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Hi, I'm building a few machines with steppers and it seems I'll go with Nema34 1090oz 4.1mH 5.6A motors.
Important thing is - they will need to run about 16 hours every day non-stop for three months.
Theoretically they should run best at 60V, but I'm thinking about lowering the voltage to 24V to play safe for longevity.
Is it a good idea? Is there a way to predict difference in heat production?
Thanks!
The motors will only be able to spin about 40% as fast on 24V.
I understand that, but I've heard heat increases as a square of voltage, but power - linearly. I have hard time understanding if I'll get any justifiable heat drop.
Actually I haven't operated steppers for such long times, don't know if they'll tolerate that.
Above 60C is when brushed dc motor start to see a reduction in life expectancy. A stepper I think can go much higher as there are no brushes. I know that we have a stepper driven rotary axis and when running all day, it gets quite hot, 77C, hotter then a hot coffee mug. It has run this way for over ten years without issue, so unless your running that hot, I wouldn't worry about it. The machinist running that machine says he had it so hot one time, he couldn't hold on too it for more then a second.
Stepper motors are pretty resilient.
That's reassuring. Thanks!
Switch 4 on stepper drives, if it is on your using 100% holding torque, this creates heat, switch it over and it will be 40 to 50%, thereby giving less heat,
Thanks, I know, but patricular motors will not stop for a second during the day (though they'll go slow), hence I'm considering voltage.Quote:
Originally Posted by Wombilly
It just occured to me that lowering current might help too. I wonder which one should help better? Keeping voltage at 60 would save me buying psus.
Hi kivig,
Actually I don't think lowing supply voltage is likely to reduce stepper motor heating much. It depends somewhat on the drives. There are two main things that generate heat in a stepper: I^2 x R losses and eddy currents.
The first depends only on the current flowing through the coils. In your case the drive will switch the supply on and off to create a relatively constant 5.6A through the coil. If for example the coil resistance is 0.3 ohms then the power would be 5.6^2 x 0.5 = 9.4 watts. This will be the same regardless of the supply voltage.
The second type of heating is caused by AC current ripple caused by switching the supply positive and negative (fast decay) or on and off (slow decay). This is sort of like applying an AC voltage to a transformer with the output winding shorted out. I had a problem once where a motor was over heating and I wasn't aware of the significance of this effect. I figured on a several amp motor what could be a big deal about 0.1A of current ripple. I kept reducing the current setting lower and lower to reduce heating. I finally reduced the current to zero and still had significant heating! Turns out an 80V supply with 0.1 Amp ripple can be on the order of 8 Watts. However most drives, when moving slowly, will use a form of slow decay mode to have very small current ripple and therefore the supply voltage should have very little effect.
HTH
Regards
@TomKerekes That explains a lot. I was wondering what decay modes mean too :) Thank you!
16 hours/day for 120 days sounds like a production run.
If you want to go reduce power consumption you could look into controllers that will operate your steppers closed loop - they will require a rotary encoder (of sufficient resolution), preferably on the rear of the motor - meaning you should buy steppers with duals shafts if you're going this route.
A closed loop system such as this could (depends on controller) be a power on demand kind of thing which would make the servos operate significantly cooler and consume much less power. Closed loop stepper controllers are usually much more advanced that standard stepper controllers, a lot of them have some pretty good mid-band resonance damping controls via DSP methods. As an added benefit, many of these controllers have significant I/O - you can connect your limit switches and even temperature sensors placed directly on the stepper - in the logic you would control what you want to do when the stepper temp is above the manufacturer's recommendation. Hold current creating high temps would no longer be an issue in one of these systems.
If you have sized your nema34's appropriately, then you probably will not be able to get away with 24V power source. At the lower voltage, you may be able to squeeze more power out of a smaller motor.
All this, or you go with a servo system.
Not really production - a motion lighting setup in an exhibition, so things should keep organisers free from failure headache.
Thanks for idea, but I guess won't be able to try it at this time, since everything is already arranged. But I need to investigate on closed loop controllers. For another project (finally building my own cnc) I had an idea to create my own intermediate step counter on Arduino with encoders that'd "press" stop or something when there is a miss/overheat/overtorque etc., but I guess a controller you described might be a better deal. Though I wonder on price, since it is more of a personal project :)
Quote:
Originally Posted by UA_Iron