April 27, 2007

Equivalent circuit of electrical generator

Equivalent circuit of generator and load.
G = generator
VG=generator open-circuit voltage
RG=generator internal resistance
VL=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 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 RGD.

- 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:

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 RGA and assume that RGAC and RGDC are equal.

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

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