I am replacing the generator in my M-37 with a NOS unit (Fresh out of the
box), I seem to remember reading in a manual that it needs to be polarized
before use.
Hi,
The operation of a DC generator depends on a small amount of residual
magnetism in the pole-pieces of the field assembly and the armature to begin
to generate a voltage on start-up and thereby produce its own field magnetism
for full output.
If left disassembled or un-run for _many years_ this very small residual
magnetism can fade away and in the de-magnetised state no output will ever be
produced, it is possible that left stored and untouched it may even become
slightly magnetised in the opposite manner to that required by the vehicle
owing to the earth's magnetic field.
Polarising or "flashing" the dynamo (generator) re-instates the internal
field and any running further re-magnetises the internal cores. There are
several ways to re-polarise but off the vehicle will also confirm that the
unit is good.
There are four connections to the generator (although usually only two are
presented externally), two for the armature and two for the field, normally
the field and armature are internally connected and strapped to the casing
with a small and large external terminal for the field and output connections
respectively.
In this instance mount the generator in a vice or otherwise clamp it down as
there is a fair torque reaction, using moderately heavy wire throughout
(similar to the size used for the vehicle generator output cable) temporarily
link the field and output terminals and connect a slave battery to the case
and the terminal link on the generator in the polarity you wish the generator
to operate, if its a negative ground and positive live system then connect in
this way with the battery negative to the generator body, if the vehicle is
the "other way up" then simply reverse.
There WILL be a hefty spark when making and breaking the final battery
connection so be aware of flammable gasses and connect at the generator end
not the battery in case of potentially explosive gasses from a freshly
charged battery. The generator should rev quite freely as a motor when
connected and this will set up the internal field and its polarity.
This can be done on the vehicle but if the drive belt is in place the
generator is stalled and will draw a very high current so the connection must
be only momentary and expect some good sparks.
Even simpler but potentially dangerous, with the generator installed and
correctly connected to the regulator/cut-out, push in the _cut-out_ blade by
hand in the regulator box.
BEWARE, the stall current has to flow via the cut-out armature coil (this is
only a high current relay in reality) and will hold it in very strongly the
instant it is made so be ready to pry the cut-out armature blade open again
with a small screwdriver or otherwise arrange to break the battery
connection. If there is an ammeter in circuit this will be seriously
overloaded and needs to be temporarily by-passed by moving one wire on to its
other terminal for example.
If left for more than a second or two you can destroy the generator, the
cut-out, the wiring or fire the vehicle and it is NOT RECOMMENDED unless
you're absolutely sure of what you are doing and the expected, and sometimes
unexpected, results.
None of this applies to an alternator of course.
Richard
(Southampton UK)
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