Equivalent circuit

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure: -
Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.
Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.
Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.
Measure the load resistance RL, if you don't already know it.
Calculate the generator's AC internal resistance RGAC from the following formula:
Note 1: The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.
Note 2: If the generator is an AC type, use an AC voltmeter for the voltage measurements.
The maximum power theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. This is inefficient since half the power is wasted in the generator's internal resistance; practical electric power generators operate with load resistance much higher than internal resistance, so the efficiency is greater.