I have an old Husqvarna type 12 sewing machine with a small AC motor which speed was adjustable with a knee-controller using a rheostat, as on most other machines with foot-pedals at the time, like Singer, etc.
The rheostat is dead and I was thinking about an Arduino with the appropriate control to build a foot pedal instead.
For the moment I have removed all the old "electronics" with the motor (mainly capacitors and this rheostat) and connected the motor directly to the 220V, but as expected without rheostat it runs at full speed.
What would be the most appropriate way to achieve speed control? PWM?
Is it possible to replace the motor with something more suitable for control by "modern" electronics, like a DC or stepper motor of comparable mechanical power?
If it's running on ac mains supply, which I suspect it is, then look up thyristor drill speed control.
They are fairly simple to build or you could buy as a kit.
Maybe check on Ebay.
A standard AC light dimmer can be used with small electric motors.
Maybe try a "Heavy Duty" version.
Depending on your electronic construction skills, you can make an Arduino controlled light dimmer / motor speed control, but a ready made item will be cheaper.
I don't know if an ssr will do what you want, they are not really designed for that use.
What you need is a real AC motor speed controller, with a triac and a zero-crossing detector circuit.
Maybe something like this, but I'm not sure if it has the features I'm talking about (the descriptions of the items on eBay & AliExpress are all not very technical or in terrible "Chinglish").
I can now see very similar things on Aliexp, rater for 2000W and beyond, but they use thyristors instead of triac and say nothing about zero-crossing: is that an issue?
I remember there are two main types of AC motor. There are synchronous AC motors, who's speed is governed by the mains frequency, like the one in my garden shredder. And there are asynchronous AC motors which can vary their speed, so I guess the sewing machine motor must be that type.
Thyristors, I think, are also called Silicon Controlled Rectifiers (SCRs). Most of those controllers on eBay and Ali use those, I think. They will only let one half of the AC current through, the positive part of the cycle, and block the negative half. So at the maximum setting you will still only get half the power through, I suspect. Maybe this will be enough, what do you think? Could you keep up with the machine with the motor going at full power? Would half power slow you down too much sometimes?
Triacs let both positive and negative parts of the AC cycle through, so should give maximum speed to the motor.
The zero-crossing thing: this only switches on the Thyristor or Triac at the point in the AC cycle where the voltage/current is zero. This reduces the stress on the motor and other components, and reduces the electrical interference produced as well, I think, because the PWM frequency is only 2 x the AC frequency (and in phase with it) rather than several Kilohertz PWM used in those cheap controllers. So it's a nice-to-have if it's not too much extra cost. But I'm not sure how you would achieve that with Arduino. I'm sure it can be done...
EDIT: no, I must have got that zero crossing explanation wrong. Triacs and Thyristors only turn off when the voltage & current is zero.
For controlling a motor you must add a snubber circuit or you have a good chance the triac won't switch off. I haven't found any modules so far that had both the snubber, the ability to control a motor, and do zero crossing detection on a single board... so just ordered the PCB to build my own instead.
Tryristors are indeed for DC (one direction), TRIACs for AC (both directions).
Industrial sewing machines usually use servo motors - not those little plastic 90 degree motors, but big ones that turn all the way around. Like this one. They usually have a controller built in.
