Introduction to the Lucas voltage regulator unit
This article is a follow up to my earlier post describing ‘How the Lucas charging system works.‘ This previous article described how the system as a whole (i.e. the dynamo, battery, ammeter and voltage regulator) function to keep the battery topped up and the lights glowing.
But now I’d like to look a little bit more in depth into exactly how the regulator unit (also known as the AVR (Automatic Voltage Regulator) or CVC (Compensated Voltage Control) box actually works. I gave a brief description previously, but in order to be able to attempt to fix or adjust the regulator, a more in-depth technical understand is required.
I’m going to use the Lucas ‘MCR2′ device, as fitted to my 1951 Matchless G3LS motorcycle as a basis, but the same principles should apply to other car and bike units of the same era (although note that alternator charging systems are completely different).
Table of contents for this article:
Terminology: Shunt versus series windings
Firstly just to clarify the meaning of the terms ‘shunt’ and ‘series’ which are often used when describing DC dynamos and regulator units. These each refer to a particular way of connecting the windings (coils) with the dynamo or regulator units.
Lucas dynamo units are referred to as ‘shunt-wound’ DC dynamos as the armature and field windings are wired in parallel to one another, each with their own independent connection to earth. They are therefore also sometimes referred to as parallel windings.
Other manufacturers, most notably Bosch, used ‘series-wound’ DC dynamos in their charging circuits in which the armature and field coils are wired in series, one after the other in the same circuit.
There are also coils inside the Lucas regulator unit, four in total. Two of these coils are series-wired in circuit with the main current path to the ammeter and so are also referred to as the ‘current windings’. The other two are shunt-wired in parallel to the main current flow to the ammeter. These have a higher resistance than the series windings and so a smaller current flows, hence they are also referred to as the ‘voltage windings’.
Overview of regulator operation
The voltage regulator is a digital device in that its two primary internal circuits may each be either ON or OFF. There is no sort of variable output from the device, although as we shall see, it does have a couple of simple but clever features that allow it to adapt its output within certain limits to the operating conditions.
For this overview, I’m going to start by repeating the description I originally gave in the ‘how the Lucas charging system works’ page, thus:
The voltage regulator, as it names suggests, regulates the fluctuating voltages supplied by the dynamo into something more usable by the motorbikes electrical system. In the most basic terms it consists of two electrical contacts (switches) which are opened and closed (turned off and on) by two sets of solenoid coils. These electro-mechanical switches work in the same way as a relay; current passes through the coils creates a magnetic field which, when it is strong enough, pulls the lever of the switch towards it, thus completing (or breaking) the circuit and turning it on (or off). The automatic bit comes in to play because the regulator knows (by means of how the contacts and springs have been adjusted) at which voltages to open and close the contacts, thus automatically maintaining the desired output voltage range.
The first of these two switches is the cut-out. When the dynamo is giving less volts than the battery, the regulator disconnects it from the system so that it cannot draw any current. A DC dynamo is basically a simple electric motor working in reverse, therefore if you connect it to a suitable power supply it will try to spin just like a motor (see my article ‘Testing a Lucas dynamo‘ for more info on this). The large current drawn would quickly sap battery power if the dynamo was left connected when it was not generating, so the cut-out isolates it from the rest of the electrical system.
When the engine attains sufficient speed that the dynamo output voltage exceeds the preset charging voltage, the cut-out switch closes to connect the dynamo into the charging circuit in order to recharge the battery and power any lights that are on. However, when the voltage coming from the dynamo gets too high, the regulating switch opens connecting a resistance into the field winding circuit. This reduces the magnetic field strength in the dynamo which reduces its output voltage, thus regulating the charging voltage supplied to the battery.
So now let’s look at each of these two circuits in a bit more detail.
The cut-out circuit
The ‘cut-out’ circuit should perhaps be more accurately called the ‘cut-in’ circuit since its default position is open with the contacts being held apart by a spring. Therefore with the engine stopped, the dynamo is electrically disconnected from the battery.
