Contributed by Tom Metty

Generating electricity is actually a simple process, you need 3 things … a magnetic field, a conductor, and motion. Take a spool of wire and a magnet, and throw one of them at the other and you’ve generated electricity.

To create the magnetic field you can use a permanent magnet or an electromagnet, what you want is a strong field with lot’s of “lines of force” (it’s coming back to you, isn’t it?). For the conductor you can use a couple loops (turns) of wire, but if you’re going to the trouble, you might as well use a couple thousand turns. That’s the key, a big magnetic field and lots of turns of wire in the coil. The bigger the magnetic field, the more “lines of force” you have. The more turns of wire, the more electricity you generate as each “line of force” cuts through each wire. To get even more electricity, increase the “motion” (speed).

As a wire passes through a magnetic field a voltage is induced with positive potential on one end of the wire and negative on the other. In rotating generators and alternators like ours, that same wire will have to go back through the same magnetic field in the other direction, which generates a voltage again, but this time with the opposite polarity, then the process starts over. Since the polarity reverses continuously it’s called alternating current (AC). A generator has a rotating armature (conductors) spinning inside a magnetic field generated by the field coils. An alternator has a rotating electromagnetic field spinning inside the armature windings. As you can see, generators and alternators both generate AC initially but they differ in the way they turn the AC into DC (direct current), which is what we need.

Alternators take their AC and simply run it through solid state rectifiers and presto-chango … DC. Generators were invented before silicon was discovered so a mechanical method was devised to change AC to DC, commutator and brushes. The armature coils are connected to a segmented commutator that spins with the armature. Fixed brushes “connect” the voltage generated by the armature coils to the outside world. By virtue of the placement of the brushes and number of commutator segments, the ends of the armature windings are “rewired” back and forth to keep the polarity at the brushes the same.

So remember:

* Generators use stationary magnetic fields, moving conductors, rotating motion, and mechanical rectification.
* Alternators use rotating magnetic fields, stationary conductors, rotating motion, and electronic rectification.
* Conventional alternators use rotating electro-magnets.
* Harley alternators use rotating permanent magnets.

To increase the output of a generator, the output from the brushes is fed back into the field coils (electromagnet) to increase the magnetic field … which in turn increases the output … which increases the magnetic field … etc., etc. But Tom, where does the first magnetic field come from to start the process?? Good question, it’s residual, the field poles are made out of a soft iron material that “remembers” the last polarity it had. If your generator sits for several months or years the residual magnetism may dissipate and there won’t be a “seed” to start the generating process. Worse, if for some reason the polarity of the residual magnetism gets reversed it will actually try to charge backwards! Remember, the commutator and brushes are designed to keep the output at the same polarity, but, unlike silicon rectifiers, they don’t care what polarity it is. That’s why generators need to be polarized.

The polarization process places the proper polarity voltage across field windings, turning the pole shoes into electromagnets briefly. When the voltage is removed the iron field poles are left with the proper residual magnetism. There are 2 generally recognized generator/regulator circuits that have been used over the years, fortunately Harley used the one that makes it easy to polarize. Since one end of the field winding is already grounded through the voltage regulator, all you have to do is connect the battery (B) to the armature (A) terminal to put voltage across the field coils. It only takes an instant which is why they often say to “scratch” or “flash” a jumper across the terminals. Most books, including my 1963 high school auto shop text book, recommend polarizing the generator every time it has been disconnected. In my years I’ve had to polarize 2 airplanes, a dump truck, and a farm tractor, to get them to work, but I have never witnessed a generator that got “reverse” polarized.

A generator regulator typically has 3 functions, regulate voltage, limit maximum current, and disconnect the generator when it’s not running so the battery doesn’t go dead. An alternator regulator is similar but uses isolation diodes instead of a cut-out relay to save the battery. Battery?? Normally a generator system is connected to a battery but not always, I remember every time I hit 6000 rpm on my ’61 XLCH the headlight would pop. A battery is an ultra-low impedance device that hates to change voltage levels. Put a voltmeter across your battery and rev up your bike, notice how long it takes to increase the voltage? That’s the “brute force” filtering effect of the battery. Another “brute force” filter is a large capacitor, it also hates to change voltage levels and keeps the short-duration high-voltage spikes from killing your lights. However it’s no battery, it may hold a charge for several minutes after you shut the bike off but it’s not capable of storing enough energy to re-start your bike, they’re exclusively for magneto bikes.

An interesting thing about Harley’s alternator system, normal alternators use battery voltage to generate the field. So if the battery is stone dead, no amount of pushing will bring the system to life. With Harley’s permanent magnet field system, a full field is always present so if you can push it, it should start.