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,

Jason

 

Subject: Driving stepper motor over maximum rated current?

Hi, Jason. If you're concerned about torque at higher speeds, you might want
to look into current drive rather than voltage drive. This is covered in the
great web tutorial on the subject, Jones on Stepper Motors

http://www.cs.uiowa.edu/~jones/step/

particularly Chapter 4. Current Limiting for Stepping Motors

Also, if you're in a torque bind even then, you might want to look at a larger
motor. Size 23 and Size 34 are the next steps up.

In answer to your immediate question, overdriving the coils at 120% of rated
current for a very limited duty cycle should be OK, but only if you compensate
by reducing the holding current in the coils during the rest of the cycle. See
if your drive has provision for reducing holding current (a lot of them do).
If it doesn't, you will be forcing 1.4A through the coils even when it's
stopped, which is a no-no. Either that or, possibly, your drive has a shutdown
signal which will turn off power to the coils when there's no motion. If
you're microstepping or half-stepping, the motor might bump when shutoff is
released and the motor is turned back on. Remember to unassert the shutdown
signal before you send motion pulses or a "GO" command to the drive - many
drives need time to settle before they can start moving.

Most manufacturers of steppers and drives have apps engineers who are familiar
with that particular stepper and drive, and should be able to give you a
definitive answer. Look to them for help - that's what they're paid for.

Good luck
Chris

 

I have seen some post suggesting that running steppers at currents

Hi

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

 

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.