by Dr. Deuce  2006

used by permission

          Most people know little about electricity in any form including HMVs. When restoring or just trying to make your HMV drive down the road, a little understanding is very helpful.  

          The 1^st thing to remember is that all generators and alternators, dynamos etc generate Alternating Current (AC) power. It is a fact of having one or more coils either rotating inside a magnetic field or the magnetic field revolving inside the fixed coil. If you want AC like you have in your house, great! If however you want to be associated with a battery, you need Direct Current (DC). To change AC to DC is relatively easy and with generators and alternators, two different means are employed.  

          Automotive Generators have stationary Field coils mounted to the inside of the cylindrical housing of the Generator itself. Inside the Generator is the Armature which is turned by the engine usually via a belt and pulley system. The Armature has a number of individual coils made into it. These coils are connected to the Commutator at the back end of the Armature by the rear bearing/bushing. The Commutator has a number of slightly rounded copper bars attached to it with a small insulating gap in between them. These bars are not attached electrically to anything but to one end of a coil. Each coil is attached to two bars that are exactly opposite by 180 degrees on the Commutator. Riding on the Commutator and attached to the Generator housing or back cover and electrically isolated are two brush holders. The brushes are held against the Commutator by springs that maintain a constant force to keep the brushes in contact with the Commutator bars. The whole system is designed so that the brushes will mostly touch only one set of Commutator bars which are connected to one coil at any one time as the Armature rotates. As the Armature rotates, the brushes move from one set of bars attached to one coil to the next set of bars attached to the next coil. One of the brushes is connected to the ARMATURE terminal of the Generator and the other is connected to the housing or back endplate (ground).  The FIELD terminal on a Generator is connected to one end of the Field Coil(s) and the other end is connected to the ARMATURE terminal on most of the General Motors Generators.  With the Autolite type Generators, the other end of the Field coil(s) is connected to the case (Ground).

            As the Armature rotates through the magnetic field generated by the Field windings, if you attached a meter to the two ends of just one of the Armature coils, you would see a positive voltage that rose then declined to zero then a negative voltage that grew more negative then declined back to zero where the whole process began. Alternating Current (AC).

What the Commutator does is make sure that the output voltage presented to the ARMATURE terminal is only either always positive, or always negative depending upon whether your vehicle is positive or negative ground. It can be zero, but never the opposite polarity in normal operation.

The strength of the Field coils (amount of current passing through them basically) and the speed of the rotation of the Armature determines the output voltage presented to the ARMATURE terminal.

If the FIELD terminal had the full vehicle voltage applied, in this case 6 volts, the ARMATURE terminal would have about 8 to 9 volts at it with the engine running above idle. The ideal voltage to charge a 6 volt battery fast is about 7.7 volts. To reduce this output, the voltage through the Field winding must be reduced and regulated. In the old days for HMVs, this was accomplished by turning the Field voltage on and off relatively fast. When you apply power to a coil, any coil, it takes a certain amount of time for the coil to ‘charge up’ so to speak. In the voltage regulator, the Voltage Regulator Relay monitors the output of the ARMATURE terminal and when it approaches 7.7 volts, the relay pulls in and disconnects power to the FIELD terminal on the generator. As the magnetic field collapses in the Field(s) of the generator, the voltage at the ARMARURE terminal starts to fall. When it reaches a voltage somewhat less then 7.7 volts, the Voltage Regulator Relay releases and reconnects the FIELD terminal to power again and the process starts all over again. The voltage at which it connects and the voltage at which it disconnects have to be different by a small amount. This is called hysteresis

Most people do not realize that the ‘points’ (contacts) in the Voltage Regulator Relay are turning on and off pretty fast all the time that the engine is running above idle. They are not opening and closing as fast as the ignition points, however they are buzzing right along. This is why you need to check the output of the generator regularly to make sure that the points have not worn, spring stretched, etc.

A generator voltage regulator also has a Current Relay in it. This relay measures the current that the load (battery and electrical devices) are putting on the Armature connection. Generators have the ability to try and please the load and destroy themselves in the process. To prevent damage from trying to make too much electricity, or in this case current (Amps), this relay pulls in and releases the Field winding power source. After the current falls to a safe level, the relay releases and allows the normal Field Voltage relay to control the output of the generator.

The third relay in a voltage regulator enclosure is the Cutout Relay. This relay allows the Armature terminal on the Generator and voltage regulator to connect to the Battery terminal on the voltage regulator only when the Armature voltage is greater than the Battery voltage (charging). If the Armature were connected to the Battery terminal all the time, the generator would try to run as a motor when the engine was not running. This would both discharge the battery and it would also burn up the generator.

Everything talked about above is the same for a 12 volt system, except the 7.7 volts maximum is 14.4 volts in a 12 volt system.  

An alternator is built similar to a generator except inside out. The ‘Field” winding is built onto the rotating part inside the Alternator. It is connected to the voltage regulator by two slip rings. These slip rings are continuous bands of copper that look like a wedding ring. Like the generator, a brush rides upon each of these rings. Unlike the generator, these slip rings and brushes carry a small amount of current and never get connected to anything as the Generator does with the brushes jumping from one commutator segment to another one.

The fixed coils (usually 3 of them) are called the Stator. They generate alternating current voltage on each coil (3 phase). Because the Battery operates on Direct Current (DC), a method must be used to change the AC into DC (rectification). In an Alternator, high current Silicone Diodes are used. Diodes act like one way valves. The electricity can only pass in one direction. By connecting 6 of these Diodes to the 3 coils, a pulsating DC voltage is created. This is outputted from the Alternator on the BAT(tery) terminal. Because the Diodes only let the electricity pass in one direction, these Diodes act as both the Cutout Relay of a Generator regulator and the Commutator inside the Generator.

            Diodes pass a lot of current in one direction, but they also pass a very small amount in the opposite direction. If you leave a vehicle with an alternator connected to the battery for more then a month, the battery may be discharged enough to give you a tough time starting. With an Alternator, a battery disconnect is highly recommended in an HMV application where it may sit for a long period of time especially in the winter.