How to do a Generator to Alternator Conversion


In this video we describe the similarities and differences in a generator and an alternator and how you would have to rewire your car to install an alternator.

Some of us have given up our old generator and moved ahead to an alternator. Some of us have moved from an old style alternator to a more modern and more efficient alternator. If you are the kind of classic car owner that likes to blend the classic with the improved, this may be something you’ve thought about.

What’s the difference between a generator and an alternator?

They both make the same kind of electricity. One doesn’t make “better” electricity than the other. The biggest differences are…

  1. Although they both make alternating current, AC, they differ in how they convert that current to the direct current, DC, that our cars use DC.
  2. The roles of the armature windings and the stator windings are reversed.
  3. Generators use external regulators. Alternators usually use internal regulators.

In a classic generator, the stator windings (statue, stationary – fixed in place) affixed to the inside of the case serve as the field windings. The windings on the armature (the part that rotates) are the current generating windings.

As you learned in school, when you pass a magnet past a coil of wire, (or you pass a coil of wire past a magnet) you create electricity in the coil of wire. The field windings (stationary) in the generator serve as an electro-magnet. The windings on the armature spin past the magnetism of the field windings. This creates electricity in the current generating windings mounted on the armature. At the end of the armature we have a series of conductive bars. Traditionally we have a pair of brushes that make contact with the bars. That’s how the electricity is directed out of the generator and to the rest of the car.

Generators are heavy, and often large. They also have another shortcoming. Having the current generating windings on the armature means there is a limit to how fast you could spin it without harming it. To keep the RPM’s of the generator under control, generators were usually set up with a large pulley to lower their RPM’s. That protects the generator from high RPM damage, but it leaves the generator almost valueless at low RPM’s. Also, running all that current through the bars at the end of the armature and through the brushes is tough on the brushes. This calls for additional regular maintenance.

An alternator was a logical evolutionary step. A breakthrough was an inexpensive electronic device called a diode. With diodes the stator windings could become the current generating windings. The smaller lighter field windings could be put on the armature. Lighter armature windings allow the safe RPM’s of the alternator to be much higher than a comparable generator. A smaller pulley could be put on the alternator. It could spin faster which made it capable of generating current even at low RPM’s. Also, the relatively small amount of power needed so the field windings can become an electro-magnet, can be sent to the armature via a pair of small brushes that would ride on slip rings on the armature instead of bars. The brushes last a very long time. Sometimes the life of the car.


Installing an alternator in place of a generator, or upgrading an older style alternator to a more modern alternator, is usually not hard to do. The challenge for many of us is how to wire the upgrade. Generators had external voltage regulators. Alternators usually have internal voltage regulators. That means the alternator is a nice neat all-in-one package, but it leaves us with a number of wires and the old regulator that need to be dealt with.

If we go from an old style alternator to a more modern one, we may be surprised to find some of the early alternators had external voltage regulators like generators did. Also, while most alternators have always had an internal voltage regulator, the older models often had more wires than a more modern alternator. So, we are left with dealing wires again.

Let’s talk about wiring. A relatively modern alternator only needs two wires to do its job. It needs a small wire that goes to the light on the dash, and it needs a heavier wire that carries the current out of the alternator and to the rest of the car. Two wires isn’t intimidating at all. We can deal with two wires.