Stepper motors' drive is AC-like if moving, but DC when stopped. There is a common misconception that you target average current, but that's not correct.
There are two windings in bipolar steppers. If you have a motor rated at 2 amps with 2 ohm DC winding, that rating allows 2A, 8W total, because it can stop on a winding. The motor is rated for 8W dissipation, presuming it is free-air with a reasonable ambient. Also the construction guarantees you won't demagnetize the magnets or cause core saturation at 2A on a winding. Alternately, same bipolar, it may stop between phases in a microstep situtation. That's 45 deg, so with a 2A peak, a microstep driver delivers 1.414A^2 *2ohm * 2 windings =8W total spread between two windings. It's spread out so the hottest winding is cooler because it's only doing 4W, but that's not where the rating comes from. Still 2A per winding. There is no case where a driver set for a 2A peak will put 2A on EACH winding. THAT is the mysterious, confusing part of the spec if you overthink it. It says "2A/phase", 2 phases, doesn't that add up to 4 amps? Not really, that number is based on the assumption that you drive only one phase or a phase A=sin(angle)*2A + phase B=cos(angle)*2A. Industry std assumption. There's ONE catch. In the case of UNIPOLAR, you break each phase into 2 windings. With the same above motor tapped like that, each winding ould be 1ohm and still rated for 2A. You could drive the two half-windings in series, making a 2ohm 2amp winding and that's the exact same motor the bipolar case above already is. Or, unipolar is only pulling down one half-winding at a time (the only reason to do this is it's a simpler driver, and mostly irrelevant now because driver costs are quite low for bipolar drives). In that unipolar case, you can still ONLY apply 2A/winding, even though you're using half-windings and only 1 of 4 windings is being used when it's on-pole. Only 4W total motor heat, but only 1/(2^0.5) the torque is possible. You might say "well the motor can handle 8W, so I'll give it 2.83 amps and get 100% of the bipolar torque again". But this will exceed the winding's rating, even if it's only a half-winding. This is why unipolar doesn't make much sense, a larger motor to meet a torque requirement is expensive, whereas even the bipolar driver is pretty cheap. Danny On 9/3/2016 3:02 PM, Kirk Wallace wrote: > On 09/03/2016 11:32 AM, Andy Pugh wrote: >> >>> On 3 Sep 2016, at 21:12, Danny Miller <dan...@austin.rr.com> >>> wrote: >>> >>> l approximation of current targets whose PEAK value is equal to the >>> motor current rating. >> Are you sure? I don't have any information at all but would expect it >> to be average current. > My understanding is that one should check the motor and driver specs for > voltage limit, and supply with the lower of the two voltage limits. > Then start with a low driver current limit, let the motor come up to > temp, test the temperature, then turn the current up and repeat until > the motor runs at the desired temperature (usually barely touchable). I > believe that the motor sitting at idle will produce the highest temp. > > In addition to the Jones link, this should also have some good > information: http://www.geckodrive.com/support.html > > ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
