- by Pete Snidal (C) 2001

A reader of the Collected Works emailed me the other day and suggested a piece on the older generator/dynamo systems found on the pre-'60's Brit bikes would be helpful. I sent him back a note telling him I didn't know much about that old stuff, and then got to thinking, "Hmmm, maybe I do......." So here it is - as usual, part theory, and part experience.

Generators, or Dynamos as they were called in England, were used on Cars and Britbikes until the early '60's. The major reason they were replaced by alternators, regardless of other claims to fame, was simply cost - alternators are cheaper to produce, requiring less tolerance and handwork, and having fewer moving parts.

The Generator also produced Direct Current, and since the rectifier required to make the DC needed for the vehicular system (charging battery, mostly) was a complicated thing prior to the days of solid-state electronics, generators were a given in the electrical systems of all vehicles until that time. The Selenium Rectifier, and later, it's more efficient cousin, the Silicon Diode, changed all that, and so we universally have Alternators on vehicles today.

However, those of us fortunate enough to have landed a pre-1960 motorcycle may have inherited a Generator/Dynamo system, and thus may wish to understand its foibles. Hence this little rant.


An understanding of the principles involved will go a long way toward preparing one to figure out the foibles of anything mechanical, so let's look at what the various mechanical Elves and Fairies do in these systems.

First, they depend on 3 facts:

Oho! AC, or Alternating Current. First one way, then the other. So far, I have described the Permanent Magnet Alternator. This, in a very simple form, is exemplified by the bicycle "generator" (actually a simple Alternator) which rubs on the tire to make lights - remember them? They work fine, but can be improved upon with some complications, which were required to solve two problems in Automotive/Motorcycle applications:

First, the AC. AC won't charge a battery. The current flow must only be in one direction. Since before the '50's, rectification could be done only with Vacuum tubes, which required serious voltages, and which were too delicate for vehicular apps anyway, the Commutator was developed. This is a series of copper bars on one end of the Generator armature (the spinning part) which were connected in opposite pairs to coils in the armature. As the armature rotated, the connection to the coils was reversed each 180 degreees as the armature turned, thus the output current was only in one direction. The connection to the spinning armature was maintained by carbon brushes which ran against the commutator.

Second, the output current varies with engine rpm, Furthermore, the current demand of the system varies with load - the state of charge of the battery, the number of accessories being driven (heater, lights, defrosters, electronics, etc.), and thus the output of the generator must be capable of being varied to match these parameters.

The output is varied by three things: the number of turns in each coil of the armature, which of course is fixed once the thing is built, the rpm of the engine - a variable which in itself must be accounted for, and the strength of the magnetic field within which the armature is rotating.

Aha! It is of course the third variable which the designers sought to control. They did this by using not a permanent magnet field, but an electromagnetic one, which would vary with the voltage being applied to it. The higher the voltage, the stronger the field, the greater the output voltage, (and therefore the current output) of the generator. This field voltage was supplied by a device which sensed the system voltage, applying a varied field voltage as necessary, called the Voltage Regulator.

So that's all there is to it. And what this means to the owner of such a system is that all s/he needs to do is first to get the generator working, and then to get a field voltage controller in place to keep things running at the correct levels.


Testing the ability of the generator to produce current is fairly simple - connect a load of some kind, such as a headlight sealed beam, get the thing spinning somehow - holding the drive gear against a drill press belt, or buffing wheel, or even starting the bike with it in place will do this just fine -, and applying some voltage to the field windings to get it started generating will do the trick.

If the system is still in one piece, to check the system, all one need do is measure the system voltage at the battery. It should increase from around 12V to 13.8 (or so) when the motor is started. For a quick check, starting the motor and momentarily disconnecting the battery will serve to check. generator output. If the unit has coil ignition, and it keeps running, then the charging system is obviously working. With magneto ignition, turning on the lights will give you a check. If the indication is that there's no output, first check the regulator by looking for field voltage between the generator's "F" terminal and ground (it's a negative voltage on (+) ground systems, remember.) If no field voltage is present, proceed as described below. If one is, you have trouble. Read on, as we discuss testing the generator out of the bike or car.


A generator which has been used recently will begin to produce current on its own, with no field voltage applied. This is because of a phenomenon called residual magnetism - some magnetism remains in the soft iron field core between uses, and this will "kick off" a rising spiral if the field is simply connected to the armature output for testing. But if the generator hasn't been in service for some time, this "jumping" of the A and F terminals won't be enough to tell you if you have a generator. You'll have to connect some battery power to the field while testing for armature output.

So here are the test connections: First, connect the load ( a headlight high beam is fine. One side to the case of the generator, the other to the generator's "A" terminal. Now for the field winding. Connect the case of the generator to the battery (+) terminal. You want control of the output, so you will only momentarily connect the "F" terminal to the battery (-) terminal to provide field voltage when you have the generator spinning. Get it spinning and do this, and your headlight should light up. Remember that supplying a full 12V to the field will result in full field voltage, which will easily blow the headlight, so a momentary connection is all that will be required - or you could have connected two headlights in series - they will handle 24V this way, which the generator probably won't put out, even with full field voltage.


