No, generally we can’t determine efficiency of alternator by direct loading, that is because of owing to difficulty in finding suitable load. Rather of direct loading efficiency is determined from losses.
Efficiency is the defined as the ratio of electrical power output 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.
Both the NEMA and IEC standards allow efficiency to be measured using the indirect method, rather of direct loading method. This methods is known as summation of losses measurement.
In this method, instead of measuring power in and out of the machine, separate tests are used to quantify the five types of losses in the generator.
| Loss Type | Fixed/Variable |
|---|---|
| Friction and Windage Losses | Fixed Losses |
| Core losses | Fixed Losses |
| Stray load losses | Dependent on Load |
| I2R Losses in Armature and Field | Dependent on Load |
| Brushless Exciter Losses | Dependent on Load |