DYNAMO

The 24volt M201 was fitted with the following dynamos: Paris-Rhone G15R39, G15R48, G15R51 or Marchal BPG24 all rated at 28.5 volt, 22 amp. They are physically large for this rated output due to the fact that they are a sealed design and there is no forced air cooling provided by a fan that would normally be found behind the pulley. This large dynamo would overheat if it were allowed to operate at a the higher output that it could certainly produce.

REGULATOR

The 24volt M201 was fitted with the following regulators: Paris-Rhone ZT3115a, ZT3118a, ZT3120a or Marchal BPR24. As far as I know all of these types are of the three coil 'current voltage regulator' design which consists of a cut-out, current regulator and voltage regulator coil. The photos, diagram and following detail is based on the Marchal type but the principle of operation will be broadly similar in all cases even if the internal layout is different.

CUT-OUT: this acts as a one way valve allowing electricity to flow from the dynamo to the battery (charging) but not in the other direction. Without it current would flow back out of the battery when the engine is turning slowly or has stopped causing the dynamo to operate like an electric motor in which case it would try to turn the engine and quickly flatten the battery.

CURRENT REGULATOR: Under certain conditions (e.g. when the battery is flat) the dynamo is capable of producing a very large charging current. The current regulator makes sure that the current is limited to a maximum of 22 amps to prevent the dynamo overheating or the batteries being damaged.

VOLTAGE REGULATOR: This provides the fine control of the charging current. As a lead-acid battery becomes charged the voltage at its terminals rises. This voltage is sensed by the voltage regulator coil and used to reduce the charge rate to a small trickle by the time the battery is fully charged. Without it the batteries would over-charge causing the electrolyte to 'boil' and the plates to buckle.

(N.B. for simplicity the ammeter, battery isolator and connections
to other circuits have been omitted from the above digram)

CUT-OUT OPERATION

As the dynamo begins to turn there is enough residual magnetism in the pole pieces for it to generate a small output. (Unlike an alternator, a dynamo needs no help from the batteries it can 'self-excite'.) This small output is fed back to the field coils in the dynamo making the magnetic field stronger which in turn will make the output stronger and so on.

When the dynamo is turning fast enough its output becomes greater than 24volts and it is ready to charge the batteries. This is sensed by the main shunt cut-out coil (several hundred turns of fine wire) and at this point the magnetic field that it creates is strong enough to pull the contacts closed connecting the dynamo's output to the batteries. The charging current passes through a second series cut-out coil (a few turns of thick wire) on its way to the batteries. This adds to the magnetic pull of the main coil to make sure that the contacts snap shut cleanly without chattering.

When the dynamo is slowed down a point will be reached when its output voltage falls below that of the battery. Current will start to flow in the wrong direction. At this point the magnetic field produced by the shunt coil would still be just strong enough to hold the contacts closed if it were not for the series coil. Current flowing through this is now in the wrong direction and its magnetism works against that of the main coil making sure that the contacts snap open cleanly without chattering.

CURRENT REGULATOR OPERATION

Current from the dynamo passes through a 'series' coil of thick wire on its way to the batteries. The contacts are normally closed but are pulled open by the magnetic field if the current in the coil exceeds 22 amps. These contacts form part of the dynamo's field circuit. When they are open the field coil is connected to the dynamo's output through a 200 ohm resistor which drastically reduces the field current and hence the dynamo's output. As the output falls the contacts will close again effectively by-passing the resistor and restoring full output only to open again. In practice they 'buzz' to produce the desired limiting effect.

VOLTAGE REGULATOR OPERATION

The main 'shunt' coil (several hundred turns of thin wire) is connected to the dynamo's output and monitors the charging voltage. When this rises above a pre-determined level the magnetic field it creates together with that of the series coil becomes strong enough to open the contacts. Like the current regulator's contacts these form part of the dynamo field circuit and have the same effect when they open. The difference is that they open if the voltage is too high rather than the current, again these 'buzzing' to achieve the desired effect. The second series coil helps to ensure that the contacts open and close in a narrow voltage band to provide accurate control. As soon as the contacts open the magnetic field is immediately weakened by the loss of current through this coil. The voltage only needs to fall slightly before for the field produced by main coil is no longer strong enough to hold the contacts open and they will close again.

DIODE

This is an unusual feature in a three coil regulator which certainly makes the regulator unit suitable for negative earth operation only. It's principal purpose isn't exactly clear as it could have been included for more than one reason and I don't know if it was fitted to all types of regulator. It does not protect the regulator against the batteries being connected the wrong way round as it would be destroyed the moment that the cut-out contacts closed. However, it will effectively kill off any back-emf produced in the field coil and therefore help to prevent damage to the contacts. The french refer to a diode used in this way as "diode de roue libre" (thanks for this information go to Thierry Lefrancois who also believes that this is the most likely reason for its inclusion). Another possibility that I can think off is that the types of dynamos fitted on M201s may also have been used on other positive earth vehicles. If one of these were inadvertently fitted to an M201 the diode would prevent the residual magnetism (with the wrong polarity) exciting the dynamo. It is important to realise that if the dynamo is wrongly polarised or the batteries are connected the wrong way round then severe damage to the regulator cut-out contacts will be caused in a dynamo / regulator based charging system.

SERVICING & ADJUSTMENT

There is no regular serving required so leave well alone. If it ain't broke don't try fixing it!

The Most common problem I have come across is that the contacts of th voltage regulator coil can become coated in a fine hard to spot corrosion. There is no effective electrical connection between them and no charge at all as a result. Except in the case of a real emergency never use emery cloth or paper to clean them! A fine points file should be used (and this is a quote from from the French maintenance manual!)

The main problem is that the abrasive particles shed by emery paper are difficult to remove. These can grind away at the surface of the points during future use or even prevent the contacts connecting at all.

The contacts, particularly of the cut out,  can become 'welded' together, usually as a result of a fault elsewhere that will need investigating. It is sometimes possible to separate them and clean up the surfaces but they are likely to cause future problems requiring cleaning on a fairly regular basis. Where this has occurred it is important to check that overheating has not caused damage to any insulating features linked to the contacts. (e.g. Heat damage to the tiny plastic insulator on both the current and voltage regulator coils on the BPR24 could easily result in the contacts becoming effectively shorted out)

Precise adjustment of the coil units is a pretty tricky business and as I stressed before you must know what you are doing and have a good reason for doing it. You will need a decent accurate current / voltage measuring device e.g. AVO meter. Before going too far down this route it is well worth checking that the fault does not lie elsewhere. The diagram below shows the connections made between the dynamo and regulator through the link cable. It is also well worth checking that the dynamo brushes have not worn down to the point that they no longer make good contact with the commutator. Replacement brushes can be ordered from Lucas in the U.K. (Thanks to Baz Davis for this information). The set needed is Lucas 140410 (Replacement for Hesselmann 40/0201) I cannot verify this but I am told by Baz that they are a very close to the originals and work well.