Of course, maybe your load won't light. If not, check your connections, ensure that the headlight is't burnt out (use the battery to test them both) and if all these check out, then your generator's in trouble. It will in all likelihood be an open field, since there's only one field circuit, but the armature circuits are multiple, each being connected to an opposite pair of commutator bars - spinning in a magnetic field, each good pair should produce some output, and the likelihood of them all being blown is low.

If you're getting no output, the first thing to do is to check the field continuity - although if the field is shorted, it will of course show conductivity. You might try taking the generator apart, and applying voltage to the field windings, to see if they become a magnet - do they draw power (look for a spark when disconnecting) - will they attract steel objects to their poles when energized? If not, it's safe to assume it's time to go looking for another winding or set of windings.

You might try another field, ie case, if you're fortunate enough to have a collection of bits. If not, look for a good and old automotive electric shop, and have them test the generator. Anyone without a bit of silver hair should be regarded with suspicion, since it's been quite a while since the days of generators. The two individual field windings are replaceable, if you can find some OEM parts somewhere, and in the negative case, should be rewindable.

If the fields check out, then the problem would appear to be something to do with the armature. Are the brushes properly connected, and of sufficient length to contact the commutator properly? Are the commutator bars insulated from one another, or is there a buildup of carbon between them (you can clean the spaces with a blade of the proper width.) Finally, a silver-hair repair shop may still have a "growler" - a device which is used to check armatures of motors and generators.

(Note: in the case of automotive apps, the obvious answer to generator problems is to replace the generator and regulator with a later-model alternator system from the same kind of engine - see your local auto wrecker for a bolt-on retrofit solution. But this can't be done with motorcycles; the setup was completely changed.)


If the generator checks out, congratulations! Now all you need is a Voltage Regulator - actually, a Field Voltage Regulator. The only ones I've ever seen were, of course, mechanical - no solid-state electronics. Fortunately, since they all work on the same principle - that of varying the field voltage to correspond with sensed output voltage (itself varying with load and rpm,) many different brands of voltage regulator can be used. In The Day, the Lucas regulators, being expensive and a trifle delicate, were often substituted by American Delco here in Canada - I used some from mid-'50's Chevrolets with good results. Again, you should find an OLD automotive electrics shop to source a suitable one of these. Avoid Ford parts - many models of Ford, at least here in North America - grounded the field terminal to increase output instead of applying voltage. Different setup.

Once you've got a suitable regulator, and you're generator's working, all you have to do is make the connections. D on the Dynamo to A on the regulator, F to F, and BAT to battery hot terminal, through the ammeter if you have one.


There may be a solid-state field voltage regulator which can be used with generator/dynamo systems - in fact, it's possible that one from an automotive alternator system would do the trick. (Unlike the permanent-magnet alternators found on our favourite motorcycles, automotive (and many Hitachi) systems use a variable-field alternator - these are superior to PM alternators, but take up more space, and aren't really necessary to basic motorcycle applications. An external regulator, such as found on the early Chryco alternator systems, just may work with a generator system - the principle is the same; the regulator just senses system voltage, raising or lowering the field voltage as necessary to maintain an SV of 13.8. Not having a generator system around to try one on, I can't say, but I think I'd give it a try.

Positive and Negative Ground

There will be one major kink in trying to use an American regulator with a Lucas system, however - the American regulator will be negative ground, and since solid-state gear is so sensitive to these things, you'd have to convert your system to negative ground as well.

This is not insurmountable. You can easily repolarize a generator. Simply reversing the battery terminals (watch the wire colours - no sense driving all subsequent owners and mechanics into early graves) and/or then connect the generator case to the (-), and the armature output to (+). Get the thing spinning, and momentarily connect the field terminal to (+) once or twice, and bingo! - your generator is now negative ground! Ready for a primitive American solid-state VR.

I'd be interested to hear from anyone who tries this - Click here to tell me if you find it works - thanks, but there will still be one more problem to surmount:

The "Cut-Out"

When a generator armature is producing a higher voltage than the battery, current will flow to the battery. But when the opposite is true, the current will flow in the other direction, causing the generator to "motor." (in fact this is one way to test a generator - apply battery (-) to the field and armature terminals, and (+) to the case, and it should spin.) This means of course, that when the motor isn't running, the generator armature must be disconnected from the system. The old mechanical regulators were in fact two devices in one - the field voltage regulator, and the cut-out. Since an alternator system is protected from this reverse flow by the very nature of the rectifier (passes current in only one direction), an alternator VR would not in itself be enough; if the enterprising tuner were to find that an alternator-style VR worked with the generator, s/he would also have to provide against this reverse flow possibility. No problem, solid-state electronics to the rescue once again! All would be taken care of by the incorporation of a 15 or 20A diode in the charging circuit to prevent this reverse flow. See your friendly local electronics supply store for such a gizmo.


I hope this little treatise will help you to understand your generator system - and to make it work if it's broken. Now you have all the information I have.