bumping holding current - how it affects torque?

[dsc]

Thanks for the reply Jonathan. I'm actually quite happy with less speed on the output, which is why I'm currently running the motor at 240RPM and the shaft (output) at 90RPM. At 240RPM there's plenty of torque, around 5.5Nm at 68VDC, this over 2.66:1 ration gives around 15Nm on the output (at 90RPM). This is on the verge of stalling, sometimes it runs fine, sometimes stalls. Max voltage both the HBS86 or the AM882 can handle is 80VDC, would that give enough bump to the torque? Here's the power / torque vs. speed data from Excel:



With a Nema 34 12Nm, which is only 25mm longer than the HBS 8Nm I'm using now (it's got an encoder fitted which I don't need at all), I'm planning to run that at the same speed of 240RPM. This gives around 7.5Nm, which over 2.66:1 ration gives 20Nm on the output (at 90RPM). Hopefully that's enough to get past stalling. Similar data to the 8Nm motor:




I've thought about geared motors, fitted with a planetaty gearbox, but those seem to be rather expensive and overall the cost was similar. I know size is different, but length is comparable and that's the only thing that limits me at this point anyway. With a planetary gearbox I could run with the same pulley size as the gearbox handles the ration, but even for a 10:1 planetary, I'd need a motor which can deliver 2Nm at 900RPM (to stick with the 90RPM on the output and 20Nm torque). This means typical smaller sized motors are out of the question, steppers probably as well, and it leaves me in the AC motor territory, which tends to go huge, bulky and loud. Unless of course I've missed something which is quite possible.

Regards,
T.

[Jonathan]

Based on your new graphs, the peak power is at about 600rpm, yet you're currently running at 240rpm. So for the 8Nm motor you're currently getting 130W (@240rpm), but if you changed the ratio to 600/90=6.66, then you would have about 175W (@600rpm) available - so 35% more. That corresponds to 18.6Nm on the output shaft, which is a much bigger difference than you're going to get from changing to 80V from 68V. Changing the motor to the 12Nm version and leaving the ratio the same gains about the same percentage, but presumably at much greater expense...

I've thought about geared motors, fitted with a planetaty gearbox, but those seem to be rather expensive
[...]
Unless of course I've missed something which is quite possible.

2nd hand cordless drill motor and gearbox.

Edit: When selecting the motor and gearbox it's sometimes easier to think of things in terms of the required power output. If you need 20Nm at 90rpm, then look for a motor that can output a little over 20*90/60*2pi=188W, then see if a gearbox is available to match the speed requirement. In general higher speed motors are more efficient (and therefore compact) - hence why cordless drills use relatively small DC motors which operate in the 1000's of rpm.