This article is first draft of theory and operation of generators and Lucas voltage
regulators.  The generators on old english cars seem always to be marginal when
driving at night with running and head lights on.  Probably the best answer to this
problem is exchanging the generator for a more modern (and higher output)
alternator.  The Vintage Triumph Register website has details about much of the
nuts and bolts of an exchange.  In addition, it has an article by Dan Masters about
the theory and operation of alternators (  I have borrowed heavily
from Dan's work when writing this article.  I refer you to his wonderful diagrams
used in his alternator article as they apply directly here too.  Anything you find
useful I owe to him, and all errors are of course my own.

Generator - Theory of Operation

In order to understand the theory and mechanics of mechanical voltage regulators,
you must understand how a generator makes electricity.  The basic principle is
that moving a wire through a magnetic field induces electical current flow in the
wire.  The faster you move the wire, the greater the voltage that is induced.  A
generator essentially moves a loop of wire through a magnetic field around and
around.  Through one half of a full revolution a positive voltage is created, in the
second half a negative voltage is created.  Since generators were created before
semiconductors were readily available, there had to be a mechanical way to avoid
making the wrong polarity voltage.  This was readily answered by reversing the
connection on the wire loop as it entered the negative phase.  The commutator
inside the generator performs this function.

The magnetic field could be created by permanent magnets within the generator,
but they are expensive and bulky and heavy.  Additionally, they can not be
modulated to maintain a constant output voltage.  It is more effective, cheaper and
lighter to use some loops of wire to do the same thing.  Just as it is possible to
create a current in a wire by moving it through a magnetic field, it is also possible
to create a magnetic field by moving a current through a wire.  In order to make
the field stronger you can loop the wire back on itself many times.  This way a
small current can create a large magnetic field.  This is an electromagnet.  In a
generator this electromagnet is called a "field coil".  It is a series of loops of wire
(a coil) which creates the magnetic field.  

Now we have a device that creates electrical pulses of the correct polarity.  The
voltage coming out is proportional to the speed of the turning of the generator, and
the magnetic field strength of the field coil.  As the engine RPMs increase, the
voltage from the generator also rises.  Unfortunately, the electrical systems in a
car like a fixed voltage.  If the voltage is too high the battery will overcharge and
boil over, light bulbs will burn out and the coil will melt.  There must be some
way to control the voltage output.

Voltage Regulator - Theory of Operation

For the purpose of an automotive generator the use of a field coil makes it
possible to control the voltage output.  We can not easily control the speed of the
generator since this is directly linked to the engine speed.  We can control the
current in the field coil.  Increasing the electrical current in the field coil will
make the magnetic field stronger.  Decreasing the current will reduce the magnetic
field.  The voltage regulator performs this function.  The voltage regulator also
performs some associated functions.  It cuts the generator out of the circuit when
the voltage from the  generator is less that the battery voltage.  This prevents the
battery current from running backwards through the generator, discharging the
battery.  There is also a mechanism to prevent too much current being drawn from
the generator which might overheat or otherwise damage the generator.  In some
models of regulators there is one relay (called a "bobbin" in voltage regulators) for
each of these functions.  In the TR2-4 series, and other models as well, there are
only two bobbins.  One of the bobbins serves two functions in this case.

The first bobbin we will discuss is the simple "Cut-out relay".  It should be more
properly described as the "Cut-in" relay because it keeps the generator out of the
charging circuit until it is generating sufficient voltage for the bobbin to close
contacts cutting the generator into the charging circuit.  It is set to cut-in at 12.7 to
13.3 volts.  Once in the circuit it will stay in until the generator output actually
drops well below battery voltage (11 to 8.5 volts).  This may never happen even at
a very low idle.

In the TR2-4 series the second bobbin provides two functions.  The primary
function is to regulate voltage by reducing generator output by reducing the
current in the field coil.  The secondary function is to prevent excessive current
output from the generator, again by reducing output.  There are two separate
windings on the bobbin to provide these two functions.  In three bobbin regulators
the voltage and current regulators are separate but functionally identical to what is
described here.

When the voltage is below a certain set point there is a direct connection of the
field coil to battery (actually battey plus generator) voltage.  This gives the
maximum magnetic field strength possible and thereby allows the generator to
produce the greatest voltage possible.  When the voltage exceeds a set point, the
bobbin opens a contact which puts a resistor in line with the field coil and reduces
the current running through the coil.  This reduces the magnetic field strength, and
in turn reduces the generator output.  The contact is opened and closed frequently
so the electrical system essentially sees the average of the duration of high and
low voltages.

In the TR2-4 two bobbin system, the current regulation is performed by a separate
winding on the same bobbin as the voltage regulator.  This winding carries the full
current output of the generator.  The wire is wound so that increasing current
through the wire will tend to open the contacts and lower the current in the field

Regulator Adjustment

The only adjustments that you can make to the regulator are the contact gaps and
the set points.  I will quote the Triumph workshop manual regarding the
adjustments.  Their description is concise and thorough.  I will add my comments
in italics where additional explanations may be in order.

