Are these stall torque figures misleading?

[Evodyne]

Hi all.

A while back I purchased some "1200" oz-in steppers from a CNC related site off the 'net. These were rated 6 Amps / phase, which is within the usable range for my Geckodrives. I've hooked them up and they run fine; plenty of speed, and seemingly ample power. But no real way to quantify it at present.

When I received the motors, the only paperwork was the single sheet that I've scanned and uploaded. No torque curves. I mailed the site owner and he "got these motors from Asia and they didn't provide him with that information".

What I'm curious about is the note in the Holding Torque column of the specs. It reads "Both phases on". Hmmm. How is holding torque typically defined? One phase energized? Both? Given that I have no curves to go by, is this properly advertised and actually a 1200 oz-in motor, or would it normally be considered a 600 oz-in unit (with only one phase on)? Legitimate or deceptive advertising?

I recently pointed some other 'zone members to these motors in another thread. Now I see the note, and I'm worried. I don't want to see anyone get ripped off, especially due to me.

Can anyone help/clarify? Thanks in advance.

Lance

[CJL5585]

Lance,
Most of the steppers I have looked at all have specified the Torque as with both phases energized.

The reason is this:
When using a drive like a Gecko, both phases will be energized at all times. I do not believe that there is ever a time when only one phase is energized, although I could be mistaken.

I do not believe you got ripped off if the spec sheet is for the steppers that you have. Just remember that any stepper torque will drop with increases in rotational speed. Also, the torque is proportional to the applied drive voltage.

I don't know what your drive voltage is, but it should be something greater than 24 Volts. My recommendation would be something like 35 VDC @ about 14 Amps.

Hope this helps.
Jerry

[ger21]

I believe that with Gecko's, both coils are NEVER fully energized at the same time. They actually never draw more than .7x rated current at any one time, I don't think. This is due to microstepping. When microstepping, the two coils are always using different amounts of current. That's why Gecko's amp requirements are only 2/3 of the motors rated phase current, or 4A in your case. I could be wrong on this, but I think I'm correct.

Jerry, are you saying he needs 14a for all 3 motors? Also, Since those are roughly 3V motors, I'd look for a 48V supply. Although I didn't see Lance mention 24V anywhere?

[CJL5585]

Originally Posted by ger21 I believe that with Gecko's, both coils are NEVER fully energized at the same time. They actually never draw more than .7x rated current at any one time, I don't think. This is due to microstepping. When microstepping, the two coils are always using different amounts of current. That's why Gecko's amp requirements are only 2/3 of the motors rated phase current, or 4A in your case. I could be wrong on this, but I think I'm correct. Jerry, are you saying he needs 14a for all 3 motors? Also, Since those are roughly 3V motors, I'd look for a 48V supply. Although I didn't see Lance mention 24V anywhere?

What you are saying about the coils never being FULLY energized is true. What I was saying is THAT THERE IS NOT A SITUATION IN WHICH ONLY ONE COIL IS ENERGIZED. I did not elaborate on the operation of the Gecko drive.

I thought about recommending 48 volts. The reason that I did not was because my system is using 48 Volts, and the steppers get too hot in my opinion. I can only run my system for a period of 4 hours before shutting it down to keep from damaging my steppers. That is why I recommended the 35 Volt system. It will not have quite the speed or torque one would have at 48 Volts, but then the steppers should operate somewhat cooler .

Jerry

[CJL5585]

Jerry, are you saying he needs 14a for all 3 motors?
------------------------------------------------

I forgot to respond to this.

To be honest, he could get by with a 12 Amp supply. However, I am from the old school, and in electrical engineering I was taught that one should ALWAYS design for a 20 percent safety current margin, So I just stated that he needed 14 Amps for all 3 steppers. With a 14 Amp supply he should never experience any voltage or current problems for the life of the equipment.

I design equipment to last like the Model A Ford...... A Lifetime.
Jerry

[ger21]

Originally Posted by CJL5585 Jerry, are you saying he needs 14a for all 3 motors? ------------------------------------------------ I forgot to respond to this. To be honest, he could get by with a 12 Amp supply. However, I am from the old school, and in electrical engineering I was taught that one should ALWAYS design for a 20 percent safety current margin, So I just stated that he needed 14 Amps for all 3 steppers. With a 14 Amp supply he should never experience any voltage or current problems for the life of the equipment. I design equipment to last like the Model A Ford...... A Lifetime. Jerry

That's what I'd do. Just wanted to be sure you meant for all the motors.

[Evodyne]

Hi guys. My question has more to do with being an informed purchaser. We see so many motors being sold from a variety of sources and the (stall) torque figure, phase current, and type (bipolar, unipolar, etc.) are what we use to compare one type to another. Ideally we would have torque curves, but they are not always available.

Well, O.K, some things are what they are: phase current and motor type, for example. But if the stall torque might be specified for one phase, or might be specified for two, there is a gray area. Unless you know how two different motors are specified, you can't reliably compare them.

