Real Classic


Stuart Francis has been fault-finding and fixing electrical systems on old bikes for more than four decades. Dynamos, he says, can be very reliable when correctly set-up and maintained…


Stuart Francis has been fault-finding and fixing electrical systems on old bikes for more than four decades. Dynamos, he says, can be very reliable when correctly set-up and maintained…

Dynamos, like magnetos, are unfairly blamed for all kinds of electrical problems. The dynamo can be a very reliable device, but as time has gone by so owners have found novel ways of abusing and misusing these trusty devices.

The first dynamo was demonstrat­ed by Faraday in 1832. It used fixed magnets to create the magnetic field and the voltage was varied by how quickly the generating coil rotated in the magnetic field. The first breakthrou­gh in creating a practical dynamo for industrial applicatio­n was replacing the fixed magnets with an electrical-magnetic (field) coil, powered by a separate small dynamo. Then came the discovery in the 1850s that a dynamo could self-generate. They noticed that with the dynamo disconnect­ed and the field coil disconnect­ed a small residual magnetic field in the dynamo still generated a small output voltage. Connecting the small output voltage to the field coil increased the field strength which increased the output voltage, which created a cascade effect (self-generate) until a balance was achieved.

By the start of the 20th century, when innovators were fitting electrics to motor vehicles, the dynamo was well understood. However most dynamos were driven by a constant-speed prime mover, like a steam engine, and the output could be manually connected and adjusted to match the load. The new challenge was a prime mover that was constantly changing speed and the load could change faster than could be manually adjusted.

The following is a practical guide to conducting checks and limited repairs before calling in the profession­als. It’s a guide for the

things to look for, it is not definitive and no doubt other dynamo dabblers will identify further issues.


THE ARMATURE is the rotor that carries the coils of wire which generate the current. The coils of wire are joined together like a daisy chain, the end of one coil and the start of the next are joined at the commutator segments. The coils are buried in the laminated body of the armature and insulated from the body. The thin steel lamination­s of the body reduce electrical / mechanical losses and increase the output

FIELD COIL/S creates the magnetic field which the armature rotates through and generates the current. The field coil is usually a large number of turns of fine copper wire wound in a roughly rectangula­r shape. The field winding(s) are insulated and encircle the field coil pole piece(s)

THE COMMUTATOR is a series of insulated copper segments connected to the armature coils which, through the carbon brushes, provides the direct current output. It acts as a mechanical rectifier, turning the alternatin­g current being generated in the armature into DC

CARBON BRUSHES take the current from the armature. Early dynamos had three brushes, two main output brushes either side of the armature plus a thinner moveable brush for the field coil. The most common British dynamos had a pair of main output brushes either side of the armature. Multiple pole dynamos, like the Lucas MC45, had two pairs of linked brushes at 90 degrees to each other or like Bosch had a single pair of brushes at 90 degrees to each other

THE BODY holds all the mechanical parts together and has the vital job of completing the magnetic circuit. To create the most intense magnetic field, the magnetic core should be an unbroken loop, however to accommodat­e the armature there has to be a couple of thin air gaps


The dynamo’s challenge is to control its output to match the electrical load without overchargi­ng the battery or putting too much voltage through the electrical system, while coping with an ever-changing engine speed. There are many types of dynamo but the following four are the most common on old bikes.

THIRD BRUSH: one of the earliest types. A movable third brush picked off the field coil current from the commutator. The third brush is usually thinner than the main brushes. The third brush had summer and winter settings to compensate for the increased use of lights in winter. These types were usually connected to a very large battery, as it was usually impossible to achieve the ideal output from the dynamo

SWITCHABLE COILS: the next generation retained a third brush system with an additional field coil that was connected when the lights were switched on, providing additional current to balance the load and charge the battery. The Harley-davidson 32E generator, fitted to their 1940/50s machines, was one such system. These types were also connected to a very large battery, although it was still impossible to achieve the ideal output from the dynamo it was a great improvemen­t

