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

Regarding item 2, I do not see this in practice. I suspect this is based on
the motor theory for normal operation, where the rotor poles do not pass
over a zero vector with the stator. If you stall a motor while the field is
spinning rapidly there is very little torque developed as the coils are
pushing forward 50% of the time and pushing backward 50% of the time.
Similarly, if you spin the rotor while the field is basically stationary,
there is very little torque required. Once I got to a certain speed I found
the load torque went almost to zero, much like it does when a motor is
stalled. This was a very discrete transition point.

Regarding item 3, I think it is more practical to measure current while it's
still in AC form. I have a cheap AC clamp meter which is easy to use to
measure current. I also have a very expensive Tektronics DC clamp meter but
it's on the fritz and it will probably cost $1000 to get it fixed. Hall
effects are difficult to calibrate, precision shunts to measure volt drop
are not common, and I suspect bad things would happen if you used a
resistance much more than a shunt. When my Tek clamp died I was looking for
ways of measuring DC current without adding resistance and I came up short,
suggestions would be appreciated. On a side note, when I worked for an ISO
company we had all of our instruments calibrated. The unfused 10A current
measuring capability of most multimeters was often very far off, but maybe
it was just a fluke (pun intended: Fluke meters).

-----Original Message-----
From: Mariss Freimanis [mailto:mariss92705@...]
Sent: Thursday, January 01, 2004 1:29 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: RE: stepper driver-transformer sizing
question


Couple of comments:

1) Backdriving a stepper with its windings shorted is the reverse of
having a stepper delivering mechanical power to a load while being
driven by a drive. The motor is a bi-directional transducer that
converts electrical power into mechanical power and visa versa.

2) The motor does not "suck up lots of shaft power when you turn them
slowly"; the torque is high exactly because the RPM is low. A step
motor is a constant power trasducer. This means the product of torque
and RPM stays constant. As you turn it faster, the torque required to
turn it drops because the absorbed power (converted to electrical
power and dissipated in the winding resistance) stays constant.

3) You can try to measure the AC RMS input power to supply but that
is going to a lot of unecessary trouble. You can safely assume an
unregulated supply has a very high efficiency (>95%) and that
efficiency is relatively constant. Simply measure the DC amps from
the supply and multiply it by the DC voltage to get the Watts
supplied to the load. It will be a very close approximation of the AC
power drawn.

4) Assume the power a step motor can deliver to a load at a given
supply voltage is constant when the motor is operating past
the "knee" of its speed-torque curve. Safely assume the motor has a
certain conversion efficiency. The conclusion is the current drawn
from the supply will be independent of speed at a stall load (motor
delivers 100% of its available power just before stalling).

You can empirically test this hypothesis by stalling your motor at a
variety of speeds while noting the supply current at the moment of
stall. The current will be the same in every case past the knee speed.

Mariss