Re: Stepper current - how bad is underdriving a motor?

Hello Tony,

OK, now I've read both 50% and 71.5% from the newsgroup here.
Still there are some issues that I would like clarified if you will.

John said,

>840 oz-in,
>1.45V
>15.2amps !!
>It is six wire. So full coil turns it into a 2.90V, 7.6A motor.
>That's almost perfect. With a 48V supply, I'll have 16.5x voltage,
>so I should get good high speed performance, and 7A into a 7.6A
>motor will deliver nearly full torque.

The book I have says this would turn into a 21.49 Amp, 2.05 Volt
motor when run in bipolar series. John is appently doubling the
voltage and halving the current in his calculation above. This seems
as if it would be fine if the above motor rating was for bipolar
parallel, but the rating shown would be unipolar as it is a 6 wire
motor.

You seem to be in agreement with John that at 7 amps, running the
motor in bipolar series mode, he is going to get nearly full torque
rating. How do you figure this? I came up with about 65% full
rating.

I appreciate any and all comments as I consider this to be not only
a "newsgroup", but an "eductationgroup" as well (as long as Mom
doesn't say it's OT).

Thanks,

Jeff


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tony Jeffree <tony@j...>
wrote:

> Jeff -
>
> As Mariss keeps telling us, derating by 71.5% is not a great idea

with

> microstepping, as you tend to get bunching of the microsteps. So

half the

> current is probably the right answer here.
>
> Regards,
> Tony
>
> At 07:52 23/12/2002 +0000, you wrote:
> >Hello John,
> > If the motors are rated at 15.2 Amps UNIPOLAR, then if you use
> >them in full-coil BIPOLAR series mode, I beleive you want to derate
> >them to 71.5% of the UNIPOLAR rating, not half. This comes to

10.87

> >amps/phase. 7 amps/10.87 gives a 64% of maximum total torque
> >rating. I would double check this (my evaluation) and get another
> >opinion though.
> >
> >Jeff
> >
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jmkasunich
> ><jmkasunich@y...>" <jmkasunich@y...> wrote:
> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, Tony Jeffree <tony@j...>
> > > wrote:
> > > > At 12:18 21/12/2002 +0000, you wrote:
> > > > >I just acquired a NEMA42 stepper,
> > > > >
> > > > >840 oz-in,
> > > > >1.45V
> > > > >15.2amps !!
> > > > >
> > > > >I was wondering how much torque I
> > > > >sould expect out of it when running
> > > > >on a Gecko at 7A? I do know that
> > > > >whatever torque I get will stay
> > > > >there to a rather high speed, thanks
> > > > >to the very low voltage.
> > > >
> > > > You would expect to get approx 1/2 the torque at low
> > > > speeds, but as you say, the low voltage gives plenty
> > > > of headroom to get good performance at high speeds.
> > > >
> > > > You haven't said whether the motor is 4, 6, or 8 wire.
> > > > If it is 6 or 8 wire, the alternative is to use it in
> > > > full coil (6-wire) or series connected (8-wire) at 7A;
> > > > you should then get close to the rated torque at low
> > > > speeds.
> > > >
> > > >
> > > > Regards,
> > > > Tony
> > >
> > > Duh! Why didn't I think of that! It is six wire. So
> > > full coil turns it into a 2.90V, 7.6A motor. That's
> > > almost perfect. With a 48V supply, I'll have 16.5x
> > > voltage, so I should get good high speed performance,
> > > and 7A into a 7.6A motor will deliver nearly full
> > > torque. Thanks Tony!
> > >
> > > John Kasunich
>
> Regards,
> Tony