REGULATED FIELD COIL: in 1936 Lucas introduced a 2-brush 40-Watt dynamo with a single field winding, linked to a new control box called MCR1 that automatica­lly regulated field coil current and therefore the output, with no rider input required. Lucas subsequent­ly introduced the longer body 60W dynamo and MCR2 regulator in 1949. The 75W C35SD for the Ariel Square 4 used a pair of interconne­cted field coils

REGULATED MULTIPLE FIELD COILS: commonly adopted in Europe. A Lucas version (the MC45) was fitted to the S7 and S8 Sunbeams and Birmingham Scotts. The most common arrangemen­t was two pairs of field coils mounted at 90 degrees to each other. More power could be extracted from a multiple field coil arrangemen­t and they could more easily be packaged to fit within the engine’s cases. They were superseded by alternator­s when relatively cheap, good quality, semiconduc­tor power rectifiers became available


Is it actually the dynamo that is causing the problem? Does the dynamo rotate when the engine is running? Problems like loose or slipping drive belts, broken drive chains, slipping sprockets, stripped teeth on the Mag-dyno fibre drive gear or a slipping Magdyno clutch can cause problems. The difficult ones are where the armature drive is slipping but it looks OK. The flat leather belts on early Velocettes are good at this; fit replacemen­t rubber drive belts. If the dynamo is making a noise as it goes round then remove it immediatel­y.

If the drive side looks OK then disconnect the dynamo connectors from the cut-out and voltage regulator, and connect the F&D terminals of the dynamo together. With a bit of throttle opening, the dynamo should produce more than 6V between the connected F&D terminals and the dynamo body. Once running and generating at a good speed, it should be able to light a headlight bulb connected to the same points and earth. Do not immediatel­y connect the bulb as it will kill the selfgenera­tion process. If this works there is a problem with the wiring or the cut-out and voltage regulator, the CVR.

If the machine has been standing for some time the residual magnetism may have seeped away to a point where the normal self-generation process cannot start. Try giving it a big handful of revs: this sometimes works. Alternativ­ely, momentaril­y close the cut-out to put current into the field coil. Or momentaril­y connect the live side of the battery to the dynamo F terminal to put current into the field coil.

This would work with a standard dynamo. If an electronic regulator or 12V conversion has been installed, consult the fitting instructio­ns. Some of the early 12V conversion­s had the field coil wired differentl­y.


With the dynamo removed, take a long hard look at it before taking it apart. Is it damaged, is oil dripping out of it, are all the screws there holding it together, does it rattle and is there any end play or side play on the mainshaft? Removing the end cover – use your senses again: does it smell of burnt wiring (once smelled never forgotten), do the wiring and connection­s look tidy and tight (not bits of twisted wire insulated with Sellotape!), is there a lot of carbon brush debris or oil or both, are the carbon brushes badly worn, does the commutator look badly worn or burnt?

If there is engine oil inside the dynamo then the oil seal in front of the drive end bearing is damaged or the shoulder it is sealing against is damaged.

Before stripping down, give the commutator end of the dynamo a good clean out with brake cleaner to get rid of all the muck that builds up over the years. Next check the carbon brushes. Unhook the springs from the top of the brushes: are the springs broken, bent or catching on the brush holder? Do the brushes slide up and down in the holder without sticking?

The brushes should be at least as tall as they are wide. Buy brushes from a reputable source as carbon brushes come in a variety of densities. Over the years I have bought some duff ones. One set were so soft they turned to powder in three months and another set were so hard they refused to bed down and started wearing the commutator.

Inspect the commutator: is it mechanical­ly sound? Is the commutator badly worn where the brushes touch? Commutator­s can be cleaned up in a lathe with a sharp tool and light cuts. If the commutator is slightly worn or just tarnished then a quick clean-up with very fine wet-and-dry paper is in order. Between each segment of the commutator is a thin insulation strip, the top of which should be just below the copper segments. The small gap between the segments can fill up with carbon dust over time causing problem. A broken hacksaw blade ground to create a thin hook is ideal for cleaning out this muck.

If the commutator has been skimmed, the insulation strips need to be undercut. A short piece of hacksaw blade with the clearance ground off the sides of the teeth can be used for this.

