Driving stepper motor over maximum rated current?

I recently purchased some stepper motors and drives. The drive has an adjustable max current setting of .25 to 1.4 amp RMS or (.35 to 2.0 amp peak). It has an input voltage range between 12 and 24 VDC. I realize that increasing voltage won't change the max current through the windings, only speed up the time to reach max current.

The motors in question are size 17, bipolar, hybrid stepping motors. The documentation sent with the motors says they were tested at "1.7 amp peak" and there are stickers on the motor which say "1.25A" which I'm assuming means RMS. I'm assuming these are the maximum ratings. All the amperage figures are per phase. The fastest I conceivably would like to turn the motors would be about 2800 pulses/sec, or roughly 7 rev/s ? they were tested far past that. I think that for my application I may want to suck every bit of torque out of these motors as possible and I'm already assuming that I'll need to supply them with 24V. This is all very theoretical at this stage though, everything is still in the box.

I have seen some post suggesting that running steppers at currents higher than the rated current is sometimes done. Is this advisable??? Is an increase from 1.25 amp to 1.4 amp/phase (the max available with my drives) a big deal? Am I likely to see a big increase in torque? Am I likely to see it more at slow speed, high speed, both??? Say if I were to slowly increment the current up from 1.25 amp to 1.4 amp to test, is there any way I could tell when I'm nearing the danger of burning out the motor? How would duty cycle play into this? For instance, if I expect to move the motor for only a 1 or 2 seconds at a time and possibly stop for most of a minute in between moves will overheating be an issue (although, I may be using partial or full current for holding torque some of the time)?

Any help would be greatly appreciated. Thanks,


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I'd say it was fine, so long as you don't cook it. OTOH, have you considered a bigger motor?

best regards

Robin G Hewitt

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Robin G Hewitt

You can feed very large currents in provided they're balanced by enough off or low current time that you don't exceed damaging temperatures inside the motors.

You can get considerable improvements in torque while running fast by feeding the motors at a high voltage from a large capacitor with a capacity designed in conjuction with the driving characteristics so that damaging temps are not exceeded. The reason for this is that conventional voltage regulated drive is designed for the worst case of static load, and is heavily degraded by back EMF at high load.

Another way of doing a similar thing is to use current controlled power supplies operating at high voltages.

The virtue of the traditional design is that it's fail safe. The problems of my suggestions are that malfunctions can easily produce damaging conditions, so you need to design in your own fail safe circuits. But the advantage is much higher performance.

-- Chris Malcolm snipped-for-privacy@infirmatics.ed.ac.uk +44 (0)131 651 3445 DoD #205 IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK

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Chris Malcolm

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