Gary Parsons at the SVR gave me this explanation of the workings of the Wolverton regulator - I believe it came from a BR maintenance manual. This is available on request - just email me.

The Wolverton equipment is a single battery system utilising a plain shunt wound dynamo. The dynamo is controlled by an automatic field regulator which senses dynamo current and voltage. Drive is derived from an axle mounted pulley. The belt must be tight enough to prevent any slip.
There are two sizes of the equipment, viz.

The dynamo brushes, pulleys and suspension lugs are the same on both sizes. The physical dimensions of the MD and CMD regulators are also identical.
The MD regulator is a development of the earlier AR and with the exception of the lamp resistance, which in the MD is divided into two sections, is interchangeable with it.
The operation of the MD regulator is described below. Assume that the lights are OFF.

When the dynamo has reached sufficient speed to generate 26.5 to 27.5 volts, the Dynamo Shunt Relay (DShRe) is energised, closing the Dynamo Shunt Relay contact (DR/1), this in turn energises the Cut In Relay (CIR). When CIR is energised, contacts CIR/1, CIR/2 and CIR/3 close , and contact CIR/4 opens. Current from the dynamo will now pass through the Regulator Shunt Coil Resistance (R6) and the Regulator Shunt Coil (RShC). Current from the dynamo also passes through the Dynamo Series Relay (DSeRe), Regulator Series Coil (RSeC) and Regulator Series Coil Stabiliser Resistor (VRI) to the battery. It is mainly due to the action of current passing through RShC that the regulator performs its - function. The addition of Rectifier (DI) is to prevent arcing across contact DR/1 when CIR is de–energised. With the lights OFF, the whole of R6 is in series with RShC. The dynamo voltage will be automatically regulated so that it is slightly greater than the battery voltage until it reaches 32 volts. At this time the charging rate will be reduced to a very small value.
When the battery is completely discharged, the rate of charge will be very high. A very large current will flow through VRI and RSeC. RSeC is so arranged to ASSIST RShC so that the rate of charge of the battery remains within acceptable limits. The function of the Regulator Series Coil Stabiliser Resistor (VRI) is to damp any, oscillation of the regulator. The resistance of VRI can be changed in relation to that of RSeC such that the maximum flow of current to the battery is limited to any desired value.
As the battery becomes fully charged, its terminal voltage rises, current in VRI and RSeC falls. Simultaneously, due to increased battery voltage, current in RShC rises such that it reduces the current to the battery to a very small value. The Dynamo Field Regulator (R7) is varied according the position of a piston. The piston position is determined by the interaction of the magnetic effects produced by Four regulator coils RSeC, RShC, RTC and RDFShC. RSeC, RShC and RTC are wired to ASSIST one another. RDFShC is wound to oppose them.
Assuming the dynamo revolves below "cut–in" speed, that is the speed at which CIR is energised, the regulator piston is nearly static due to the balancing action of RShC and RDFShC. As soon as CIR is energised, CIR/1 closes, assisting RShC. The regulator piston moves, opening the regulator contacts (not shown), increasing the value of R7. The regulator resistance (R7) is arranged so that the voltage produced by the dynamo is just greater than that of the battery so that the charge is maintained. As the dynamo speed increases, its field current is decreased by the increase of resistance R7. RDFShC has LESS influence on regulator operation. As speed decreases the regulator resistance R7 is at a minimum and CIR/1 opens again, disconnecting the dynamo from the battery.
With the lights ON, the operation of the dynamo is as follows: With LR/1 closed. Assuming CIR/l to have closed and CIR/4 to have opened, current from the dynamo will flow to the battery as before, but will also flow through the lamp resistance (Rl), the Lamp Resistance Shorting Relay (LRShoR/1), the lights relay contact (LR/1) and on to the lights. The value of the lamp resistance (R1) is such that there is a 2 volt drop across it which allows sufficient potential across the battery to secure correct charging rate while simultaneously preventing the lights from excessive voltage. A section of lamp resistance (R2) is introduced into the lighting circuit when the volt drop across the lights reaches 25.5 volts. An additional volt drop of 4 volts ensures that the battery will be fully charged when the lights are on.
The conditions whereby the Lamp Resistance Shorting Relay (LRShoR) is energised is determined as follows. Consider gradually increasing dynamo speed.
The Voltage Control Relay (VCR1) is coupled to the circuit such that when the lights are ON, VCR1 becomes energised when the voltage at point "A" has risen to 25.5 volts. When this occurs, contacts VCR1/1 and VCR1/2 close. Thus if the Regulator Contact (RC/1) is closed, LRShoR will be energised, but RC/ 1 is operated by the regulator piston itself and will only close when the regulator has reached a specific stage in the regulation cycle.
Additionally when VCR1 is energised (voltage control stage l) current also passes to the Regulator Shunt Resistance (R6) or to be more specific, the junction of R6/1 and R6/2. The additional current in the Regulator Shunt Coil (RShC) thus strengthens RShC which inserts more Dynamo Field Resistance (R7) into the dynamo field circuit thus reducing its output.
Additionally, current also passes through the Voltage Control Relay 2 (VCR2) which itself is energised when the voltage on the lights rises again to 25.5 volts, closing VCR2/l. When contact VCR2/1 closes, resistance R6/2 is shorted which has the effect of strengthening RShC causing the output of the dynamo to fall further, when the battery should be almost fully charged.
As the dynamo speed is decreased the reverse effect occurs until after the Cut In Relay has de–energised. The lights remain across the batteries as long as LR/1 is closed.
The Retaining Resistance (R5) permits sufficient current under certain operating conditions to maintain VCR1 in its energised state. Consider a fully charged battery, small lighting load and the dynamo running at average speed. The regulator will produce an output LOWER than that of the battery. Consequently there will be a brief battery discharge current which will cause CIR to de-energise until the battery voltage has fallen to about 25 volts when CIR will energise again. When CIR de-energises, CIR/2 opens which would cause VCRl to de-energise opening contacts VCR1/1 and VCR1/2. The action of shorting R5/1 causes sufficient current in VCRI to retain it in energised state. As the dynamo slows down, VCRI remains energised due to the current passing through R5/2 to enable VCRl/1 and VCR1/2 to remain closed until the voltage on the lights falls to 24.5 volts. With all the lights on this occurs at the same time as CIR is de–energised, but with small lighting load and a fully charged battery a few minutes passes before VCRl/1 and vcR1/2 open after the dynamo stops.
The Toggle Coil (RTC) has its circuit completed by the Regulator operated contact (RC/1). RTC's action is to ASSIST the closing and opening of RC/1, while RShC remains to regulate the dynamo output as before.
When the dynamo is not generating, current flows from battery positive through Regulator Series Coil (RSeC) and VRI via contacts CIR/4 and LRShoR/1, LR/1 to the lights and back to battery negative.