Re: RE: stepper driver-transformer sizing question

Well, I try to develop theory from empirical experiments instead of
forcing results to match a preconcieved theory. It's like the horse
and carriage thing.

I neglected to mention I used an adjustable (active circuit) constant
power load because I thought it would be a complicating factor in the
explaination. I was refering to my lab notes when I crafted the reply
and I overlooked the significance of the obvious. As a result you
correctly caught me out. My reply was muddled mess by leaving that
out; chalk it up to a New Year's hangover.

After the clarity aspirin and some time brings I still defend the
last point. Look at it in these terms: who cares how the supply
works? It's job is to supply DC power. DC power out is easy, an
ammeter, a volt meter and a calculator. That is easily measured and
that is what you want after all.

I generally use a Tek TDS 360 scope and a Tek Hall probe for quick
and dirty work because it's nice for data logging to a floppy. I use
a real scope (analog) for precise work.

Too bad about your Hall probe. I have a couple of Tek 503s that
continue to give good service despite their age. I too don't trust
mulimeters on the ammeter setting. For accurate stuff I use 0.01 Ohm
3W 1% precision resistors (LVR) on which I have soldered calibrated
Kelvin taps. I use the multimeters (Tektronix of course) set to volts
for current measurement.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Greg Jackson" <greg@t...>
wrote:

> 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@y...]
> 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