Hi folks,
I'm pondering changing the motor on a light commercial sewing machine from the original small (54W) brush-type universal AC motor to a stepper motor. I want to do this for two reasons - first, to get better very-slow speed control (lets not ask about my sewing machine skill...) and better low-speed torque, and second, to allow it to be used aboard a boat with only 12V DC power. (If it matters, the machine is belt-driven with a 3:1 motor/machine pulley ratio.) The 12VDC part is my concern today.
First a bit of background:
The machine layout limits me to a NEMA 17 stepper. The stepper I'd like to try is a 17HS19-2004S1 (I've seen mention of it in several Arduino posts). It claims the following specs: 1.8 deg, 2.0A, 1.4 ohm, 3mH, 2.8V and 84 oz.in. torque (and I assume this torque is only realized at fairly low speeds - but that's OK - that's where I need it). I plan to drive the stepper with a prepackaged TB6600 driver (don't know yet if I'll try microstepping to avoid low speed resonance issues) and control the whole thing with an Arduino Uno.
OK, on to today's question - (more to follow, I'm sure...) - since not everything will be done at slow speed, I'm looking for a rough guess from the assembled gurus about the max unloaded RPM I can expect if powering this stepper and driver on nominal 12VDC (typically 12.2 - 12.7VDC actual).
To get a stepper motor to operate at speed you need a high voltage supply - 24v or 36v or maybe more if the TB6600 can accept it. Of course you must be careful to adjust the current limit on the driver to match the 2 amp motor current.
Be aware that the torque from a stepper motor falls off rapidly as speed increases. The better motor manufacturers post graphs showing speed and torque - I suggest that you study some of them.
If you want a stepper motor to operate at high speed you must accelerate it. Like any motor they cannot go instantly from stationary to high speed.
Do you know the speed (RPM) that the existing sewing machine motor works at?
I wonder if a larger stepper motor without a gear reduction might be a better option.
Hi Robin2. Thank you for replying so quickly, and your points are well taken. But there are several limitations I have to deal with.
First - and perhaps most importantly - I'm limited to 12VDC for power. I understand the advantage of higher voltage but providing it would a major problem.
Next, I'm limited by the shape and dimensions of the sewing machine itself. There is a finite space and location for the motor and anything bigger (i.e., NEMA 23) won't fit.
Further, while changing the pulley ratio would help gain higher speeds, it would reduce the available torque at the low speeds where I need it most. (Most of my work is done at very slow speeds and, when dealing with heavy material at very slow speeds, the machine lacks the momentum to help push the needle through on the next stroke - it depends more on continuous toque from the motor. At higher speeds, momentum is greater and everything works better.)
I just put a hand tach on the existing AC motor and it looks the range I use is from about 180 RPM to perhaps 750 RPM - and I suspect that is achievable with the stepper motor on 12VDC. My original question was just to see what I might expect for a max RPM (unloaded - still using only 12VDC). It's not that I expect to work at the max RPM - and I understand the great reduction in torque there - I'm just wondering what the upper limit might be.
So still hoping for a rough guess from folks' experience about what the max unloaded RPM might be with this stepper and driver on only 12VDC?
I doubt if you would get any useful torque from the stepper motor at 180 RPM with a 12v supply. But do search out and study manufacturers' performance graphs. If you can't find them for your exact motor look for something similar.
Stepper motors are intended for precision rather than speed. They do have the useful feature that at any given speed they operate at their full torque all the time, unlike a DC motor for which you have to increase the current to maintain speed when the load increases.
If you can get a 12v DC motor that has sufficient torque that would, IMHO, be a better choice. The motors in car windscreen wipers spring to mind and should be available cheap from breakers' yards.
Hi again Robin2 - and thanks for your most recent reply.
I pondered using a regular DC motor but I'm concerned about their poor low-end torque and lack of inherent speed control under varying loads.
Your point about relatively constant torque from the stepper and its precision motion (which can be viewed as precise speed control under varying loads) are the elements that attracted me to them. Hoping that one could operate successfully at 12VDC may be my downfall...
So, of course, I will take your advice to look further for curves more appropriate to the particular motor I have in mind. Hopefully that will shed some light on the 12 volt problem.
Thank you again - I appreciate the time and help you're giving me.
fitzpad:
I pondered using a regular DC motor but I'm concerned about their poor low-end torque and lack of inherent speed control under varying loads.
You can build a feedback speed controller with an Arduino that will hold a DC motor's speed very steady with varying loads. I built one for a very small DC motor (for a model train). The only difference for your project would be the need for a much beefier motor driver.
I used a QRE1113 reflective optical sensor to detect a spot of white paint on a black disc on the motor shaft to detect the speed. The low speed for my motor might have been much faster than yours so maybe print a disc with 4 or 8 white stripes.
fitzpad:
Hi again Robin2 - and thanks for your most recent reply.
I pondered using a regular DC motor but I'm concerned about their poor low-end torque and lack of inherent speed control under varying loads.
Don't think that's warranted, AC universal motors are basically like DC motors.
Your point about relatively constant torque from the stepper and its precision motion (which can be viewed as precise speed control under varying loads) are the elements that attracted me to them. Hoping that one could operate successfully at 12VDC may be my downfall...
For low speed control you ideally need a feedback (servo) loop. You might even find one of the servomotors
sold for CNC use (as stepper-replacements) could serve you here.
So, of course, I will take your advice to look further for curves more appropriate to the particular motor I have in mind. Hopefully that will shed some light on the 12 volt problem.
Thank you again - I appreciate the time and help you're giving me.
Hi MarkT, and thank you for replying. I understand your point about DC motor torque. My remark about low speed torque dealt more with the running the motor at relatively low supplied current to obtain slow (open-loop) speeds rather than the motor's max torque potential. (The little universal AC motor on the machine now, if given enough current, has plenty of torque to operate the machine, and it runs quite smoothly - and steadily - at higher speeds. My issue is trying to operate it smoothly at very slow speeds.)
However, considering the nature of the problem, especially the way it expresses itself at slow speeds, your point - and that made by Robin2 earlier - about a closed-loop system are well taken and may wind up being the best solution from a control standpoint. The realities of motor size and electrical demands may pose other problems - but this is certainly an approach I will investigate.
Thank you (and Robin2) again for your thoughts and your help with this.