Re: [CAD_CAM_EDM_DRO] Running Uniploar motors as Bipolar motors.

In a message dated 11-Jan-02 14:47:58 Central Standard Time,
j.guenther@... writes:

> I have some Pacific Scientific 8 wire unipolar motors rated at 5.47 volts at
> .489 amps. I am currently running these at 38 volts 1 amp in a parallel
> configration on a Bipolar Chopper driver. The motors don't seem to be
> getting hot, only slightly warm to the touch. Is this current setting Ok
> or am I asking for trouble?

John: I am not the know-all whiz I suspect several in this group actually
are, but I have had some relevant personal experience in recent days with
this very topic, so don't laff too loudly at my poor offerings:

First, you say you are "running these at 38 volts...". You mean your
POWER-SUPPLY puts out 38 volts? The motors surely NEVER see this much
voltage, or they'd COOK! The "chopper drive" may apply chopped DC at 38
volts, but if you AVERAGE the on-time and off-time, and multiply the result
by 38 volts. you'd have a much lower "effective" voltage, which, using Ohm's
law, would result in only 1/2 amp per winding, I suspect! If the motor is
only perceptibly warm to the touch, even after quarter hour of being
so-powered, then all is OK! (or, at least not "dangerous to the motors'
health.)

Mariss corrected my feeble thinking about a basic engineering consideration
of a chopper-drive: I was freaked out that a motor I was testing with a
Gecko drive had "one amp per winding" two windings, so, two amps, right?
Only, the power-supply was delivering only 300 mA! How so? Duh... the
VOLTAGE was 24 VDC, and it was being CHOPPED! The inductance of the motor
was INTEGRATING the electrical energy, and the "effective" current WAS 1 amp
per winding, at about 5 VDC, though the power supply was delivering nearly
five times as much voltage to the drive. (Reminded me why I just never
managed "Junior Circuits"!)

Now, if you wanna go poking voltmeter-leads to the motor's windings, to CHECK
if the [effective] voltage is only about 5 VDC, do NOT use a DVM, as those
read the PEAK voltage of pulsed-DC (usually, anyway)! But a good "old
fashioned" d'Arsonval movement "analog" voltmeter will "integrate" 20 kHz
"mechanically" just fine, and "fool you" into seeing 5VDC across a winding,
when the instantaneous "peak" voltage is 24 to 38 VDC! Seems SO simple when
a sage like Mariss explains it to a dolt like me! Duh... I guess, same
with current. But I wouldn't suggest you go disconnecting leads to insert
ammeters "in there". If you know the winding resistance and "effective"
voltage, you can use Ohm's law for I !

By the way: An "eight lead motor" is NOT a "unipolar motor", but an "either
kinda" motor! That's why eight leads! You can lash it up to be five, six,
or four leads, high-voltage/low current, low-voltage/high current, four-wire
bipolar, five or six wire unipolar! (this is always so confusing to my
feeble tiny mind, I now stick with four-wire bipolar and G210 drives!)

Hope my palaver is of use! Questions???? Jan Rowland


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