Primary operation of the cut-out contacts is via a shunt coil which is connected directly between the dynamo armature input (D) terminal and the earth (E) terminal of the regulator unit. This coil is formed of thin wire with a relatively high resistance so that little current flows, however the large number of turns on the coil means that it produces a reasonably strong magnetic field. When the output voltage from the dynamo armature coil exceeds a certain preset voltage (6.3 to 6.7 volts), this magnetic field is strong enough to overcome the spring which normally holds apart the contacts, completing the circuit and connecting the dynamo to the ammeter.
This setup would be fine by itself if no battery was to be included in the circuit. However once the battery and dynamo have been connected by the regulator, current is free to flow in either direction. This means that when the engine speed and dynamo output voltage falls, current could flow backwards through the regulator from the battery into the dynamo. This reverse current would flow through the shunt coil in the same direction as a charge current, and hence would keep the contacts closed even though the battery is being discharged.
To overcome this, a second coil is also included in the cut-out circuit. This time it is a series coil located just after the cut-out contacts, in-line between the dynamo armature (D) input and ammeter (A) output terminals of the regulator. It is made from thicker gauge windings since it must carry the full charging current from the dynamo. When the dynamo output exceeds the charging voltage, current flows through the coil in such a way that the magnetic field produced assists the main shunt cut-out coil with keeping the cut-out contacts firmly closed.
However when the dynamo output voltage falls below the preset value (4.5 to 5.0 volts), the current flows back through the series coil in the other direction (from battery to dynamo) which sets up a magnetic field in opposition to the shunt coil. With the help of the spring, this opposing magnetic field separates the cut-out contacts and disconnects the dynamo from the battery, preventing its discharge.
The voltage regulation circuit
The above section described how the cut-out circuit connects the dynamo output to the battery once the charging voltage has been exceeded, and disconnects it once the dynamo voltage falls to prevent the battery discharging. However there is no regulation of the charging voltage above this minimum threshold and so the full maximum output of the dynamo (which could be up to 20 volts) would be applied to the battery. The maximum charging voltage for a 6v battery is normally limited to less than 7.2 volts (see the ‘Motorcycle battery voltages’ page for more details) as anything higher will lead to gassing and damage the cells. Some form of limitation (regulation) of the charging voltage is also required.
Regulation of the voltage output to the battery is achieved by rapidly opening and closing a set of contacts within the regulator unit, maybe up to 50 or 60 times per second. These contacts are operated by a shunt coil which is connected between the dynamo armature (D) and earth (T) terminals. Since the contacts have only two states (open or closed) there is no analogue control over the output. However by varying the proportion of the time that the contacts are closed, the average voltage over a period of time can be determined.
When the contacts are closed (the default state) then the full voltage output from the dynamo armature is applied across the field coil giving a maximum magnetic field strength within the dynamo and hence full output. When the contacts are open then the dynamo field current must take an alternative path through the regulator and the only one available is via an in-built resistor. This resistor uses up some of the supplied voltage so that a lower voltage is applied to the field winding, resulting in a lesser magnetic field and hence a reduced dynamo voltage output.
As an example, lets assume that the dynamo is producing 10 volts. Then if the contacts are constantly closed the full 10v output will be fed to the battery. However if the contacts are open constantly then the field winding current and dynamo output will be reduced, and so a reduced dynamo output voltage (for the sake or argument, let’s say it’s 6v) will be fed to the battery. In reality the contacts will be constantly opening and closing at high speed. When the contacts are open and closed for even durations, then half the time the battery will get the full 10v output and half the time it will get the reduced 6v output. Because the switching between the two states happens very fast, what the battery actually sees is an average 8v charge (50% of 10v + 50% of 6v = 8v).
If the contacts are closed for a quarter of the time then the battery sees an average of 7 volts (25% of 10v + 75% of 6v = 7v). If the contacts are closed for three-quarters of the time then the battery receives an average of 9v (75% of 10v + 25% of 6v = 9v). By varying the proportion of the time that the contacts are open and closed it is therefore possible to produce an average voltage output to charge the battery that is anywhere between the upper and lower limits. (Note that the voltages I used in this example were completely arbitrary so that the numbers work out easily; the actual voltages will differ).
