RE: [CAD_CAM_EDM_DRO] stepper driver-transformer sizing question

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


-----Original Message-----
From: Scott Riddle [mailto:sariddle@...]
Sent: Wednesday, December 31, 2003 12:37 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] stepper driver-transformer sizing question


I am a little confused on sizing a transformer to drive some small 30-
Oz-in bi-polar steppers. They are rated at 3 amps/phase 2.1Vdc and
the IMS IB462 drivers I have are good for 12-40V 0-2Amps 160Watts.
What I am thinking is that the motors can take 6 amps (3 per phase).
But can the motors only take 12.6 watts (6A x 2.1Vdc). I would like
to just buy 1 transformer to supply power to 2 or 3 steppers. I am
not looking for high speed mainly low speed high torque. Can someone
help me on sizing the transformer required? Would I also need to run
resistors on series with each driver to limit the current each driver
would be able to recieve?

Thanks
Scott Riddle


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