I've seen a number of posts a few years old on this, but none that seemed to have any successful details on driving a motor.
I need to find a motor to replace the regular AC motor on the sewing machine. I need to be able to accurately control the speed of the motor. I've worked a little with stepper motors, but I can't get the rpm I need for the sewing machine motor. (About 1200 RPM). It not real fast, but it seems faster than the working range of steppers.
So is my only other option a DC motor with an encoder attached? I'm not having much luck searching for a small motor like this. Does anyone have any experience with accurate control of a motor hat might work for this application?
Sewing machines generally use speed reduction by virtue of the different pulley sizes on motor and machine wheel. So if the motor is 1200 and the drive ratio is say 3:1 then the machine actually runs at 400 RPM.
You might therefore get sufficient speed if you increase your stepper motor pulley diameter to effectively reduce the speed reduction ratio.
However the speed reduction also gives an increase in torque between motor shaft and machine wheel so you need to ensure your stepper delivers the torque requirements of the actual machine.
You can get to 1200rpm with a stepper, but you have to do things right. 48+V supply, 0.5 ohm NEMA23 motor
with an industrial stepper drive should have a reasonable chance of working, but not guaranteed (the
torque requirement needs to be measured and compared against graphs for the actual motor and supply
voltage).
However you don't want a stepper, you want a well engineered DC motor and H-bridge, that will be good
enough (certainly as good as an AC motor).
Once you get to reasonable sized DC motors (a few 100W), then the speed and PWM duty cycle are strongly related if you use the right decay mode.
your probably right. I probably don't need the extreme accuracy of a motor with an encoder, or a stepper. I've used these motor drivers quite a bit, so I'll have to try that. Pololu - ST VNH2SP30 30A Motor Driver
You could make a very simple speed detector from a slotted-optical-switch and something attached to either the motor shaft or the sewing machine shaft to give a pulse every revolution.
...R
Wouldn't it be simpler to attach an encoder to the existing motor and control the speed with a typical PID method? You'd need to interface or replace the existing driver, which I assume is a triac driver, but that would still be a lot simpler and cheaper than replacing everything with a DC motor, driver, encoder, and power supply.
triac control isn't great at the low-power end, its very sensitive to mains waveform shape and component
tolerances as the circuit spends most of the 1/2 cycle before triggering. DC control is inherently more
stable, though whether it matters for this application I have no idea.
Chagrin:
Wouldn't it be simpler to attach an encoder to the existing motor
That's what I had in mind with the slotted-optical-switch.
...R
I've picked up a nice little 3000 RPM DC Motor. High quality, so I'll see how controlling it with a motor driver does. If it's not accurate enough, I'll add an encoder on the back of it.