Driving stepper with more/less than rated amps?

[MoparJon]

I've noticed that many stepper motor drivers allow you to select the amount of amperage to use for your motor, and of course there are all sorts of motors with different amperage ratings available. However, given the limited amperage options of some drivers, it seems likely that it will not always be possible to exactly match the amperage setting of your driver to a particular motor.

What happens when a chopper type driver is set to a higher or lower amperage than the current per phase rating of the motor it is driving? If you have to err to one side or the other, is it better to drive with a lower or higher amperage than what the motor is rated for?

Thanks!

[ger21]

Higher will damage the motor, so lower is the better option. You'll lose a little torque by going lower, though.

[LeeWay]

Most drivers will only allow a certain amount of amps through. Most are adjustable withing a certain range. Setting a drive higher than it's rated amps is sometimes not even possible, but shouldn't be done in case it is.

If the motor has a higher amp rating than the driver can provide, it won't harm either, but will lower torque rating on the motor.
If the motor has a lower amp rating, you need to lower the drivers rating to match as close as possible.
Not good to run motors at higher than amp rating.
Voltage is a different story and some motors can be wired several ways to accommodate different types and sizes of drives.

[MoparJon]

Thanks very much for the replies and info! I'm glad to hear that underdriving the motors will not damage anything.

Is there a direct relationship between the current and the torque? For example, just to use round numbers, say I were driving a 4A rated stepper with 2A from the driver; would the torque across the board be exactly half what it would be if it were driven with 4 amps? ...and dropping down to 1A with that same motor would result in 1/4 torque?

Or is it not that simple? If it isn't, is there a formula or general rule of thumb to figure actual torque versus amperage?

Thanks!!

[ger21]

Mabe not exactly, but it should be roughly proportional. Half the amps = ~half the torque.

[craigspc]

Originally Posted by LeeWay Not good to run motors at higher than amp rating. Voltage is a different story and some motors can be wired several ways to accommodate different types and sizes of drives.

Hello,

Sorry to jump in on this question, but I have a similar issue/question...

The guy that set up my cnc controller set the controller to 3.0A per motor. My stepper motors are rated 2.8A (2.5V DC). If I turn the power on and jog the machine around, the motors are hot enough to burn your hand off after only 1 minute. Even if I don't jog the machine and just leave it standing for 5 minutes they get extremely hot.

Looking at my controller, the current for each motor can be adjusted between 1.0A and 3.0A by setting DIP-Switches on the board. 2.8A isn't available, but 2.5A and 2.75A is possible. Would lowering the current to one of these settings really make a significant difference with regards to heat generation and the life of the motor?

Regards,
Craig.

[LeeWay]

Possibly. It would likely extend the life for sure depending on how hot they are getting.
I am actually doing the same thing with a set of motors. 2.8 amp rating, but running @ 3 amps. Gecko 201's. They don't have current idle reduction like the 203's. These were the first motors I bought. I was looking for an excuse to get better ones, but so far, these are still kicking. They do get warm. One in particular. These are mounted on a lathe. The one I have on the tailstock for drilling gets the hottest @ 145F degrees.
The other two are 125 and 130F. They just won't seem to die for me.

That doesn't mean it's good practice to do. High temperature will effect the magnetism in the motors over time. I think it needs to be higher than what I mentioned though, but they are probably getting in that vicinity.

I don't think the .2 amps is enough to really kill the motors though. It would only run at that rating when going full bore, so not all the time.
The 2.75, however would be a better choice for increasing the motors lifespan. That is if that's what your intentions are.

I will order two larger motors for my new lathe build and will just likely donate these to some newbie who has already started a build. I would get more satisfaction out of helping someone else out rather than seeing what the destroy point is.

[craigspc]

Originally Posted by LeeWay Possibly. It would likely extend the life for sure depending on how hot they are getting. I am actually doing the same thing with a set of motors. 2.8 amp rating, but running @ 3 amps. Gecko 201's. They don't have current idle reduction like the 203's. These were the first motors I bought. I was looking for an excuse to get better ones, but so far, these are still kicking. They do get warm. One in particular. These are mounted on a lathe. The one I have on the tailstock for drilling gets the hottest @ 145F degrees. The other two are 125 and 130F. They just won't seem to die for me. That doesn't mean it's good practice to do. High temperature will effect the magnetism in the motors over time. I think it needs to be higher than what I mentioned though, but they are probably getting in that vicinity. I don't think the .2 amps is enough to really kill the motors though. It would only run at that rating when going full bore, so not all the time. The 2.75, however would be a better choice for increasing the motors lifespan. That is if that's what your intentions are. I will order two larger motors for my new lathe build and will just likely donate these to some newbie who has already started a build. I would get more satisfaction out of helping someone else out rather than seeing what the destroy point is.

Thanks lee and good luck with your own project.

I think I will go ahead and reduce the current on these motors. Unfortunately I don't have a thermometer for measuring the temperature, but they do seem remarkably hot (i.e they're impossible to touch without burning yourself).

Assuming I don't blow something up in the process of messing with the controller, I'll let you know if it makes any real difference

Craig.

[craigspc]

Originally Posted by craigspc <<cut>> Assuming I don't blow something up in the process of messing with the controller, I'll let you know if it makes any real difference Craig.

Well, I tried dropping the current from 3.0A to 2.75A, then down again to 2.5A. To be quite honest, the change from 3.0 to 2.75A didn't seem to make any difference at all, although the further drop to 2.5A does seem slightly better. I guess that what I originally considered to be "too hot", may be quite normal after all...

[ger21]

What voltage are you running them at. Excessive voltage can cause heating as well.