On 06/08/2010 08:26 AM, Ignoramus8975 wrote:
On 2010-06-08, Tim wrote:
It may work great, but if you didn't remove any resistors from it you
may find that it has poor regulation under a varying load. Most battery
chargers need some sort of current limiting; you can do this in the
transformer by designing in a healthy amount of leakage inductance
(microwave oven power transformers usually have a slug of transformer
material wedged or spot-welded into the core between primary and
secondary for this purpose). So you may find that the transformer it
inherently incapable of good regulation.

And poor regulation to your servo amplifiers could cause all sorts of
weird problems.

I'd try it, but I wouldn't be surprised if it wasn't good enough.

And yes, put some filter caps in there.


Tim, how can I test it? Can I, say, make it produce, say, 15 amps of
current, and I would watch the voltmeter to see if it sags a lot?

Yes. That's the test for voltage sag. Testing it without the filter
caps will measure the sag from the transformer. With filter caps you
have an effect that can either be described as "ripple causing sag" or
"ripple that looks like sag" depending on your point of view, but which
is, in either case, a ripple whose high-voltage point doesn't change
much but whose low-voltage point sags more and more with increasing current.

You'll be measuring a pulsating DC voltage; some (really cheap) digital
voltmeters will jump around a lot measuring that, others will be steady
(analog voltmeters will be rock steady). If you voltmeter jumps around
a lot then try another one, or post again and I'll show you a low-pass
filter that'll steady it out.

The servo amplifiers will be able to tolerate some sag -- they're
designed to work with power supplies that have considerable ripple, and
to them ripple and sag are the same thing. But they'll also have some
input voltage limit below which they're just not going to work right,
and that input voltage is going to be reached when you're asking for the
most torque from your motors.

Basically, you want the lowest instantaneous voltage out of the supply
to be above whatever the amplifiers needs, without letting the highest
instantaneous voltage out of the supply exceed whatever the amplifiers
can stand. You can compensate for some amount of sag by putting in
honking big filter caps -- that'll reduce the ripple, which gives you
more room for sag. The best part is that the more current limiting you
have in the transformer, the more you can just load in the filter caps
without worrying about the power factor of the current, because the
transformer will be taking care of that.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com