Need a little experiment carried out....

[Jonathan]

Using the 1Nm motor, 700mA, 42rpm (calculated from graph, seems reasonable as lathe was set to 10Hz using 205rpm ratio so 10/50*205-slip etc)



Many more graphs attached:

MY103Test.zip

That's with 700mA through one phase and a 1K resistor in parallel with the other phase. I also added a 1nF capacitor in parallel with the resistor to filter out the many spikes on the waveform, which the oscilloscope reckoned were at about 20kHz. I think this motor may be damaged though, as to get 700mA through it in bipolar parallel required approximately twice the voltage on the phase I used, compared to the other phase, which implies one coil is open circuit. I should try again with a more reliable motor! This does mean that the test at 1.4A was exceeding the rating somewhat, so I didn't do it for long... may cause a problem due to saturation.

I'm not sure that we can glean much from these results, as I'm not measuring the torque, so we there's not enough information to plot power vs phase current, or P vs V. I could measure the torque using a longish bar and a force meter (= hanging scales)...awkward to calibrate though, but still fine for comparing readings. Alternatively could assume that (pretending for the moment I'm not using a dodgy motor) at rated current the mean torque will be rated divided by squrt(2) and assume torque (and therefore power) is proportional to current but that method clearly has problems.

Thinking about it I should have measured the input voltage to the energized phase then at least we'd have input power...just done that and at 42rpm and 84rpm input power is 14.8W with 1.4A, similarly 3.7W for both speeds at 0.7A. Still 3.7W at a substantially higher speed, so looks like input power isn't a function of speed. At higher speed the waveform peak splits into two, and if you look at the FFT the harmonics are substantially greater. If the speed is greater still (tried 1kHz, so 1000/50*60=1200rpm) the output is a very much purer looking sinosoid, power is still 3.7W ... might be going a bit off the topic here so I'll stop.

[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