In reality it is the voltage output from the dynamo that is continuously varying with engine speed (up to 20 volts) and we wish to main an approximately constant output to charge the battery (around 6.5 to 7.2 volts for a 6v battery). Therefore the regulator is continually adjusting the proportion of the time that the contacts are open and closed for in order that the average regulated voltage output remains within the desired range.
The current regulation circuit
Neither the cut-out or voltage regulation circuits discussed so far place any limitation on the maximum current that can be drawn from the dynamo by the charging battery. This is not normally an issue with a charged battery in good condition since it will self-regulate the charge that it will accept depending upon the voltage of its cells. However a flat or damaged battery (or maybe a wiring fault) could potentially draw a very high current from the dynamo that may cause it to overheat and melt the internal windings and connections. The current flowing out of the dynamo therefore also requires regulation, and this is the ‘compensation’ aspect of the regulation referred to in its official Lucas ‘CVC’ (Compensated Voltage Controller) title.
Just like in the cut-out circuit, there is a secondary series (current) coil alongside the main shunt coil in the regulation part of the circuit. The series coils for the cut-out and current regulation circuits are actually part of the same connection between the dynamo armature (D) input and ammeter (A) output terminals of the regulator. The current from the dynamo flows first through the cut-out series coil then through the current regulation series coil before going off the the battery via the ammeter connections.
However unlike in the cut-out circuit, the series and shunt regulation coils both operate in the same direction against the spring such that the required current flowing through either coil will open the contacts and hence bring the regulating resistor into play.
Under normal charging conditions the voltage regulation shunt coil controls opening and closing of the regulator contacts in order to maintain the output voltage in the required range. However when the current output is increased beyond a certain level, the series (current) coil assists the shunt (voltage) coil in opening the contacts such that the limiting resistor is connected much sooner and for a greater proportion of time. The current drawn from the dynamo is therefore also limited alongside the primary voltage regulation.
The voltage characteristics of a lead-acid battery vary depending upon the ambient temperature, fluctuating by around 0.2 volts per 10°C rise or fall. Similarly the resistances of the copper windings in the regulator unit will also vary with temperature. The dynamo regulation circuits described so far will operate the same under all climatic conditions which means that the voltages supplied to charge the battery will be too high in summer and too low in winter.
The Lucas regulator units overcome this problem by using a temperature compensation device which makes its output conform more closely to the voltage characteristics of the battery. The device takes the form of a bi-metallic spring that is located behind the tensioning spring of the regulator contact armature. Being made from two different metals (each with different thermal expansion characteristics), the tension of the bi-metallic spring varies with temperature causing the charging voltages to be increased in the cold and reduced when it is hot.
Temperature compensation of the regulator output voltage is therefore automatically achieved to match the battery charging characteristics, without any input or adjustment by the rider.
Adjustment of the voltage regulator
The voltages at which the cut-out and regulation contacts described in the preceding sections open and close are set by adjustment of the respective spring tensions and contact gaps. The correct procedure is described in the original ‘Lucas MCR2 regulator service manual‘ which can be downloaded from the Resources section of this website.
Having changed to using a modern electronic voltage regulator unit on my own bikes, I have never needed to make these adjustments myself although I’m sure that the service manual should tell you everything you need to know.
Conclusions and your comments
So hopefully this article has explained in simple terms how the sometimes rather confusing Lucas electro-mechanical voltage regulator unit works. It is based upon the Lucas MCR2 unit fitted to my bike, but is the same for the earlier Lucas MCR1 units and also for the Later RB107 Control Box bar a few re-ordered connections.
If you found this ‘how it works’ guide useful then please show your support by leaving me a message below. Similarly if you have spotted any glaring errors or omissions, or think that something is not as clear as it could be, then please also do get in touch.