Steppers don't want to get up to speed
Hi,
So I hope it's ok to make a post where I believe I have the answer, but it bugs me too much for me not to ask experienced people to get the definitive answer...
So i have a complete kit from HobbyCNC.com. It consists of:
- A "Pro" board equipped with SLA7078MPR stepper drivers
- HobbyCNC's most powerful steppers (3A/phase, 2.15Nm holding torque, 3.2mH inductance, Vcc= 56VDC <--- I'll get back to this)
- The components to build a non-switching power supply of 36V. HobbyCNC recommends 36V for their board, but minimum 12V and maximum 42V is also within tolerances.
So, my problem is:
regardless of load, which axis I'm running, acceleration, microstepping setting, Vref setting (currently set to exactly what's recommended), my motors will not get up to speed. Anything above 720 RPM and they stall. with the gear/sprocket ratio of my EMCO F1P, this results in a not very impressive travel of 720mm/min (1:5 gearing, and 5mm pitch on ballscrew).
Now, back to the "Vcc" value for the motors, of 56VDC. Doesn't this value tell me that the motors would be better off with a higher voltage? And if so, and this bugs me the most: why did even HobbyCNC sell these motors together with a board that can only handle 42V (and recommend 36V in addition to that..)?
I could switch to a power supply with a 42V output, but how many rpm's would I gain with the extra 6V's?
Might there be anything else I've missed? Or am I expecting to much from my steppers, is 720rpm not too bad? Up to and including 720rpm, they run very smooth, and feel quite torquey.. the reason I expected more was that the machine is from somewhere in the late 80's and was specced to rapid up to 1200mm/min, and it is painful not to at least get the same out of my "modern" setup.
Thanks in advance guys :)
Re: Steppers don't want to get up to speed
1:5 ratio is a bit much. You need to change that if you can.
Extra 6v won't do much.
I went from 36v to 60v on a mill and it now flies but I had to change a lot of stuff.
If you can't change the gearing then steppers are a waste of time for you.
They max at 1000rpm and rarely get that.
You need 'proper' AC servo's that can run at 1500-3000rpm for the ratio you have I'm afraid.
Re: Steppers don't want to get up to speed
Hi,
Quote:
Anything above 720 RPM and they stall. with the gear/sprocket ratio of my EMCO F1P, this results in a not very impressive travel of 720mm/min (1:5 gearing, and 5mm pitch on ballscrew).
Believe it or not but 720 rpm is not too shabby for a two phase stepper.
All steppers lose torque the faster they go, that plain physics. The lower the inductance the less degradation in torque. A stepper of 1mH inductance and 60-70V drive should retain about 45% of
its torque at 1000 rpm.
The higher the voltage (drives and supply) the faster you'll get any given stepper to go.
I have just taken my Vexta 5-phase steppers out of service after seven years use. The Vexta drives supply about 150VDC and so they really fly. I had them tuned for 2400 rpm, and they never missed
a step unless I did something stupid....a common enuf occurrence! Point being that low inductance AND high voltage are required to make them go fast.
Note that my steppers had a low lash 10:1 planetary gearbox so while the motor speed was 2400rpm the output shaft was only 240rpm. Direct coupled to a 5mm pitch ballscrew resulted in rapids
of only 1200 mm/min. I lived with that for seven years. Given that my mini-mill had only 180mm x 180mm x 180mm it wasn't a huge issue.
My new mill has 750W Delta B2 series servos and they eat any stepper ever made. Their rated speed is 3000rpm, but I have then tuned for rapid traverses of 5000 rpm. It uses a clever mode
of operation called 'field weakening mode' and is only applicable to Field Oriented Control motors. Direct coupled to 5mm pitch ballscrews results in 25m/min rapids....which is scary fast for me!
Craig
Re: Steppers don't want to get up to speed
Thank you so much for your answers!
I have then come to the conclusion that I will live with the steppers and speeds I have for now, and when the time is right to upgrade, servos will be the only option.
There's not much I can (or want) to do about the gearing, but I do realize that if I got it down to say 1:3, I would be where I wanted to, speed wise.
I still think/wish i should be able to see some higher zero-resistance speed with the motors I have, but "investing" in a 56V power supply would also mean ditching the whole HobbyCNC control, which is for another day.
Thanks again :)
Re: Steppers don't want to get up to speed
I really do not know what i am doing.
Just another reason to leave it slow and steady for the time being. Thanks again :)
Re: Steppers don't want to get up to speed
Hi,
changing the gearing is not the answer. Reducing it to 3:1 will demand more torque from your steppers so that instead of stalling at 720rpm they would
stall at 500 rpm, net zero gain.
If you replaced your drivers with 80VDC units, which are the highest voltage readily available drivers, and used an 80V power supply, your existing steppers would
perk up and probably do 1000-1100 rpm. If you replaced your steppers with low inductance units, say 1-1.5mH AND 80VDC drivers hen you could probably
get to 1500-2000 rpm. If you want more than that then servos are the answer.
Craig
Re: Steppers don't want to get up to speed
But lack of torque is definitively not what's happening, as all 3 axes are stopping at the exact same rpm, and regardless of direction. Take the Z axis, on its way down, it's getting help by the mill head, but on the way up it has to lift it. Those two resistances are way apart, and still it stalls at the exact same rpm.
Also checked disconnecting all stepper motors from their pullies and pull screws, and still the same.
Also: when running the motors at the absolute highest rpm it will run, there is nothing I can do to force it to stop.
I am really leaning towards a resonance issue. But still, I have no idea what I am doing.
Envoyé de mon SM-G970F en utilisant Tapatalk
Re: Steppers don't want to get up to speed
Quote:
Originally Posted by
EddySPalm
But lack of torque is definitively not what's happening, as all 3 axes are stopping at the exact same rpm, and regardless of direction. Take the Z axis, on its way down, it's getting help by the mill head, but on the way up it has to lift it. Those two resistances are way apart, and still it stalls at the exact same rpm.
Also checked disconnecting all stepper motors from their pullies and pull screws, and still the same.
Also: when running the motors at the absolute highest rpm it will run, there is nothing I can do to force it to stop.
I am really leaning towards a resonance issue. But still, I have no idea what I am doing.
Envoyé de mon SM-G970F en utilisant Tapatalk
Torque drop off will be what's happening. You're actually lucky you're hitting 720rpm!.
Once you get to a certain current for a given voltage there will be next to no torque in them and they'll stall.
If you double the voltage with larger drivers you may get close to an extra 20% curve but that will be it.
There is nothing wrong with them, it just... is what it is... That's steppers for you.
On ny mill I have fairly decent ones, nema24 size running off 60v. They hit stall at 700rpm which gives me max 3500mm/min direct drive with 1605. Gibs are snug and the table/carriage add up to around 30kg weight.
Like I said, with steppers, it is what it is. They have dramatic drop off after 500rpm.
If keeping the machine in the long run as is and do eventually want to go faster. Don't waste time/money messing with stepper systems. You need servo's of 2000-3000rpm or more (depending on what rapid speed you want). You won't be reliable trying to get steppers close to 1000rpm and above.
AC servo's such as this:
https://www.aliexpress.com/item/3304...10986290%22%7D
No needed extra expense for power supplies either. Mains breaker direct.
(I have the 1.8kw version of one of these for my spindle).
