Equivalent circuit of electrical generator

Equivalent circuit of generator and load.
G = generator
V_G=generator open-circuit voltage
R_G=generator internal resistance
V_L=generator on-load voltage
RL=load resistance

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's V_G and R_G parameters, follow this procedure: -
- Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance R_GD.
- Start the generator. Before connecting the load R_L, measure the voltage across the generator's terminals. This is the open-circuit voltage V_G.
- Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage V_L.
- Measure the load resistance R_L, if you don't already know it.
- Calculate the generator's AC internal resistance R_GAC from the following formula:

RGac = RL(VG/VL - 1)

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 R_GA and assume that R_GAC and R_GDC are equal.

Note 2: If the generator is an AC type (distinctly not a dynamo), use an AC voltmeter for the voltage measurements