Need a little experiment carried out....

[irving2008]

Jonathan

Thanks for doing that. not sure i got the answer i was hoping for :(

Basically I was hoping that applying dc to one coil of a stepper it would act as a brake. the lathe clearly has much more grunt so no braking effect was seen (frequency is near identical) and the output voltage from the second winding is essentially the same so isnt a measure of braking force.

i wonder if turning it by hand would empirically show the braking effect and maybe measure the effect using pulley and falling weight approach

[Jonathan]

Basically I was hoping that applying dc to one coil of a stepper it would act as a brake. the lathe clearly has much more grunt so no braking effect was seen

The stepper motor is definitely applying a torque opposing the lathe and if I just hold the motor it feels like that torque is a function of the phase current - presumably they're proportional. The torque ripple is high though since we're operating it as a single phase motor.

i wonder if turning it by hand would empirically show the braking effect and maybe measure the effect using pulley and falling weight approach

I think measuring the torque using the method I suggested earlier would be easier?

[irving2008]

The stepper motor is definitely applying a torque opposing the lathe and if I just hold the motor it feels like that torque is a function of the phase current - presumably they're proportional. The torque ripple is high though since we're operating it as a single phase motor.

Hmmmm hadn't though about that. Should be powering both windings in parallel then?

I think measuring the torque using the method I suggested earlier would be easier?

Maybe, but a dropping weight and pulley is easy to calibrate

[Jonathan]

Hmmmm hadn't though about that. Should be powering both windings in parallel then?

If we're just using it as a load, since it doesn't look like we can get useful information from the other phase, then yes I tend to agree. Also you wouldn't get rated power with just one phase energized.

Maybe, but a dropping weight and pulley is easy to calibrate

Yep, could even use dropping a weight to calibrate the other system.

[WandrinAndy]

It's great seeing you two "talking dirty" with one another again

[Lee Roberts]

Just spotted something interesting...

I measured the back-emf voltage from the 3.1Nm stepper motor at 1200rpm and it's 66 volts, so using those numbers k=0.525 Vs/rad. The unit Vs/rad is dimensionally equivalent to Nm/A ... i.e the 'torque constant' of the motor, so we can use this to find the torque for a given phase current. Rated phase current is 4.2A, so for both phases T=2^0.5*4.2*0.525=3.12Nm. That matches the holding torque specification rather nicely, so maybe this could be a simple way to find the rated torque of stepper motors. Or more usefully, use the formula for phase current (something like i(t)=V/R-(V/R-I)*e^(-t/(L/R))) multiply by k to get torque as a function of time then integrate to get the mean torque and then the torque vs speed curve for the motor. I think this should be more accurate than the usual approximation.

Yea i would have to agree with what he said....because i don't have a clue....what he just said....

It's great seeing you two "talking dirty" with one another again

Agreed, all we need now is the shaft size equations to come into it...

.Me