Next, check the commutator with a magnifying glass, looking at the connection­s to the armature coils. Each segment of the copper commutator has two copper wires (from the windings) soldered into them. If you can see the copper wire in the slot then the solder has

melted and been thrown out, caused by too much current – a common problem before current compensati­on.

It’s harder to diagnose a ‘dry’ joint. They exhibit a grey grainy dull appearance, usually caused through too much current or just being badly made. If you spot one of these faults there is invariably another. These joints can be re-soldered; I use an old fashioned copperbloc­k soldering iron heated with a gas torch. I re-solder any connection­s that don’t look perfect, taking care to clean the surface with brake cleaner and an abrasive fibreglass brush, and use flux paste in the joint.

Electrical­ly checking the windings can be a challenge because of the very low resistance of the individual armature coils: 0.15 Ohms – 40W and 0.2 Ohms – 60W, for each individual coil. Most multimeter­s cannot accurately or reliably measure at this low level. Also multimeter­s only put a very small current through the windings, and some faults do not appear until a decent current is flowing. Never measure through carbon brushes – they can produce misleading results at low currents.

I built a very simple test rig to check out the windings. The armature sits in a V-block and a pair of electrical contacts touch either side of the commutator. I use a variable current power supply set at a nominal 5Amp, which usually results in about 2-3V across the electrical contacts. This can also be achieved with a good 6V battery with a 30W bulb wired into one of the supply leads or a 12V battery with 60W bulb. The bulb works as a current limiter to stop the winding being cooked!

With a multimeter, check the voltage across adjacent commutator sections. They should be identical at about 1/6th of the voltage (about 350 to 400mv) across the whole armature. If the voltage drops vary significan­tly across the commutator there is probably a dry joint at one or more of the commutator connection­s. If there is no voltage or the full voltage, then there is a break in the winding. To find the break leave a multimeter probe attached to one of the supply contacts. Starting at the other contact, check the voltage at each commutator segment. The voltage will drop from 3V to 0V after passing the defective armature coil. Have a good look at the commutator connection­s: thrown solder can leave the connection open circuit.


The battery is a critical part of a dynamo system. It supplies current when the dynamo is not generating enough, it stores the excess current, it smooths out the output voltage, and can provide a small current to initially excite the field coil. The very early systems relied upon relatively large, lead-acid batteries to smooth out the peaks and troughs of current generation. Battery technology has improved and dynamo outputs better controlled, allowing smaller batteries to be safely fitted. The key criteria to look for is the Ampere Hour rating, it specifies how much current the battery can supply for an hour at the rated voltage. Invariably a battery will have it marked as part of the battery number: ie. 6V-8AH (6 Volt 8 amp hour).


The cut-out is usually an electrical relay that connects the dynamo to the load at a given voltage, and disconnect­s when the output voltage falls. However it is not unusual, even for a properly adjusted cut-out, that when the voltage falls the contacts remained closed even when battery current was being fed to the dynamo. A small amount of back current can be expected, but larger amounts can burn out the relay contacts or worse weld them together.

Cut-outs were initially a separate unit which were then incorporat­ed into the dynamo body (like Miller) or became part of the voltage regulating unit. Modern power diodes can easily replace the relay and its contacts, creating a more efficient and safer arrangemen­t. Check the cut-out contacts for burning. I always give the contacts a quick touch up with a fine diamond nail file or fine wet-and-dry.


The final electro-mechanical device for controllin­g the dynamo output was the automatic voltage regulation and current control box. The new device took the cutout and added a voltage control relay and incorporat­ed current compensati­on. Miller chose to keep the voltage regulator as a separate unit that sat on top of the dynamo

As the output voltage of the dynamo exceeded the correct charging voltage (usually around 7.2V) the voltage regulator contacts open, adding a resistor (around 30 Ohms) into the field coil circuit, reducing the voltage output and which closes the voltage regulator contacts. This cycle of opening and closing the contacts maintains the correct voltage and ‘buzz’ when working properly.

An additional winding was incorporat­ed on the coils that conducted the output current. As the current output increased, the field generated by these additional winding would open the regulator contactors, reducing the output voltage and current protecting the dynamo from an overload.

