses depend upon the load. The temperature rise therefore depends upon the load also, and the maximum allowable temperature rise determines the maximum permissible load that the machine may carry. The limit of output occurs at the load for which the temperature rise becomes high enough to endanger the insulation of the windings.
Thus the consideration of machine losses is important for the following three reasons:
1.Losses appreciably influence the operating cost of the machine.
2.Losses determine the heating of the machine and hence the rating or power output that can be obtained without undue deterioration of the insulation.
3.The voltage drops or current components associated with supplying the losses must be properly accounted for in a machine representation.