The control box (regulator) contains two units - a voltage regulator and a
cut-out.  Although combined structurally, the regulator and cut-out are electrically
separate.  The voltage regulator relay (bobbin)  can be identified as the coil
with just a few turns of heavy gauge wire around it.  The cut-out relay has many
more turns of the same heavy gauge wire.

The regulator is set to maintain the generator terminal voltage between close
limits at all speeds above the regulating point, the field strength being controlled
by the automatic insertion and withdrawal of a resistor in the generator field

Cleaning Contacts

(i)  Regulator Contacts:  used fine carborumdum stone or silicon carbide paper
     (sandpaper 400 grit or finer).
(ii) Cut-out Relay Contacts:  used a strip of fine glasspaper, never
     carborundum stone or emery cloth.

Voltage Regulator-Electrical Setting

It is important that only a good quality MOVING COIL VOLTMETER (0-20
volts) is used when checking the regulator.  The pulsing nature of the voltage will
prevent a digital voltmeter from settling on a single reading

Remove the cover and insert a thin piece of cardboard between the armature and
the core face of the cut-out (contacts) to prevent the contacts from

Remove and join together the cables from the control box terminals A and A1. 
Connect the negative lead of the voltmeter to the D (output) post on the

Start the engine and slowly increase its speed until the voltmeter needle flicks and
steadies, at about 2,000 RPM.  The voltage reading should be between the
appropriate limits given in Table 1.

If the voltage, at which the reading becomes steady, occurs outside these limits,
adjust the regulator by turning the adjusting screw 1/4 turn at a time clockwise to
raise the voltage or counterclockwise to lower.  The adjusting screw can be
found on the back of the regulator facing the firewall. 

Adjustment of regulator open-circuit voltage should be completed within 30
seconds otherwise heating of the shunt windings will cause false settings to be

Remove the cardboard.

NOTE:  The voltage that you see in Table 1 is not the actual operating voltage of
the generator and electrical system.  It is the a voltage that is only used for setting

Voltage Regulator-Mechanical Setting

A copper separator, in the form of the disk or square, is welded to the core face of
the voltage regulator (the coil with just a few heavy gauge wire windings)
and affects the gap setting between the core-face and the underside of the
armature as follows:

When a round separator is used, the care gap should be 0.015" (0.38mm).

Win a Square separator is used, the inner gap should be 0.021" (0.53mm).

To adjust the air gap:
Slacken the fixed contact locking nut (on top of the bobbin) and unscrew
the contact screw until it is well clear of the armature moving contact.

Slacken the voltage adjustment spring-loaded screw (on the back of the
regulator) until it is well clear of the armature tension spring.

Slacken the two armature assembly securing screws.

Insert the gauge (feeler gauge) of sufficient width to cover the core face,
and of the appropriate thickness, between the armature and copper separator.

Press the armature squarely down against the gauge and re-tighten the two
armature assembly securing screws.  Without removing the gauge, screw in the
fixed contact adjustment screw until it just touches the armature contact.  Re-
tighten the locking nut.

Re-check the electrical setting of the regulator.

Cut-Out -Electrical Setting

If the regulator is correctly set but the battery is still not being charged, the cut-out
may be out of adjustment.  To check the voltage at which the cut-out operates,
remove the control  (regulator) box cover and connect the voltmeter
between the terminals D and E  (the right-hand-most two spade
terminals).  Start the engine and slowly increase its speed until the cut-out
contacts are seen to close, noting the voltage at which this occurs.  This should be
12.7 to 13.3 volts.

If operation of the cut-out takes place outside these limits, it will be necessary to
adjust.  To do this, turn the adjusting screw (found on the firewall side of the
regulator) in a clockwise direction to raise the voltage setting or in a counter
clockwise direction to reduce the setting.  Turn the screw only a fraction of the
turn at a time and test after each adjustment by increasing the engine speed and
noting the voltmeter readings at the instant of contact closure.  Electrical settings
of the cut-out, like the regulator, must be made as quickly as possible, because of
temperature rise effects.  Tighten the lock nut after making the adjustment.If the
cut-out does not operate, there may be an open circuit in the wiring of the cut out
and regulator unit in which case the unit should be removed for examination or

Cut Out - Mechanical Setting

Slacken the adjustment screw (on the fire-wall side of the regulator) until
it is well clear of the armature tension spring.

Slacken the two armature securing screws.

Press the armature squarely down against the core face (copper sprayed in some
units, fit with a square of copper in others) and re-tighten the armature securing
screws.  No gauge is necessary.

With the armature still pressed against the core face, adjust the gap between the
armature stop arm and the armature tongue to 0.032"(0.81 mm) by bending the
stop arm (the stop arm is the metal all arm on the very top against which the
moving armature contact arm (called the "fixed contact blade") rests).

Adjust the fixed contact blade so that it is reflected 0.015" (0.38mm) by the
armature moving contact when the armature is pressed against the core face.

Re-check the electrical setting of the cut-out.

Table 1.   Open Circuit Settings

     Ambient Temperature             Open Circuit Voltages

     10C (50F)                     16.1 - 16.7
     20C (68F)                     16.0 - 16.6
     30C (86F)                     15.9 - 16.5
     40C (104F)                    15.8 - 16.4