> I have an old 18V battery powered drill which is missing its battery
> pack. I'm pretty sure the motor is a DC brushed motor, judging by the
> sparks and smell of ozone. I haven't opened it (or any other drill, for
> that matter) up yet to determine the trigger-switch mechanism, but it
> seems to be two-speed.

> Question: is it okay to build a simple PWM motor controller and drive
> this thing at 24V, with a reduced max duty cycle to prevent burning out
> the motor? Since it's a simple DC brushless motor, I'm assuming that it
> should be safe as long as the average power is kept below its max power
> at 18V.

You MAY be happily surprised with running the drill on 12V. Try it
first just in case.

It may work OK with straight PWM. Speed wise you will be able to make
the motor "see" the correct voltage and the windings are inductive so
arguably you are creating a buck converter. You'll want a suitable
diode across the motor terminals  in reversed polarity to allow the
motor current to "recirculate" when the PWM is off.

There is a modest chance that the commutator does not like the
potential increased current but as long as PWM frame rate is small
compared to motor time constant then there should be no great extra
current.

Try the PWM and look at the results on a scope. If there is more
current rise on occasion than you are happy with you can add a small
series R with a cap at the motor end to add rise-time to the circuit
BUT should not be needed.


         Russell
--=20
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