Stepper stalls, resonance, and voltage questions

[boyvox]

I was using some small nema 23 size 110oz stepper motors on my hobby sized cnc machine. I was pushing the limit with the cutting I was doing in hard maple and starting to stall out the motors due to cuts that were too deep and too fast. Running it slower was giving me really long cut times and wearing out my CMT bits in no time and burning the wood. All of this may be arguable and I could certainly go for lighter cuts at high speeds, but after trying it didn't give me what I wanted. I decided to try some larger stepper motors, so I bought some of these: http://www.kelinginc.net/KL23H286-20-08B.pdf

My drives are some gecko g201's. I do not know the voltage of my power supply. Given that my old motors were 3v 3A and the new motors were 4.7V and 2.8A when wired in parallel, I thought the transition would work fine. It did not. (probably shows what i know )

The motors worked fine on several different parts but then I came upon one part file that resulted in the X-axis motor stalling every time. Over and over. It would stall even if I was just test running in air. The thing that came to my mind was mid-band resonance, and I was also hearing a kind of groaning sound from the motors as it would get closer to the point at which it would stall. I tried adjusting the trim pot on the G201's but at one end of the adjustment the motor would stall very quickly after starting the cutting file, and at the other end of the adjustment it took a long time, but it would eventually stall. It would typically stall and then start up again once there was a direction change sent from the computer. It would run fine if I ran it really slow, or if I ran it as fast as my setup will allow. If I ran it at my usual cutting speed it always stalled in that particular file which had the x-axis doing continuous parallel movements at a nearly continuous speed for about 30 minutes.

How do I know if the cause of the stalling was or was not mid-band resonance? What else would cause a motor to stall when doing nothing but moving in air?

I also have some Slo-Syn SS2000D6 drives from warner-electric. I hooked the motors up to those drives to see what would happen. The motors sound very different and they have not stalled at all. The motos are running much much hotter though. The first thing that came to my mind was voltage. Are the motors getting a different voltage than with my gecko setup? I do not know for sure but the power source for my Gecko's may be : Watts power supply. 24VDC unregulated (~30V) 8 Amps. This was my first CNC and it's a K2, and this is what is currently listed on their site for my machine. I do not know what the voltage supply of my Slo-Syn SS2000D6 is. I could not find the specs for my exact unit but I think this is the same unit with more programming features: http://www.electrosales.com/warner/p.../WarpDrive.PDF
I am not sure where to find the voltage spec in that pdf. Source mode? huh?
The motor compatibility specs for the SS2000D6 are:

Number of Connections 4, 6, 8
Minimum Inductance 8 millihenrys
Maximum Inductance 64 millihenrys
Maximum Resistance 2 ohms at 6 ampere setting

It looked like my motors should work fine and so far (tested a few times) they have, but they just run so much hotter than with the geckos and I want to know why, and I want to know how hot is OK?

How can I determine the voltage? I don't really understand what determines the voltage the motor is running at. Can I just measure the voltage somewhere (like one of the coil phases) while the motor is running? (I am a total electrical novice btw.)

So to sum up: Why do these motors stall with the Gecko G201's? Why do they work with my Slo-Syn's? Why do they run so much hotter with the Slo-Syn's?

I have my theories but I'd like to hear yours and I would like to learn how to shop for motors that match my drives and power supplies, or at least get a better understanding of how it all works. I've read the "step motor basics" from the Gecko website, and I have a fair understanding of that, but I need some help tying that information back into my current situation.

Thanks,

Ben

[ger21]

Could run hotter with the Slo-Syns due to a higher supplied current or voltage, or both. I didn't see the voltage rating in that .pdf either.

To get the best performance from Geckos, you usually want to run about 20x the motors rated voltage. With a 4.7V motor, you're only supplying about 5x the rated voltage. A 60-70V supply would probably work much better. Any higher and you could risk damaging the Geckos. Usually 2-3V motors are your best bet with Geckos.

[boyvox]

Ok, so will running the motors with such a low supply current (only 5x their rated value) lead to stalling? I know that it is best to get the most out of your motors by running them with as much voltage as they can take, but I'm surprised that 5x is low enough to cause poor operation. I don't understand what is happening when the motors stall. I understand mid-range resonance but I am still not clear as to whether that is what is happening. I am starting to think that I was just running into failure because of erradic operation due to low voltage. I keep reading things at various manufacturers sites that say things like "a minimum ration of eight to one is required for best motor performance". I am still interested to know what actually happens in the motor when it is not being supplied enough voltage over it's rating, that would cause a stall.

I found this link: http://bbmotor.com/products/superior...s/ss2000d6.htm
That has a spec of : Line operated high voltage 180 VDC bus
for the Slo-Syn drives. Is that the voltage of my drive? In that case I'm 38 times over the motors rated voltage, which is too much. The current is defintely limited to 3 Amps on the drive,(same as the Gecko) it's 2.8A rated motor. How can I be sure what voltage my motors are getting(?)using? Is there a way I can take a "live" measurement to get the definitive answer. I just wonder if the drive automatically "senses" the demands of the motor and keeps it in range. I have run it for a few hours with no problem now.

Here's the operating manual for my drive but I still can't find information on the voltage supply:http://mcsupplyco.com/uploads/images...pdf1/D3-D6.pdf

[ger21]

The higher the voltage, the more torque you'll have at higher rpm's. Steppers lose torque as rpm's increase, and with low voltages, the drop off can happen very quickly.

[boyvox]

In my case the motor was running fine at 75ipm (sorry I haven't done any rpm calcs) but when I ran it at 40ipm it would lock up. This was with no cutting load, so I was getting enought torque to run at higher speeds but at a mid-range speed lock-up was imminent. Running at 20ipm would also work fine. Continuous running at 40 always caused a lock-up.

[harryn]

To get the best performance from Geckos, you usually want to run about 20x the motors rated voltage. With a 4.7V motor, you're only supplying about 5x the rated voltage. A 60-70V supply would probably work much better. Any higher and you could risk damaging the Geckos. Usually 2-3V motors are your best bet with Geckos.

This actually answers a question I have been searching for, but it is not the answer I expected (newbie here). I have been reading spec sheets from Danaher (former pacific scientific) stepper motors. For the same general stepper motor model / torque, they often offer 3 sub versions, same torque rating, but different voltage / current per phase.

I was trying to figure out why anyone would not just buy the higher voltage version, as it took much less current to do the same thing, and the voltage of the drivers / power supplies are plenty high for any of them.

So, with Geckos, it is common to drive motors at 20 X the rated voltage - ok, so is that true of microstep drivers in general ? I am guessing there is a relationship between micro drive method and this driver voltage optimization.

Feel free to just point me toward a link / thread / or web site.

Thanks

Harry

[Mike Nash]

While this is not an answer to the question you were asking, watch the videos in this post:

My take on a stepper damper

Then look at the start of the thread:

My take on a stepper damper

to see how to make your own dampers that may help with your stalling. I haven't made any yet, but I do intend to.