Stepper Power

I've attempted some calculations on the stepper motor vs power supply
size issue.

I took as an example Microkinetics 34M470, nomivally a 470 oz-in motor
in a Nema 34 frame. I haven't bought one yet, so I'm working entirely
off the data on their web site:

http://www.microkinetics.com/34m470.htm

The various wiring voltage/current combinations all indicate that they
rate this motor for just under 15 watts of I^2R type power.

I looked at their graphs for performance with their various drives,
and used 7.4 x 10^-4 as the conversion from oz-in-RPM to mechanical
watts (it seems to be right, can anyone verify?). Their data and my
calculations give results like this:

500 ozin 300 rpm = 111 watts
100 ozin 2300 RPM = 170 watts
125 ozin 2100 RPM = 194 watts (1/4 HP)

These are of course marketing numbers obtained under ideal conditions
with the best drives they make, and quite possible overheating the
motors (the were running at 120% of rated current) But needless to
say, 194 watts is a lot more than 15W (or 21W), and getting this kind
of performance clearly requires a high voltage supply sized for the
larger number.

Working backwards, 300 watts (mechanical, winding heat, switching
losses, etc) at 50 volts requires a supply that can deliver 6 amps. Not
quite the sum of the rated current for the two windings, but only off
by a factor of 2 or 3.

It would thus appear that sizing the chopper drive high voltage supply
current based on the rated motor current results in a large but not
unreasonable safety margin. Obviously this is bad news cost wise, so
I'd be thrilled if anyone can poke holes in my logic...

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
http://www.mdc.net/~stratton
(617) 492-3358 home/shop