Info Manual

This project is a voltage regulator for a 3-phase, permanent-magnet alternator found on motorcycles. The same design can be used for single-phase alternators simply by eliminating some parts.  This project only describes the regulator, even though a rectifier is typically integrated into the unit.

Please refer to the circuit diagram during the analysis of the circuit.

The way this type of regulator controls the alternator’s output voltage is by simply shorting out the stator winding for one cycle of the ac waveform. This is called shunting. This is done because it is much easier to short-circuit an inductor (the stator winding is an inductor) than to open-circuit an inductor. Very high voltages are induced when an inductor circuit is opened. This may cause a breakdown in the winding’s insulator.

The device used to shunt one cycle of one phase is an SCR. A silicon-controlled rectifier acts as a diode when triggered at its gate. It stops conducting when the current drops below a very low threshold value. Once it stops conducting, it will not conduct again until it receives another gate signal.

A voltage detection circuit is used to trigger the gate on each of the three SCRs. The threshold voltage to trigger the gate is selected as 14.6volt (on the motorcycle’s system-voltage). This is when the gate would trigger if there were no capacitor as described in the next paragraph.

Since the voltage from an alternator fluctuates, the detection circuit will trigger during a peak in the waveform of one of the phases, but this causes the average voltage to be too low. To raise the average system-voltage without increasing the detection threshold, a simple capacitor is used to delay the triggering of the SCR gates.  The capacitor acts as a filter to reduce the ac ripple going to the detection circuit. If the system voltage was strictly DC voltage (with no ripple) the gate would trigger at 14.6v and the average voltage would be 14.6v. In reality, the average voltage is always less than the threshold voltage and only approaches it as the ripple reduces. The actual trigger voltage is higher than 14.6v, but the average voltage is lower than 14.6v. As the ripple reduces, the average and trigger voltages approach 14.6v.