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.