tech2

Understanding the electrical system of your vintage British bike can
relieve a lot of the confusion and concern when we take extended rides
on our 25+ year old relics. So, for this article we will cover the charging system for the battery, the heart of the system, for alternator supplied machines.

British bikes with Lucas electrical systems use an alternator that
consist of a set or rotating magnets mounted to the crank shaft and a
set of windings surrounding that rotor, that are securely mounted to
the engine cases or the primary chain case. As the magnets pass by the
fixed windings a voltage is generated at the output terminals of the
alternator. Early alternators used three sets of windings internally
connected as two set in parallel and a separate single set of
windings. The headlight switching was such that the two parallel
connected sets of windings were always connected and the single set
where switched in, in parallel, when the lights were turned on. This
was an attempt at regulating the system to keep from overcharging the
battery. Later system connected all three sets of windings in parallel
with only two output wires from the alternator, with regulation
handled electronically, which we will cover later.

The alternator generates Alternating Current (AC). As the magnets pass
by the alternator coils the output voltage slowly build to a positive
peak, then decays passing through zero, slowly builds to a negative
peak, then decays passing through zero. This is a sinusoidal wave
shape, similar to ripples on a pond. Since the system needs Direct
Current (DC) in order to charge the battery, the device Lucas chose to
convert between the two is called the rectifier. Several styles can
and have been used over the years. The first style is the selenium
stack full wave bridge rectifier. It consists of four selenium solid
state diodes connected as a electronic bridge and mounted to
individual flat plates and stacked vertically. Terminals jut out from
the stack such that two are used for AC input and two are used for DC
output. One of the output terminals may be the mounting stud on the
base of the stack. A more modern rectifier bridge uses the four
terminals, but is an encapsulated module made up internally of silicon
diodes that are much more robust electrically. The rectifier works to
isolate the alternator windings from the battery to minimize the
alternator's ability to discharge the battery when the engine is not
running, and in converting AC to DC. In converting from AC to DC it
flips the negative part of the sinusoidal alternator output to be
positive. This makes for only positive going lumps or ripples of
voltage. On the average it is DC but instantaneously it still goes
from zero to maximum positive voltage and back to zero again. But, the
peak voltage level of the voltage changes based on the engine RPM,
hardly what anyone would term regulated.

For regulation the Lucas system uses a brute force approach of a zener
diode connected across the rectifier's output terminals. The zener
does nothing until the voltage rises above its' specified trigger
level. At that voltage it conducts through itself, in the Lucas case
shorting the rectifier output, causing the alternator to supply a
large current. This large current flow inside the alternator's winding
generates a voltage loss at the alternator output terminals due to the
winding's internal resistance. The voltage at the rectifier output,
the zener diode's input, drops to the specified system voltage,
usually between 13 to 14 volts DC. This voltage level is required to
assure the battery has a high enough terminal voltage for current to
be forced into the battery to charge it. It is a balancing act-the
zener dumps current to ground only to the point where the resultant
zener input voltage matches the system designed voltage specification.

The System is more complicated than just the alternator/rectifier and
the zener diode. Other system components require current as well: the
ignition, the lights, and the battery for charging. The greater these
other components requirement for current the less the zener has to
dump to ground. The alternator operates at full bore, generating
whatever its' output can be for a specific engine RPM. The system
demands of the ignition, the lights, and the battery are added to by
the zener diode in order to make the alternator's internal resistance
drop the entire systems voltage level to the system design
specification, 13 to 14 volts, at all engine RPMs. Usually the
alternator does not have an ability to get up to the design level, 13
to 14 volts until the engine RPMs are above 2000 to 3000 RPM. All the
other component's current requirements are constant and the zener just
dumps a varying amount of current to ground to maintain the system
design voltage.

The alternator is capable of generating about 9 amps and the ignition
needs about 2 amps. The battery's needs vary based on charge state and
system voltage, but can be allocated for our purposes to about 1 amps.
Without lights, which typically need about 6 amps, the zener can be
expected to have to dump the light load of 6 amps. The zener's maximum
current load capability is about 4 amps, so you can see that the worst
case scenario for the system is lights off at maximum RPM. Obviously
we rarely operate our motorcycles this way for very long, but you can
also appreciate that a zener's life is not an easy one. The zener, in
dumping the system current to ground, generates heat. It helps to heat
sink mount it in the air stream.

Modern single modules that replace the rectifier, the regulating zener
diode, and in some cases the battery, are readily available. The most
common ones are Tympanium, MittyMax, and the Boyer Power Box.
The last two have the ability to make the battery optional. Lucas systems had this ability, as done in the 1970's Triumphs and BSAs, through the use
of a physically large capacitor wired either in parallel with the
battery or in place of the battery. In the first case it is just kind
of stored on the bike so that if you have a battery problem you can
remove the battery and be able to start and run the motorcycle. The
modern modules mentioned above, that offer the battery eliminating
feature use modern electronics to emulate the large capacitors
functionality, quite successfully. I currently have built two bikes
that use the Boyer Power Box batteryless. No problems have been
encountered either in starting or running of either. One is a 500cc
twin while the other is a 500cc single. Of course the lights must
remain off until the engine starts. The Boyer Power Box is offered in
a version that provide a time delay in the lights turn on if your
particular model has lights on all the time. Also the regulating
scheme used by the Boyer unit is a more sophisticated approach. It is
a passive method that only calls for current from the alternator based
on system needs. This means that when the lights are off the
alternator is not running full bore as in the Lucas system. But the
units still develops heat and must be place in the air stream to get
rid of that heat.

Well, that about covers this topic. I hope you learned something from
this to ease your mind about future projects component selection and
failure at the roadside.

Addendum:

Since no schematics are available for these modern devices I had to make assumptions on how they worked based only on my knowledge of how they should work, the components available, and the resultant size of the
new module. I guessed wrong on the aftermarket rectifier/regulator
modules. Apparently they are not passive regulators but shunt
regulators. I think they are using Silicon Controlled Rectifier Diodes
across the AC output of the alternator rather than a zener diode across
the DC output of the rectifier. SCR’s are much more robust devices and
that may explain why the packaging is smaller and the new modules
relatively trouble free compared to the 50 watt zeners used by Lucas.