Recently the Miller dynamo on my 1946 Velocette MAC stopped charging. I connected the D and F terminals but didn’t disconnect the CVR. As soon as the engine was running, the dynamo cut in and started charging. Removing the connection between the F&D terminals stopped it charging, then shorting out the field terminals in the voltage regulator restarted it charging. The contact faces were burned and dirty. A good clean up restored normal service.

Most manuals contain details of air gaps, settings and voltages for these units. I know through bitter experience that trying to achieve exactly the right voltages while running the machine is frustratin­gly difficult. So I recommend that before playing with the settings, you give the contacts a good clean and as see what happens. The cleanlines­s of the contacts is critical; a quick touch-up can make a world of difference.

If you are changing the CVR (or it has never worked) check the connection­s. Originally Lucas used FADE (Field, Armature, Dynamo and Earth) as the connection order but later changed to FAED


I was once told that converting Lucas 6V dynamos to 12V was easy: all that was needed was a 12V car dynamo regulator. I tried this a couple of times with initially good results but they failed with burned-out field coils after a short time. PUB told me that the 6V field coils didn’t like being taken above 8V and would cook at 12V. So to produce a reliable 12V converter, the electronic­s have to limit the field coil current to what it would normally be at 7V.

As the dynamo output reaches 14V (roughly

the charging voltage of a 12V battery), the field coil current is further reduced and regulated. The electronic­s also sense the supply current and further reduce the field current if the current supply is too great. The electronic­s also incorporat­e a power diode that takes on the function of the cut-out.

An alternativ­e approach would be to have the field coil rewound to take 12V, an expensive option, but with a small extractor fan added to the armature it could possibly double the power output. I once built a 12V converter, designed by PUB, for my Ural. It worked well to keep a small car battery fully charged and a Honda doubleende­d 12V ignition coil improve starting no end.

The downsides of converting a dynamo to 12V are rarely discussed. The main issue is the speed the dynamo cuts in and the speed the full electrical load is balanced at. To generate 12V, the dynamo and therefore the engine have to spin at a higher speed, with the same for the full load balancing speed. Manufactur­ers tried to design the dynamo drive to achieve a full load balancing speed of 30mph in top.

Experience with a couple of 12V conversion­s indicate both speeds have increased between 5 to 10mph. It is not really an issue unless you are doing a lot of winter night riding in heavy traffic and even then LED bulbs would make a world of difference.


What do you do if your dynamo didn’t come with the bike but is new to the machine? Check that the dynamo body is properly earthed. Newly painted or powder-coated frames, engine plates and dynamo bodies can make wonderful insulators. If in doubt short a piece of wire between the dynamo body and the battery earth connection. If it sparks or the engine note changes as the charging dynamo puts a load on the engine, there is an earthing issue. Check the CVR is correctly connected.

The simplest way to get right polarity output is to first disconnect the F&D terminals from the dynamo and connect a voltmeter between dynamo terminal D and earth. With the engine running, connect a flying lead to the battery output and tap it onto the dynamo terminal F. The terminal D voltage should shoot up and be in the same polarity as the battery. If the voltage doesn’t shoot up there is a problem with the field coil, its connection­s, or the dynamo is not earthed. If the voltage shoots up but is the wrong polarity swap the brush connection­s over. Repeat the initial inspection checks specified earlier in the article.


Apart from loose and dirty connection­s, occasional­ly you can come across more obscure faults. Always give the brass female connectors on the back of the dynamo and cut-out / CVC a good clean out. I use a miniature wire brush to clean them. The curly plastic insulation strips on the brush wires can get extremely brittle and disintegra­te, leaving the wire exposed to short out on the cover. I once found a loose dynamo mounting bolt, on a Magdyno, that had cut through the armature windings. Always do a resistance check between the field windings and the body, and the armature shaft and commutator. Such insulation failures are extremely rare but not unknown.

Back in the 1970s my mate Paul had a rather nice 1952 BSA A10 with a duff dynamo, so he bought a reconditio­ned dynamo from Pride & Clarke. It was fitted but did not work. Paul returned the dynamo, but it came back by return of post with a note saying it had worked perfectly on their test rig. It was refitted but still didn’t work. As I undid the cover band on the back of the dynamo it started charging! The cork insulating strip was not in the right place.

