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We know from basic theory that a generator’s output will vary depending on the input shaft speed. A means of regulating the generator’s output is therefore required.

6.1.1 Vibrating contact regulator

This device comprises voltage and current regulators. They are used on small general aviation (GA) aircraft that have relatively low generator power outputs. When the engine starts, the alternator output voltage builds up rapidly to the nominal aircraft level (either 14 or 28 V DC).

  Contacts of both regulators remain closed to allow current to flow into the field windings. When the generator output voltage increases beyond 14/28 V, the voltage coil contacts open and this introduces the resistor into the field windings, thereby reducing the field excitation current, and subsequently reducing the generator output.

Once the output voltage drops to below 14/28 V, the contacts close (by a spring mechanism) and the resistor is bypassed, allowing full excitation current back into the field.


The on/off cycle repeats between 50 and 200 times per second, or 50–200 Hz. This process regulates the generator output to a mean level, typically 14  0.5 V (or 28  1 volt).

Current regulation is achieved in a similar way, i.e. by controlling the field current. When loads are high, the voltage output may be insufficient to open the contacts. The result is that the output will continue to increase until the maximum rated current is reached.

At this point, the current regulator contacts open, and the resistor is connected to the field windings.


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