Generators and Regulators

What Generators Do and What Regulators Ought To Do

Most people first learn about generators at night on a back country road in the middle of nowhere. (Actually, about 100 yards from a house, but the middle of nowhere is so much more depressing.) You have one of those "English sports car needs minor electrical work" from the classified ads. Oh, the man who sold you the car was honest; the car was most certainly English and it did need electrical work. Anyway, after standing over the open engine compartment and alternately thumping on the generator, the control box, and the flashlight, you conclude that although flashlights improve with thumping, generators and control boxes don't.

Perhaps the best way to come to grips with old electrics is by gaining an understanding of what makes them work. Contrary to popular belief, the operation of a Lucas generator is not based on some magic incantation - it is based upon five fundamental properties of electricity and magnetism:

  1. Electric current in a coiled wire will create a magnetic field.
  2. Wrapping the coil of wire around a soft iron core will intensify the magnetic field.
  3. The strength of the magnetic field will vary with the current in the wire.
  4. Rotating a loop of wire in a magnetic field will induce a voltage in that loop of wire.
  5. The strength of the induced voltage is dependent upon the strength of the magnetic field and the speed at which the loop of wire is rotated.

A generator is composed of five parts. The armature is made up of coils of wire wrapped around an iron core and it is the armature which rotates when the generator pulley is turned. The brushes are the spring-loaded contacts which transfer current from the armature to the electrical system. The brushes actually rest against a segmented ring-at one end of the armature; this ring is called the commutator. Inside the generator body are the field coils (also called field windings) which are wrapped around the field poles (5), which are essentially pieces of soft iron. It is current in the field windings that produces the magnetic field in which the armature rotates.

When the engine is turning over, the armature is spun by the fan belt. In the presence of a magnetic field (generated by the field coils), a voltage is induced in the armature windings. When the voltage in the armature windings is greater than the rest of the system, current will flow from the armature terminal of the generator (usually "0") to the corresponding terminal (also usually "0") of the control box or voltage regulator.

The control box (or voltage regulator as most of us call it) has two main parts. The cut-out relay (6) prevents current from flowing to the generator from the battery when the generator's output voltage is lower than battery voltage. The second part of the control box is properly called the voltage regulator (7). This strengthens or weakens the magnetic field in the generator accordingto the needs of the battery or other electrical system components. Remember, the stronger the magnetic field, the greater the voltage induced in the spinning armature.

The cut-out relay consists of an iron core with a "shunt" and a "series" coil wrapped around it. The shunt windings are connected between the generator armature terminal "D" and a ground terminal (usually marked "E') on the control box. This means that the internal generator voltage is always impressed upon the shunt windings. The series windings are wired so that all the generator output current passes through them before going to the electrical system in general.

Fixed above the cut-out core is a spring arm that carries a contact which is connected to the series windings of the cut-out core. Output current from the generator can only pass on to the electrical system and the battery when the contact arms are touching. Spring tension normally holds the contacts apart so there can be no current flow in either direction.

When the armature in the generator is spinning fast enough, (about 1000 generator RPM or 750 engine RPM) the current in the shunt windings of the cut-out relay will generate a magnetic field strong enough to overcome the natural spring tension of the contact arm. The arm snaps down and the two contacts touch. Current now flows through the series windings, across the contacts and on to the battery through the output terminal (usually "A") on the control box. Current in the series windings actually intensifies the magnetic field around the core of the cutout relay, and this in turn holds the contacts even more firmly together. The point when the contacts close is usually adjusted so that the internal voltage of the regulator is about 12.7 to 13 volts.

When your engine slows to idle, the armature slows as well. This means that the voltage induced in the spinning armature decreases. Lower voltage reduces the strength of the magnetic field holding the series winding's contact arm closed. Eventually, the weakened magnetic field can no longer hold against the arm's spring tension and the contacts open. (Note: the way in which the contacts open is actually somewhat more complex, but this description will do for our purposes.) This immediately stops all current flow to or from the generator. The point at which the contacts open (around 8.5 to 11 volts) is known as the drop-off point.

If the series winding contacts did not open at low generator output, the higher battery voltage would flow back through the control box into the armature's fine wire windings. The reverse flow would melt the windings and thus, destroy the generator. Now you know one of the reasons why the control box is so important.

The other part of the control box, the voltage regulator, acts to limit the voltage in the charging system to a safe value by controlling the internal voltage of the generator. The voltage regulator, like the cut-out, has a shunt winding made up of many turns of fine wire wrapped around a soft iron core. Suspended above the regulator core are a pair of contact points, again like the cut-out relay. However, these points are normally closed, rather than open. The function of the regulator is to break this connection. When generator voltage is low, the current in the shunt windings is small, so the magnetic field is too weak to overcome the spring tension in the arm holding the contact points closed. When the points are closed, the output current from the generator (entering through the "D" terminal) goes through the regulator frame, through the regulator contact points to the field terminal on the control box (usually "F"). From the field terminal on the control box, the current flows to the field terminal ("F) on the generator and then through the field windings around the field poles of the generator.

Since we have a direct connection through the regulator contacts, current in the field windings is at a maximum. Consequently, the magnetic field (in which the armature spins) created by the current in the field windings is also at its maximum. Because the magnetic field is at its strongest, induced voltage in the armature is also at its highest. (The voltage induced is directly related to the strength of the magnetic field.) As the voltage in the generator increases, the current in the shunt windings of the regulator relay increases, which in turn increases the strength of the magnetic field trying to pull the regulator contacts apart.

When the field strength finally overcomes the natural tension of the contact arm and the regulator contacts are separated, the direct connection between the armature terminal "D" of the generator and the field terminal "F" of the control box is broken. While the direct connection has been severed, there still exists a way for the current from the generator to return to the field windings.his second path is through a short piece of resistance wire, and the built-in resistance reduces the current passing through the field windings inside the generator. The reduction in current in the field coils reduces the strength of the magnetic field in which the armature is spinning. The induced voltage in the armature windings falls, and so generator output falls as well. With reduced generator output, the current in the shunt windings of the regulator is also reduced, and the magnetic field produced by the current in the shunt windings is likewise reduced. When the strength of the magnetic field is no longer enough to hold the regulator contacts apart, they snap back together, and direct contact between the generator output and the field windings is restored.

Since current is no longer flowing through the resistance wire, the current in the field windings of the generator is increased, which strengthens the magnetic field inside the generator. The induced voltage in the armature increases, and the generator output also increases. As generator output increases, current in the shunt windings of the regulator increases once again until the magnetic field is strong enough to pull the regulator contacts apart. As before, with the direct connection broken, the current to the field windings is reduced by the passage of current through the resistance wire. The strength of the magnetic field in the generator falls, and so the generator output falls. The cycle described here takes place very quickly; so quickly that the contact points seem to vibrate.

We've now traced the system through its entirety. With this knowledge in hand, you'll be able to entertain your companions with a profound dissertation on the fundamental properties of electricity and magnetism which make thumping on the generator and control box useless. We all know that once the magnetism has leaked out, there is nothing anyone can do.

Note: If you reverse the polarity of the battery, you must repolarise the generator BEFORE starting the engine. If you have the generator rebuilt, you must also repolarise the generator. To repolarise the generator, connect a piece of wire to the solenoid battery lead and then touch this onto both terminals of the generator. If you do not, you will burn the cut-out points on the regulator - you will then need a new regulator.

Many thanks to Garth Bagnall for reviewing this article prior to publication.

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