over current steppers - ok?

[Elnino]

Hi all, i have some Vexta steppers and it seems that i'm just in need of a little more torque. Every now and then they seem to bind on the thread but it can't be by much so i'm wondering if i can run my 2.1A 'rated' motors a bit higer? say 2.5-3A?

I'm using Oatleyelectronics drivers with thier constant current devices as well so i can set whatever current i want with the abliity to go to about 3A each motor with some room to spare on my Power supply.

The motors do not even get warm at all as they are now. Is it safe to bump up the current a bit or am i going to fuse them??

[Markus53]

Its not a Good idea to Over-Current Vexta (stepper motors). It will cause the motors to run to Hot and this will shorten the lifespan of the motors and in the worse cause just burn um out. There are better and much more sensible ways to get (lots) more torque out of the motors. Cheapest solution is run um at slower speeds. Stepper motors loose torque the faster they go, also using a longer acceletaion ramp can get you better results. As you mention the 2.1 Amps i assume you are running them with a Unipolair steppermotor driver and a PK266 (or something like it) motor. By switching to Bipolair drivers you can get 30% more torque. Another step that will get you more torque is using a higher voltage with a Bipolair chopper driver. The coils in a stepper motor have to build up magnetic energy, by using higher voltages (with the same current setting) the buildup is faster allowing higher torque at the same rotation speed. Have a look at the Torque/rpm graphs on the Vexta website for your motor. You will see what i mean. A last important step is using micro stepping. Most of the new bipolair (and some unipolair) drivers can microstep your stepper motors. This means they can energize the coils in the motor partially, and thus forcing the motor to a assume a axial position between to physical steps. This gives a higher step resolution (up to a point) but also increases the torque at high RPM's.
I used to use my Vexta PK266 motors at half steps and when i switched to eighth steps (8 micro steps per physical full step) i got more than a 15% rise in top speed. With my Mills i have been able to get idle speeds of up to 4000mm/minute (157 inch/minute) using Vexta PK266-E2.0 motors and acme threads with a pitch of 4mm/rotation. I have used both TA8435H and A3977 based bipolair chopper drivers, with similar good results.

Hope this helps,

Greetz,

Markus

[altair]

Hi,
steppers are supposed to run very warm, almost hot, at their rated current.
I would say you just don't have the current you think you have in your motors. Check the settings on the drive.
Or maybe, they are just wired wrong, if they are 6-wire or 8-wire types.
Check the motor specs.

Guy

[pminmo]

elnino,
Couple of things. One, Guy is correct, if the motors aren't warm your proably not getting the current you think. Thus less torque. Two, where do you need more power, lower speed cuts or rapids? What power supply voltage are you using?

[Elnino]

Hmm, this is interesting. As i said, the motors are running cold - they do not even get warm after an hour of use.

At the moment i am using the oatleyelectronics drivers. They're not too well known but they do not have microstepping. I am also using thier constant current drivers which maintains the current in the coils at higher RPM.

I could get about 10x the RPM of the motors than at thier 'rated' specs with the CCD's. I'm maintaining 2.1A at up to 28v. (rated is 4.2v). When i built the machine i tested all the CCD's were set at 2.1A - it's possible they might need to be readjusted but they WERE at 2.1A

I'm only using 1.2mm pitch thread and i can maintain 1m/min. I have cleaned and cleaned the thread thinking that some metal shavings have got in there or something but i can't see anything that might even remotely cause it to bind. When it does, i can turn the machine off and manually turn the motor with NO feel of ANY resistance.

It even does it at 100mm/min but the difference being that it only loses 1 or 2 steps instead of a whole bunch. It is not because of the movement mech of the axis as i can not get anywhere near stopping the movement with my hand - it would take my hand OFF before it was likely to stop.

I also tried stopping the thread with pliers and it is almost impossible.

I'm possibly even putting this down to software and NOT the machine itself. I have been using Mach3 and i have heard of other reports from people of it 'losing steps'. I might have a shot at EMC2 and see if that has the same problem.

[kreutz]

I took a look at the oatley-electronics driver. It is an analog current controller and will behave as the linistepper without micro-stepping, so, the motors will work colder than with other current controlling drivers. Your problem could be related to mechanical binding. (I am supposing that your motors have the required torque to move and accelerate your load).

I don't think the problem is a MACH3 limitation unless your PC is too slow to keep at least a 25 Khz pulse rate. (since you are working in Full step "one coil at a time" mode).

Are you using the bipolar or the unipolar drive?

[tfmacz]

I have been told you risk demagnatizing the stepper motor if subjected to serious overcurrent.
That said My nema23 size steppers run very hot at the rated current.

