Repairing Two Brush Generators

In todayís throw away world it seems no one wants to fix things any more. That is probably because most of the things built today are designed as throw away items. This is not the case with Cycle Electric Inc. generators. These generators where used on Harley Davidsonís from 1958 to 1984. They are designed to be rebuilt and Cycle Electric Inc. still makes all the parts for them. If you are willing to get your hands dirty here are some tips on repairing them. 

Theory of operation

I believe it is important to understand how things are supposed to work before you can figure out why they donít work. I will start of with a little theory of operation. These generators are an A type exited field two brush DC generator. On an excited field generator the magnetic force of the field is produced by an electro magnet. This makes it possible to control the generator out put by controlling the amount of current flowing through the field coils. A type exited field generators draw the field current from the armature. B type excited field generators get their field current from a battery. 

The field circuit consists of two coils of wire wound around iron poll shoes. The poll shoes are mounted opposed each other on the inside of the generator housing. One end of the coils connects to the A terminal and the other end connects to the F terminal on the generator. The armature is mounted on bearings and held between the poll shoes. When the generator is first built it needs to be polarized. This is done by flowing current through the field coils. This current makes a magnetic field and magnetizes the poll shoes. One shoe will have its north poll facing the armature; the other poll shoes south poll will face the armature. The polarity of the poll shoes will determine the polarity of the out put voltage at the A terminal. When polarizing it is important the make the A terminal more positive than the F terminal. Flowing current the other direction will make the A terminal have a negative voltage.

When the generator starts to spine, the residual magnetism from the poll shoes will induce a voltage into the armature windings. Two carbon brushes are used to make an electrical connection between the commutater on the armature and the brush plate. One brush is positive and the other is negative. The positive brush is connected to the A terminal with a wire. The negative brush is grounded to the brush plate. When the armature voltage gets high enough to turn on the regulator, the regulator turns on and ground the F terminal. This will allow current from the A terminal to flow through the field coil to ground. At this point the generator is full fielded and producing full power. A properly working regulator monitors both voltage and current. If either voltage or current go high the regulator shuts down the field. When the field shuts down generator output drops off. Voltage and current go low so the regulator turns back on. This cycle repeats it self several times a second to maintain proper voltage or limit current.

On less you have a generator test stand it is easiest to test the generator while it is mounted on the motor. See testing two brush generators. Once you determine that your generator doesnít work, remove it from the motor. Find a suitable stand with that will support the generator with the armature shaft sticking down. Remove the chrome end cover if installed. Remove the brush strap if installed. Remove the two ľ- 24 lock nuts from the generator tie bolts. Use a soft mallet to tap the commutater end bell loose and remove it. Use a 3/8Ē nut driver or wrench to remove the nut that holds the positive lead wire to the brush plate. Remove the brush plate. If you need to inspect the armature hold the generator housing in your hand and lightly tap the commutater end of the armature shaft with a soft mallet to separate the end plat from the housing. Do not hit too hard or you will damage the ball bearing.

Excluding physical damage to the housing, end bells and bearings there are three main components to test when looking for a problem. The brush plate, field coils and Armature.

Brush plate

Inspect the brush plate assembly. Check the condition of the brushes and brush springs. You can tell a lot about the generator by looking at the brushes. When the brushes are new they do not start out with perfect contact on the commutater. After a couple thousand miles the brushes seat in and should have full contact. This leaves a shinny finish on the contact aria. The brushes should move freely in the brush holders and have a shiny contact surface. A dull satin finish is an indication the brush has been arcing. A brush that is sticking, a collapsed brush spring or a commutater problem can cause arcing. Sometimes a brush will stick when the generator gets worm and the out put will drop-off. When the generator cools down the brush loosens up allowing contact to the commutater and the generator will put out again. This will cause intermittent output.  A sticky brush may also ware at an angle. Check for swollen or warped insulators between the brush holder and plate. Replace if necessary.

Use an ohmmeter or continuity tester to test for continuity between the brush holders and brush plate. The positive brush holder should be insulated and has no continuity to the brush plate. The negative brush holder should have continuity to the brush plate.

Field Coils and terminal insulators

One end of the field coils connects to the A terminal and the other end connects to the F terminal. The easiest way to test the field coils is with an ohmmeter. Check the resistance of the field coils. Use an ohmmeter on the Rx1 scale. Probe between the A and F terminals 6-volt field coils should read between 2.5 and 3.5 ohms. 12-volt coils should read between 5 and 6 ohms. With the brush plate removed, probe between the A terminal and the case of the generator. This should show no continuity. If you get the correct readings the field coils and insulators are good. If not you will have to disassemble the terminals and check the insulators and field coils separately. A field coil that has no continuity has a broken wire or a bad connection. This almost always happens at one of the terminals or where the two coils are spliced together. This can usually be fixed. Low resistance indicates winding-to-winding shorts. A sorted coil will need to be replaced.

The easiest way to get the poll she screws out is to drill them out. Start with a center punch in the middle of the screw slot. Then use a drill a little larger then 3/8. Drill slowly until the head of thee screw pops of.


The best way to test an armature is with a growler. The growler is a tool that is designed to magnetically excite the armature. Once excited the armature can be tested for winding-to-winding output, shorts and opens. If you do not have access to a growler the only way to test the armature is to install it in a working generator and test for proper out put. You can start by removing the armature and giving it a visceral inspection. The steel lamination stack should be shiny. A little rust wont hurt any thing. If it is discolored from heat, this is an indication of a problem. The windings should be more of a Caramel copper varnish color. Black or burnt winding in another indication the armature has been over heated. Inspect the bars on the commutator for sines of uneven ware. Shorted winding can cause arcing. This will cause excessive ware on one or two of the bars on the commutator. Inspect the bearing surface on the commutator end of the shaft. The needle-bearing ride on this surface so it needs to be in good shape.  An armature that shows any of these problems should be replaced. Reassemble the generator and test for proper out put. If the field coils and brush plate check out good, and you do not get the proper out put try a new armature.

Of coerce if you donít want to get your hands dirty or just want a the reliability of a new generator with a two year guaranty we make new ones every day.

Let the road you choose to ride bring you happiness.

Karl Fahringer

Cycle Electric Inc