RE: [CAD_CAM_EDM_DRO] Re: RE: stepper driver-transformer sizing question

I did not record the speed, but it was more a problem with the load than
with the speed/torque profile. I was using stepper motors with shorted
windings as the load. If you short the windings of a stepper motor you will
find they can suck up lots of shaft power when you turn them slowly because
they will pump lots of current through the internal resistance. If you turn
them fast you will get the equivalent of "missed steps" and they are
actually easier to turn at higher speeds.

The speed/torque profile of any stepper is flat until the current starts to
get limited by the inductance, and then it slopes down. Max theoretical
power is close to the knee. The speed/power profile ramps up from the start
but then turns down or goes flat at the knee. Of course power is speed x
torque. Before the knee, power is low because speed is low; after the knee,
power is low because available torque is low.

I think that practical sizing of the power supply ends up much smaller than
prevailing wisdom because it is impractical to use steppers anywhere near
their power delivery capacity. Everybody knows that the torque demands you
place on a stepper should be a really long way from the holding torque. The
peak holding torque is really the failure point, since a missed step is
functionally a failure. If you run a system with torque demands which are
50% below the holding torque, then your power supply will only need 50% of
the theoretical power. Sizing a transformer like that would be risky if the
load of the system were viscous in nature, like stirring paint with a paddle
wheel, since the whole system would crash when the power demands exceeded
the delivery capacity of the transformer. However, the load of most of our
systems is usually not viscous, but more subject to short term peaks from
accelerations or impact forces. For those types of short term loads the
capacitor, not the transformer, are the key to carrying the system through
without loss of steps.

In other words, size the transformer for actual RMS power but size the
capacitor for peak torque demands.

It would be very interesting if some others would hang a clamp on current
probe on the AC input to their power supply transformer. I suspect that
most of them are way oversized. The factor that I do not know the answer to
is how screwed up the reading will be on an AC current meter given the
nature of current flow in a rectifier application. Current only flows in
the transformer at the points where the voltage exceeds the bus voltage.
This is a very nasty way to use a transformer, and a long way from a sine
wave that most cheap meters expect to see. It's a good excuse to buy a true
RMS meter.

Greg

-----Original Message-----
From: Ray Henry [mailto:rehenry@...]
Sent: Thursday, January 01, 2004 11:29 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: RE: stepper driver-transformer sizing
question


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