ALTERNATOR OPERATION                        
by Jim Mais

MOT_ALT.GIF   shows the internal wiring of the Motorola alternator with the external regulator. The principles apply to all alternators although there may be minor differences in internal wiring.

The stator is the fixed/non-rotating part. It consists of a laminated iron frame with 3 windings distributed around it. Electrically, these windings are 120 degrees apart.

The rotor is an electro-magnet which is energized by current fed thru the slip rings and brushes. As the rotor magnetic field sweeps past the stator windings, it induces an alternating current in the windings. There are actually three voltages; each winding has a voltage 120 degrees out of phase with its neighbor.

To convert the alternating current to DC to charge the battery, a full-wave bridge rectifier is used. This consists of the 6 large rectifier diodes shown in the drawing. Any time a winding has positive polarity, the diode on the right conducts, connecting it to battery. In the next half cycle, when the winding is negative polarity, the diode on the left conducts. A "full-wave" circuit means that all the windings are used all the time.

The output voltage is a combination of the waveforms from the 3 windings. This results in an output which is pretty close to DC.

When the alternator is not turning, the diodes prevent battery current from flowing back into the alternator.   Thus, no cut-out relay is required as with generators.

FIELD EXCITATION    To control the output voltage, and maintain the correct battery charge, the field winding current is varied. The regulator is a solid-state sensor which monitors battery voltage. When the battery voltage is low, more field current is supplied.

Excitation voltage is supplied by the alternator windings, rectified by the Trio diodes (shown as the smaller diodes on the drawing). These diodes are quite small since maximum field current is only about 2 Amps. The regulator acts as a rheostat, controlling the current from the Trio diodes to the field.

Unlike a generator, the alternator is not self-exciting at start-up. A generator has field poles made of soft iron which hold a residual magnetism. The alternator field structure has little residual and thus has almost no output unless field current is supplied.

To get the alternator going, a tiny field current must be supplied. In most designs, this current is initially provided by the dash warning light. With the Ignition switch closed, current flows thru the Lamp to the regulator and into the field winding.    If the dash warning light is burned out or disconnected, the alternator probably won't begin charging.

(Note: The drawing shows an externally regulated alternator. Other designs, and internally regulated units may place the regulator either in the DF lead or in the D- lead. Operation is the same though.)

As the alternator speeds up, stator voltage increases until the Trio diode voltage is sufficient to provide field excitation. As the voltage approaches 12 Volts, the dash Lamp goes off because it has the same potential on both sides.

REGULATION    How does the regulator sense the battery voltage when it's not even connected to the battery? The answer is that the voltage at D+ almost exactly follows the voltage at B+ because the voltage drop in the Trio diodes is almost the same as that in the larger rectifier diodes.

If battery voltage drops, the regulator circuitry senses that fact (at D+) and increases the current flowing into DF until battery voltage is restored. Most regulators also include some form of temperature compensation. A cold battery requires slightly higher voltage to fully charge. A temperature sensing element in the regulator increases output voltage at low temperatures.

Current limiting is not provided in alternator regulators since the alternator magnetic structure inherently limits the maximum current that can be produced.