AC Generator Operation Overview

When the engine is running, the drive belt spins the rotor inside the stator windings. This magnetic field inside the rotor generates a voltage in the windings of the stator. Field current flowing through the slip rings to the rotor creates alternating north and south poles on the rotor.

The induced voltage in the stator is an alternating voltage because the magnetic fields are alternating. As the magnetic field begins to induce voltage in the stator’s windings, the induced voltage starts to increase. The amount of voltage will peak when the magnetic field is the strongest. As the magnetic field begins to move away from the stator windings, the amount of voltage will start to decrease. Each of the three windings of the stator generates voltage, so the three combine to form a three-phase voltage output.

In the wye connection, output terminals (A, B, and C) apply voltage to the rectifier. Because only two stator windings apply voltage (because the third winding is always connected to diodes that are reverse-biased), the voltages come from points A to В, В to C, and С to A.

(A) Individual stator winding voltages; (B) voltages across the stator terminal A, B, and C

FIGURE. (A) Individual stator winding voltages; (B) voltages across the stator terminal A, B, and C.

To determine the amount of voltage produced in the two stator windings, find the difference between the two points. For example, to find the voltage applied from points A and B, subtract the voltage at point В from the voltage at point A. If the voltage at point A is 8 volts positive and the voltage at point В is 8 volts negative, the difference is 16 volts. This procedure can be performed for each pair of stator windings at any point in time to get the sine wave patterns. The voltages in the windings are designated as Va, Vb, and Vc. Designations of Vab, Vbc, and Vca refer to the voltage difference in the two stator windings. In addition, the numbers refer to the diodes used for the voltages generated in each winding pair.

Note: Alternating current is constantly changing, so this formula would have to be performed at several different times.

The current induced in the stator passes through the diode rectifier bridge, consisting of three positive and three negative diodes. At this point, there are six possible paths for the current to follow. The path that is followed depends on the stator terminal voltages. If the voltage from points A and В is positive (point A is positive in respect to point B), current is supplied to the positive terminal of the battery from terminal A through diode 2. The negative return path is through diode 3 to terminal B.

Both diodes 2 and 3 are forward-biased. The stator winding labeled С does not produce current because it is connected to diodes that are reverse-biased. The stator current is rectified to DC current to be used for charging the battery and supplying current to the vehicle’s electrical system.

Current flow when terminals A and В are positive

FIGURE. Current flow when terminals A and В are positive.

When the voltage from terminals С and A is negative (point С is negative in respect to point A), current flow to the battery positive terminal is from terminal A through diode 2 . The negative return path is through diode 5 to terminal C.

This procedure is repeated through the four other current paths.

Current flow when terminals A and С are negative

FIGURE. Current flow when terminals A and С are negative.

Current flow when terminals В and С are positive

FIGURE. Current flow when terminals В and С are positive.

Current flow when terminals A and В are negative

FIGURE. Current flow when terminals A and В are negative.

Current flow when terminals A and С are positive

FIGURE. Current flow when terminals A and С are positive.

Current flow when terminals В and С are negative

FIGURE. Current flow when terminals В and С are negative.