My stepper motors are 8 wire interface. I am using them bipolar with the windings on each phase in parallel. The motor specs list 2.1A per phase so I was advised that with the windings in parallel I should set the motor current limits at 4.2A.

A problem I had with the steppers "stalling" turned out to be a PC problem. I am running EMC2 and a video card was causing the PC to miss steps. We verified this by creating a tester with a pizo type speaker on each channel step pin on the PC parallel port. You could hear "clicks" in the "squeal" as each axis moved. at certain speeds the missing steps would cause the stepper to stall.
I Installed a different video card and the problem went away. Sorry can't remember what make the video was???
Try removing/replacing any extra cards one at a time.

This stuff is always interesting...
Good luck....
Ted

[Elnino]

Mine is the '6 wire' one. I can never remember whether that is uni or bi polar....

Machine is a P4 1.8 and i have tried different frequencies without change. It is however a 'brand name' box - perhaps it's parallel port is not overly 'standard'.

I agree with you that it seems like mechanical binding but it just does not make any sense. Once it 'binds' i can just take my finger off the button and put it back on and off it goes again. And like i said, i shut the machine down and manually turn it from where it bound up and i feel no resistance.

It does the same regardless of rate of movement - 100mm/min or 1000mm/min. the only difference is how many steps it loses.

[Elnino]

Originally Posted by tfmacz I have been told you risk demagnatizing the stepper motor if subjected to serious overcurrent. That said My nema23 size steppers run very hot at the rated current. My stepper motors are 8 wire interface. I am using them bipolar with the windings on each phase in parallel. The motor specs list 2.1A per phase so I was advised that with the windings in parallel I should set the motor current limits at 4.2A. A problem I had with the steppers "stalling" turned out to be a PC problem. I am running EMC2 and a video card was causing the PC to miss steps. We verified this by creating a tester with a pizo type speaker on each channel step pin on the PC parallel port. You could hear "clicks" in the "squeal" as each axis moved. at certain speeds the missing steps would cause the stepper to stall. I Installed a different video card and the problem went away. Sorry can't remember what make the video was??? Try removing/replacing any extra cards one at a time. This stuff is always interesting... Good luck.... Ted

Hmmm - SERIOUSLY interesting that!! (the latter part anyway). See, what i have noticed is that it does seem to 'pulse' as it transverses the table. it's hard to explain but it sorta chages pitch high, then low every second or so. The binding appears to coincide with the pitch change....Maybe that's it!!

Maybe it's just some stupid IRQ confict happening or something! It does make some sense.

Oh, and on your 2.1A vs 4.2A - that seems wrong to me - If you parallel the coils, it halves the resistance thus you should halve your current, not double it. But then again, i have limited experience with steppers (read: 'Nothing before building my CNC machine')

edit - Now that i think of it, 4.2A does make sense halve the resistance and the current will double but here we 'set' the current rather than having it as a result of voltage through resistance. Ignore my comment above....

[Vern48]

I might suggest going to the Mach3 site.
They do suggest that a mother board integrated video card is a bad idea.
Also if you are running Windoze, they have a down load txt of all the supid system programs running in the back ground that you should turn off and how to do it. ie. Windoze security update, etc.

[Me2]

G’Day Elinino,
I had the Oatley Stepper drives and was losing steps intermittently. I had the old drivers that used the surface mount opto-isolators on the board. I found they where playing up so I asked Oatley to send some more down. Soldered them in, same problem.
I noticed that the new boards no longer use them but instead use transistors. I removed the opto-isolators from the board and basically made up an add on board using their new arrangement. No more problems.
If you are using the old opto-isolated boards it is worth looking at.
Have you contacted Oatley direct?

Cam

[Elnino]

Originally Posted by Vern48 I might suggest going to the Mach3 site. They do suggest that a mother board integrated video card is a bad idea. Also if you are running Windoze, they have a down load txt of all the supid system programs running in the back ground that you should turn off and how to do it. ie. Windoze security update, etc.

i have done that and i have disabled all the crap that is not needed. My machine does not have integrated video either....

I'm going to have a bash with another 'generic' machine and see if that makes a difference.

Originally Posted by Me2 G’Day Elinino, I had the Oatley Stepper drives and was losing steps intermittently. I had the old drivers that used the surface mount opto-isolators on the board. I found they where playing up so I asked Oatley to send some more down. Soldered them in, same problem. I noticed that the new boards no longer use them but instead use transistors. I removed the opto-isolators from the board and basically made up an add on board using their new arrangement. No more problems. If you are using the old opto-isolated boards it is worth looking at. Have you contacted Oatley direct? Cam

Mine are the new ones. I have not spoken to oatley's yet - i was assuming that the problem was with my machine and not thier boards....

I'll see how it goes with a new machine dual boot with Mach3 and EMC and compare.