Bosch dynamos use the current from the charging warning light to kickstart the self-generation process. If the charging light doesn’t come on when the ignition is switched on, it is unlikely to start charging.

Earlier Lucas dynamos used small edgecontac­t bearings, like magnetos. They were later superseded by decent-sized ball bearings at the drive end. These small bearings are particular­ly vulnerable to wear, overloadin­g and damage. AJS & Matchless singles seem particular­ly adept at damaging these bearings. Hidden between the engine and gearbox, the dynamo is difficult to get at. Having to remove the outer primary chain cover (a messy job at the best of times) to get to the drive chain or to remove the dynamo makes maintenanc­e difficult.

Replacing yet another dynamo on my well-used G80, I stumbled on the heart of the problem. I pinched up the dynamo clamping strap once I had the chain tension correct. After double checking everything I gave the clamping strip bolt a final couple of tweaks. Suddenly the dynamo drive chain was way too tight. Subsequent experiment­s seemed to show that the act of tightening the clamping strip slightly rotated the dynamo body, so tightening the chain. I now set the drive chain on the loose side to anticipate this affect.

This isn’t a definitive list of potential problems and solutions, just my 45 years’ experience of working on motorcycle dynamos. Do you have any other problems and solutions you would add to the list?

 ??  ?? Illustrati­ons by Stuart Francis / RC Rchive
Illustrati­ons by Stuart Francis / RC Rchive
 ??  ?? What you do not want to see! Observe, if you will, the oil. Dynamos should not be oil-filled
What you do not want to see! Observe, if you will, the oil. Dynamos should not be oil-filled
 ??  ?? Taking apart a non-functional dynamo – take it steadily!
Taking apart a non-functional dynamo – take it steadily!
 ??  ?? At one time, reconditio­ning kits for dynamos were available. This may still be the case, of course
At one time, reconditio­ning kits for dynamos were available. This may still be the case, of course
 ??  ?? A useful diagram showing an exploded Lucas dynamo
A useful diagram showing an exploded Lucas dynamo
 ??  ?? An easier way to make a dynamo work is to enlist the help of a friend. These are the hands of PUB, applying skill to an elderly E3L Lucas instrument. Of course she made it work
An easier way to make a dynamo work is to enlist the help of a friend. These are the hands of PUB, applying skill to an elderly E3L Lucas instrument. Of course she made it work
 ??  ??
 ??  ?? Dry and de-soldered armature windings
Dry and de-soldered armature windings
 ??  ?? Checking the armature windings
Checking the armature windings
 ??  ?? Cleaning the commutator gap with a high tech tool (a modified hacksaw blade)
Cleaning the commutator gap with a high tech tool (a modified hacksaw blade)
 ??  ?? Re-soldering the armature connection­s. Note the large soldering iron and flux
Re-soldering the armature connection­s. Note the large soldering iron and flux
 ??  ?? A wayward dynamo bolt sliced through the windings…
A wayward dynamo bolt sliced through the windings…
 ??  ?? Shredded fibre Magdyno dynamo drive gear
Shredded fibre Magdyno dynamo drive gear
 ??  ?? Not entirely recommende­d!
Not entirely recommende­d!
 ??  ?? New brushes are available. Do not simply buy the cheapest
New brushes are available. Do not simply buy the cheapest
 ??  ?? Brittle insulators on carbon brushes and brush springs. Note the broken spring
Brittle insulators on carbon brushes and brush springs. Note the broken spring
 ??  ?? Dirty voltage regulator points from a MCR2. And then some cleaned-up points
Dirty voltage regulator points from a MCR2. And then some cleaned-up points
 ??  ??
 ??  ?? Happy together. Dynamo and magneto in perfect harmony…
Happy together. Dynamo and magneto in perfect harmony…
 ??  ?? It pays to ensure that the dynamo is snug against the crankcase before tightening it
It pays to ensure that the dynamo is snug against the crankcase before tightening it

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