Alternator with Neutral-Point Voltage

Neutral-point voltage

(1)A conventional alternator uses six diodes to rectify three-phase AC (Alternating Current) into DC (Direct Current). The output voltages generated at the neutral point is utilized as a power source for the charge warning light relay. It is known that the average voltage of the neutral point is 1/2 of the output DC voltage. While the output current is flowing through the alternator, the voltage at the neutral point is mostly DC, but it also includes an AC portion.

The AC portion is induced in each phase by the flow of output current. When the alternator speed exceeds 2,000 to 3,000 rpm, the peak value of this AC portion exceeds the DC output voltage.

(2)This means that, compared with the output characteristics of the alternator without neutral-point diodes, the output gradually increases from midway by 10 to 15% at a normal rated alternator speed of approx. 5,000 rpm.

Alternator with Neutral-Point Diodes

Circuitry and construction

In order to add the potential variation at this neutral point to the DC voltage output by the alternator with neutral-point diodes, two rectification diodes are provided between output terminal (B) and ground (E) and connected to the neutral point. These diodes are installed in the rectifier holder.

Regulate generated electricity

The necessity to regulate the amount of generated electricity

The alternator used for the vehicle rotates with the engine. Therefore, while driving, the engine speed frequently changes, and the alternator speed does not become constant. If the alternator has no regulator, the charging system cannot supply electricity constantly to the electric devices.

Therefore, even if the alternator speed changes, the voltage applied to the electric devices should be maintained, and according to the change of the amount of electricity, the output to supply the required amount should always be regulated. In the alternator, the regulation like above is conducted with an IC regulator.

Principle of regulation