There are different methods to measuring alternator efficiency, and the results may vary depending upon the method. Therefore, when comparing efficiencies at the quotation stage, it is important to make sure the values given have been calculated by the same method.
Efficiency is the ratio of electrical power out of the alternator to the mechanical power put into it and is usually expressed as a percentage.
Two standards guide the method of calculating efficiencies: NEMA (typically used in the US) and IEC (typically used everywhere else). The IEC method generally gives a higher result.
A direct measurement of efficiency is obtained by measuring the generator’s power input and output. This type of efficiency measurement is subject to larger variations caused by the accuracy of the metering used.
However, both the NEMA and IEC standards allow efficiency to be measured by an indirect method. This is also known as summation of losses measurement. Instead of measuring power in and out of the machine, separate tests are used to quantify the five types of losses in the generator. These losses can be categorised into fixed losses and variable losses.
Fixed losses are independent of load. There are two:
Friction and windage losses – These are losses due to the bearings, fans or blowers and the rotor itself. Optimising the fan design is the best way to reduce these losses.
Core losses – These are losses caused by hysteresis (a magnetic ‘friction’ caused by the alternating magnetic flux flow) and eddy currents (induced electrical currents). Core losses can be reduced by using a better grade of lamination steel or modifying the alternator design to operate at a lower magnetisation level. The latter method, however, affects the generator’s ability to handle reactive loads.
Variable losses are dependent on load. There are three:
Stray load losses – These are losses caused by the load current due to changes in the magnetic flux distribution, eddy currents and harmonics. They can be reduced by using better materials and thinner laminations.
I2R losses in the armature and field – These heat losses are due to resistance in the windings and can be reduced by using more copper or by running at cooler temperatures.
Brushless exciter losses – These are small loses due to electrical energy going into the exciter and rectifier.
The reference temperature used when determining the I2R loss is another variable that can influence the calculated efficiency. The IEC and the NEMA standards state guidelines of how to determine this temperature.
Finally, comparisons of efficiency must also take into account the ‘method of guarantee.’ Efficiency can be stated as a nominal figure that may include some tolerances on the stated losses. This is allowable in both IEC and NEMA standards.
Yet efficiency can also be stated as a guaranteed minimum number. The nominal number is higher than the guaranteed on a given machine, so it is important to know what type of efficiency number you are looking at.
Knowing how efficiency is calculated is important in order to make real comparisons between alternator designs.