The losses in any power equipment can also be classified broadly into three categories:
Voltage-related loss in ship electrical system
Varies approximately with the flux density squared or applied voltage squared. Since most power equipment operates at a constant voltage and constant frequency, this loss is a fixed loss at all load currents.
Current-related ship power loss
In diode type elements, varying with the load current or power output.
current-related loss in the conductor resistance, varying with square of the
load current or square of the power output in constant voltage equipment.
At a constant voltage operation, therefore, the total loss can be expressed as a function of the load power output Po as follows:
Total loss in ship electrical system
Total Loss = Fixed loss Ko + Linearly varying loss K1 · Po + Square variable loss K2 · Po2
The efficiency peaks at some load point, and then decreases due to high
square-variable loss at high loads. We, therefore, deduce that there has to be a load point where the efficiency has the maximum value.
Ship equipment maximum efficienty
Equipment efficiency is maximum when the fixed loss equals the square variable loss.
It is noteworthy that the linearly varying loss in diode-type devices does not matter in determining the maximum efficiency point. It just degrades the efficiency in a fixed ratio which does not vary with load.
η = Po/(Po + K1· Po) = 1/(1 + K1)
For equipment such as ship transformer, ship motor, and ship generator, the fixed loss comes from the magnetic loss in the core operating at constant voltage (constant flux amplitude), and the square-variable loss comes from the I2R loss in winding conductor resistance.
Thus, in electrical machines, an important conclusion is stated as follows:
At the maximum efficiency point:
Fixed core loss = Square-variable conductor loss
Large power users often specify the maximum efficiency point at the
power level at which the equipment is likely to operate most of the time, and specify
the rated load equal to the maximum continuous load capability needed during the
equipment service life.