Re: RE: stepper driver-transformer sizing question

> From: "Greg Jackson" <greg@...>
>
> I have yet to find a good paper on transformer sizing. I think that
> the 2/3 rule yields a transformer which is far bigger than necessary.
> It also ignores the fact that basically the same power motor can be
> wound for low current with high induction or high current with low
> induction. Real power delivered through a motor is not a function of
> motor current. I'll relate a recent test:
>
> Running 4 each size 34 motors at 6 amps per phase. These were 1240 oz
> in motors, pretty big ones. I loaded the motors by direct coupling to
> 4 other motors which had all leads shorted in order to create as much
> load as possible. I adjusted the motor speeds while watching the AC
> current into the transformer in order to achieve the highest possible
> power load. While running 4 bipolar motors at 6 amps, that's 12 amps
> per motor or 48 amps current delivered by the system. At the 70 volt
> bus, one might expect 3,360 watts if regeneration were not part of the
> equation. In fact, the highest power I recorded into the transformer
> was 300 watts. I ran this test until everything came up to
> temperature. The motors were running around 180 F even with a fan on
> them. The stepper drivers were around 130 F, but the capacitor on the
> stepper drivers was up to 180 F (bad news). The dc rectifier was about
> 209 F and the 640 VA transformer was barely 10 F over room temperature.
> The temperatures suggest that the system was running plenty hard, yet
> the transformer was the least taxed of anything in the system.
>
> Mariss suggests that the most a large stepper can deliver is about 100
> W of power. I think this is a very reasonable way of looking at things
> and probably better than motor current. My feeling is that, if using
> larger steppers, 110 VA of "rectifier rated" transformer or 125 VA of
> standard transformer per motor is probably plenty. In the context of
> the test above, I was driving 4 motors using 300 watts. Given a better
> load system I might have be able to suck up 400 watts. A 440 watt (VA)
> transformer would probably have been fine.
>
> I have not done any similar testing with any motors under 500 oz inch
> so I cannot provide any direct references there, but one might suppose
> that the system dynamics scale in a linear fashion. If anyone has
> references to a technical discussion that is a little more specific
> than the 2/3 rule, I would certainly like to know about it.
>
> Greg

I find this report fascinating because the result of the empirical,
real-world tests seem to run counter to the prevailing wisdom. The only
thing that you did not mention was the rotational speed of the stepper
motors at what you saw as max AC draw.

Others mention running the steppers as fast as possible and I think that
this is not the speed which would cause max current draw from a power
supply. Max velocity under load would be for servos but not for
steppers.

I had a discussion about some of this with Mariss a while back. My
thinking was that you needed to factor the motor voltage into the draw
question as well.

Ray