G’day; Carbon pile controllers are exactly what they sound like.
One type is a number of thin plates made of compressed carbon. Some
manufacturers add a small amount of copper dust to the carbon before
compression to decrease the resistance of the carbon plates because
carbon isn’t the best of electrical conductors. Some method of
pushing the plates together with variable amounts of pressure is what
controls the power flow. When loosely touching, the electrical
resistance is very high, so up to a certain point, the more the
pressure, the greater the power flow. One problem is that fine
particles of dust can get between the plates and reduce the power flow
by increasing the resistance. A very old type of carbon variable
resistance relied upon a container of carbon granules being pushed
together, but these suffered from several problems - the crushing of
granules being only one, moisture was another. But whatever the
method of resistance control, whether it is an arm wiped across a
coil of resistance wire (Nichrome for instance) or carbon as the
resistance, the power reduction is obtained by converting unwanted
power to heat. Which is why most older pedal controllers get hot.
The more modern controllers work by removing part of the sine wave
from an alternating current, and some convert what is left to direct
current to power a DC motor, which gives far more torque at low
speeds. Speed controllers for AC motors need more complicated
control systems if mechanical power is not to be lost with speed.
Speed controls for three phase motors is another game altogether, and
doesn’t affect users of low power tools as such motors are rarely
below 1 hp.
So, if a foot or other controller gets warm, then that is to be
expected. Unless it is the modern type which doesn’t depend solely on
resistance changes in the circuit. If it keeps saying ‘snap, crackle,
and pop’, it will be because the equipment is faulty. Unless it is
given prompt attention, it will get worse quickly. One thing that
often helps is to use the blower end of a good vacuum cleaner fitted
with a narrow nozzle to blow out dirt from any electrical appliance.
50 psi from an air compressor is even better, but most people won’t
have one. Only make sure a clean filter is in the vacuum cleaner
first! I have to do that to the 1� hp motor on my (home made) table
wood saw at monthly intervals. I also blow out other electrical
appliances (when I think of it! or when they don’t work.) Finally if
you see an almost continuous green spark spinning around the motor’s
commutator segments, give it attention soonest. It needs a good clean
with the correct solvents and probably a new set of carbon brushes.
Any appliance repairer will do this properly. At a price. – Cheers
now, and may you never drown under a sine wave.
John Burgess; @John_Burgess2 of Mapua Nelson NZ