Perhaps there is a standard. Perhaps they all are specified for both phases and I'm just confusing the issue for others out of ignorance. The point is that I don't know, and if I don't it's likely that others don't either. So CJL5585, or Ger, or Mariss, or Dan Mauch, or anyone else with more background than me, help! CJL5585's input is that motors are rated for both phases energized. Any other input?

Lance

[ger21]

I **think** that they are usually rated the same as yours. Not 100% sure, though.

[Evodyne]

Back again...

Regarding power supplies, the .pdf off of the Geckodrive site says that a drive will draw 1/3 of the motors rated phase current if wired in series, 2/3 if wired in parallel. I've got four wire motors, not eight, so I don't have an option on how to connect them, but I assumed the 2/3 figure for my supplies.

Since the alleged 1200 oz-in motors are rated at 6 Amps per phase, a four amp supply should minimally adequate, according to the attached .pdf.

My new supplies use a 48 volt secondary, 10 amp, no center tap transformer. I had some 20 Amp 400 PIV bridge rectifiers, so I used them. I've an assortment of 80-100 V electrolytic caps, ranging from 4800 uF to 60,000 uF so I'm covered there.

A quickly cobbled together bench supply put out 66.7 VDC. As my target was 70 VDC, I'm happy. Motors seem to run well.

I'm going to have one supply per motor. Theres no real extra cost as I've had all the parts laying about for some time.

Lance

[Evodyne]

Originally Posted by ger21 I believe that with Gecko's, both coils are NEVER fully energized at the same time. They actually never draw more than .7x rated current at any one time, I don't think. This is due to microstepping. When microstepping, the two coils are always using different amounts of current. That's why Gecko's amp requirements are only 2/3 of the motors rated phase current, or 4A in your case. I could be wrong on this, but I think I'm correct.

That and the chopper design. Remember Mariss says the drives ought to be powered by a supply 3 to 25 (or 4 to 20, depending what you read) times the motors rated voltage AND be between 24 VDC and 70-80 VDC. As a phase is energized, a much higher voltage is applied to it than its rated voltage-and the current QUICKLY rises. Once the current is at the drive's programmed limit, the electronics modulate the voltage on and off to maintain it. The time averaged current the drive sees is thus lower than if the phase were energized continuosly. That's my understanding of it.

Oh, and that rapid inrush of phase current is mirrored by a rapid magnitization of the coil. Torque is proportional to the field strength and so over the time that the phase is energized, more torque is available from a chopper drive than a standard drive.

And unless Mariss says otherwise, I agree with you about that two phases are never fully energized at the same time.

Lance

[miljnor]

I no nothing about steppers but from a company stand point I believe that if one company advertised 1200 in/oz with both phases energized then all of them would have to advertise this way. For one simple reason, people tend to believe the advertisement and don't tend to investigate further, And this being the case, the big mondo numbers (like 1200 in/oz) compared to half that number for the same prices, most people would buy the 1200 in/oz.

Now the people on this sight I would have to say are in the top of the IQ pool, and being a DIY's we are not lazy (well not that lazy ) so we tend to ask these types of insightfull questions.

So that being said, I think everyone advertises the motors the same way and the ones that don't probably have a huge clarification saying how all the "bad" motor manufacutres do it. Kinda like with 3 phase induction motors (20hp vector drive, ya right and I got 12"'s to prove it! )

[Evodyne]

Originally Posted by miljnor I no nothing about steppers but from a company stand point I believe that if one company advertised 1200 in/oz with both phases energized then all of them would have to advertise this way. For one simple reason, people tend to believe the advertisement and don't tend to investigate further, And this being the case, the big mondo numbers (like 1200 in/oz) compared to half that number for the same prices, most people would buy the 1200 in/oz.

Ah, but there is always that exception, that one scammer. I was troubled by the fact that torque/speed curves were not available. Why not? If they had been I wouldn't have started this thread-there would have been no need.

Originally Posted by miljnor Now the people on this sight I would have to say are in the top of the IQ pool, and being a DIY's we are not lazy (well not that lazy ) so we tend to ask these types of insightfull questions.

Yes! I am constantly impressed with the people who frequent this sight. This is a paradise for those who want to learn and need some inspiration, education, or guidance.

Originally Posted by miljnor So that being said, I think everyone advertises the motors the same way and the ones that don't probably have a huge clarification saying how all the "bad" motor manufacutres do it. Kinda like with 3 phase induction motors (20hp vector drive, ya right and I got 12"'s to prove it! )

I would hope that this is the case, but like I said before, I don't know. Which was the whole point in asking to begin with. If nothing else it points out the need to ask before you buy. Maybe, once all is said and done, it boils down to nothing more than that. The kicker is that you need to be informed enough to know what to ask! Sometimes I just feel dumb. Ugh.

Lance