Re: [CAD_CAM_EDM_DRO] Re: How much torque is enough? (Was: COMPLETE 3 axis CNC...)

At 14:19 03/05/2006, you wrote:

> >
> > The first motor was one of the "standard" 200 oz-in 1A/phase motors.
>The
> > max half-step rate with this motor before losing steps was 1250
> > half-steps/sec, equating to around 9.4 IPM (20 TPI leadscrews).
> >
> > The second was a 254 oz-in, 2.5A/phase, so with the reduced (2A) drive,
> > effectively a 211 oz-in motor, so very comparable to the first in
>holding
> > torque. This managed 4000 half-steps/sec or 30 IPM.
> >
> > The third was a 140 oz-in, 3A/phase motor; again, with the reduced
>drive,
> > effectively down-rated to 93 oz-in. This managed 7000
>half-steps/sec, or
> > 52.5 IPM.
> >
>
>Good imperical data, but one has to ask why the differences ?
>
>were you using incrementally lower inductance motors ?
>
>I have found a dearth of discussion about motors electronical
>parameters as everyone just focuses on the nameplate oz-in.

Yes, the motors were incrementally lower inductance - you can figure that
out pretty easily from the information I gave, as torque is proportional to
Amp-turns, and inductance is proportional to the square of the number of
turns. So if you have a 1A/phase motor and a 2.5A/phase motor that have
roughly the same holding torque, then all other things being equal (same
magnetic materials, construction, etc.), the 1A motor has approx 2.5 times
as many turns in its coils, and therefore approx 6 times the inductance.

Similarly, if you have a 2.5A motor that generates 254 oz-in holding torque
and a 3A motor that generates 140 oz-in holding torque, then the 2.5A motor
probably has twice as many turns in its coils than the 3A motor, and
therefore has roughly 4 times the inductance.

So, keeping the load being driven and the supply voltage the same, it is
actually not very surprising that the motor with the lowest inductance (not
the highest torque) won hands down - with a 24V supply, it was being driven
with an "overdrive" ratio of around 12 times the nominal motor voltage
(~2V), compared with a little over twice the motor voltage in the case of
the 1A motor (~9.5V). If I had been able to vary the supply voltage as
well, to keep the overdrive ratio the same in each case, then the results
would have been very different.

This is basically why there is a place for the "lowly" 2 or 3 Amp driver,
limited to 30 or 40 volts at one end of the scale, and the 7 to 10 Amp
driver with an 80 to 100V supply at the other end of the scale. The former
will do a great job of driving a low inductance size 23 motor; however,
give it a size 34 motor *with the same current rating* and it will drive
the motor just fine as long as you don't expect fast step rates out of it.

So, you have to look at the whole system, not just the motor torque figure,
if you are to stand any chance of predicting how the system will behave.

Regards,
Tony