Build Your Own Combat Robot

72 Build Your Own Combat Robot
Heat can destroy a motor in several ways. Most lower-cost PMDC motors use
ferrite magnets, which can become permanently demagnetized if they are overheated.
They can also be demagnetized by the magnetic fields produced when the
motor is running at a voltage higher than that at which it is rated. The flexible
braided copper leads that feed current to the brushes (called shunts) can melt after
just a few seconds of severe over-current demands. The insulation on the heavy
copper windings can fail, or the windings can even melt. Depending on the motor
brush mounting technique used, the springs used to keep the brushes on the commutator
can heat up and lose their strength, thus causing the brushes to press less
tightly against the commutator. When this happens, the brushes can arc more,
heat up, and finally disintegrate. You don’t want to use that expensive motor as a
fuse, so make sure it can handle the heat.
Motor heating is proportional to the current 2
× resistance. Our 18-volt motor
example has a resistance of 0.174 ohms. If you were to stall it, it would draw
103 amps. If you stalled the same motor at 36 volts, it would draw 207 amps.
Since heating is a function of current 2 , the motor would get four times as hot.
Pushing 207 amps through a resistance of .174 ohms will generate 7,455 watts of
heat, which is five times more than the heating output of a typical home electric
space heater. Now imagine all the power of your portable heater multiplied by five
and concentrated into a lump of metal that weighs just a few pounds. You can see
why survival time is limited.
The physical size of the motor that would best fit your robotic needs is in large
part determined by the amount of heat that will be generated. Some people find it
surprising that a 12-ounce motor can produce exactly the same amount of power
as a 5-pound motor. The same formula for motor power is just as true for small
motors as it is for large motors. The difference is in how long that power can be
produced. The larger motor has a larger thermal mass, and can therefore absorb a
lot more heat energy for a given temperature rise.
Pushing the Limits
Okay, so you would like to use a greater-than-recommended voltage on your motor
to get more power out of it, but you are worried about damaging it. What should
you do? First, you must realize that you always run the risk of destroying your motor
if you choose to boost its performance past the manufacturer’s specifications. Following
are some things you can do to minimize the risk.
Limit the duty cycle. If you run your motor for, say, 1 minute on and 5 minutes
off, it should survive. Cooling is critical for an overdriven motor. One Robot Wars
heavyweight (La Machine) cooled its over-volted motors by directing the output
of a ducted fan into them. This ducted fan was originally created for use in propulsion
in model airplanes because they put out a lot of air.
An easier way to accomplish this same effect is to use batteries that are limited in
the amount of current that they can produce. The problem here, though, is that you
will often be pushing your battery to output levels that will shorten its useful life.
Even the sealed lead-acid batteries can sometimes boil and leak under heavy loads.