Re: My stepper getting hotter

John,

Everything marked "Amps" was measured, everthing marked "Watts" was
calculated. I use ACAD for nearly everything; the data was keyed into
ACAD for the graphs but if you like I will enter it into Excel and
post that as well.

You pretty much have it right in your assumptions except for one
point; data indicates motor losses are independent of load, not
speed. You may have misread that.

Motor power output is the difference between the max load and no-load
currents multiplied by the power supply voltage. The result meshes
closely (+/- 2%) with dyno measurements.

Speaking of dynos for testing step motors; a DC brush servomotor
makes an excellent load. Pick a motor that has a high Kt (>50 in-oz /
amp) and about 10A for the max continuous current.

Measure its Kt accurately. This is easy to do; drive it with a step
motor set to exactly 1000 RPM and measure the motor's open circuit
voltage. That will be its Kv. The motor's Kt will be 1.351 times Kv.

Next, construct a variable current sink that can accomodate the
currents involved (10A times 50 in-oz / A = 500 in-oz). This will
require a supply with a voltage sufficient to develop 10A thru the
motor.

Mount a 1,000 line encoder (4,000 count) on the servomotor. A 40
count lag will constitue 100% load on the test motor, so use some
count less than that.

Servo the current source to maintain this phase lag while you sweep
the test motor from 0 to some maximum speed. Log the servoed current
source reference versus speed and you will have your speed torque
curve.

There are other refinements necessary but you get the idea.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "jmkasunich" <jmkasunich@y...> wrote:
> Mariss:
>
> Thanks for the data - I had no idea my curiosity would lead to
> you doing all that work! About the data, I am not clear exactly
> which variables are measured and which are calculated. I know
> that all of the "Amps" curves are measured data. "Stalled" and
> "No Load" amps are self evident. I think "Maximum Load Amps" is
> the power supply current when the motor is loaded just short of
> stalling or losing steps, right? And "Motor dissipation watts"
> is calculated, "No Load Amps" times Voltage? The one I'm not sure
> about is "Motor Power Output". Is that a calculated result (as
> the text seems to indicate), or the shaft power as measured by
> the dyne (torque times speed)? How do you acquire the data - you
> don't happen to have it in a spreadsheet do you?
>
> The part about motor losses being independent of speed is hard
> for me to get through my head. I'm programmed to think of motor
> current increasing with load, since both DC and AC motors work
> that way. I'll "get it" sooner or later.
>
> I realize that my questions go far beyond what is needed to
> simply use Gecko drives and steppers on a CNC machine - perhaps
> we should take this off the list, or perhaps I'm simply being
> annoying, and I should drop it and let you get back to work.
> If you want to go off list, or drop it, you can contact me
> at jmkasunich "at" ra.rockwell.com (work email, read daily) or
> jekasunich "at" worldnet.att.net (home email, read every couple
> of days, depending on how busy I am).
>
> This thread has gotten me curious enough about stepper motor
> speed/torque/current/efficiency relationships that I'm trying
> to figure out how to make a stepper sized dyne cheaply in the
> basement! I've got a lot of salvaged steppers, ranging from
> tincans and NEMA 17 to NEMA 34, many with no data. It would
> be nice to be able to characterize them. The Shoptask project
> wants NEMA 34 at 300 oz-in or more, microstepped. I am also
> thinking about a robotics project that would need many (18+)
> smaller motors. That project might use the steppers from 5-1/4"
> floppy drives. Cheap motors, with (cheap) unipolar drive.
> Eighteen Geckos would be nice - but too steep for my budget.
>
> Regards,
>
> John Kasunich
>
>
> --- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> > John,
> >
> > I posted "Loss and Power vs Voltage.pdf" in the Files section,
> > Circuits folder. This is the preliminary result from all
> > the "science" I have been doing.
> >
> > Mariss
> >