"Guy Berthiaume" wrote in message
...
snip
I actually didn't expect this conversation to go into the design
issues I might face. But, if you guys are willing to discuss this,
then I am more than happy to get your help.

Part of this project will be to determine the battery specs. DeWalt
has chosen not to publish specs on the batteries or the tools that use
them (at least any that I can find). I can find neither current
ratings nor horsepower ratings.

From my experimentation so far I have found the Equivalent Series
Resistance (ESR) of my remaining battery is about 0.2 ohms. This
puts the short-circuit current at about 90 Amps. Assuming a stalled
motor has an impedance of 1 ohm (a big assumption right now), then the
peak current under worst-case conditions is around 15 Amps.

Under normal load conditions, I would expect half that value. A
switching power supply with a peak output of 15 Amps and continuous
output of 8 Amps should suffice. That translates into a supply that
can handle 144 W continuous, and 270 W peak. That will be a hefty
supply. 270W is equal to about 1/3 HP.

The other issue is that most switching supplies have over-current
protection which could cause the tool to stop/start in rapid
succession if the supply can't handle the peak current demand. I
would expect that to have an adverse effect on the item being worked.

I actually have several small 18 V supplies that I can parallel for
combined output of 225 W max. This doesn't fit the exact
requirements, but it will be good enough for an experiment.

Guy

I think the motor specs would be more relevant than the battery
specs. Like you stated, the battery is capable of far more amps than
the motor would ever draw. Your plan of testing with your existing
supplies is a sound one.

A switching supply is definitely the way to go. An off line switcher
running at several hundred KHz will have magnetics which are much
smaller and lighter than the cells of your current battery pack. Your
over-current concerns can be addressed by simply designing it to go
into a constant current mode instead of shutting down and restarting.
I also wouldn't bother with PFC. A non-isolated design will be more
compact and less costly than an isolated one and the drill/saw/whatnots
are usually insulated sufficiently to avoid shock, but this is a safety issue
and is your call. And finally, given the application, the ripple spec can
be relaxed or even eliminated as the motor(s) receive PWM current
supplied by the VSR electronics anyway. If your design for the
converter allows the voltage to fall to zero 120 times per second this
would be probably be unnoticeable to the user.

Art