What are my options for controlling an universal motor?

I am restoring a vintage sewing machine equipped with an electric motor.

It's a 100W brushed motor operating at 220V (AC), unfortunately no other information is available from the plate. Having a woven stator and rotor, it is very likely to be an universal motor, unless someone else can object to that. Current speed control is achieved using a foot-operated rheostat, which becomes quite hot after sewing at slow-speed for some minutes.

I would like to replace the rheostat with an Arduino-driven speed controller to achieve higher efficiency or at least to eliminate the nasty overheating issue. Ultimately, I would also like to computer-control the thing, but that will be trivial as soon as I have the microcontroller in place.

I have researched two possible approaches, with some pros and cons:

  • phase-angle partialization (AC): simpler and cheaper circuit, but lower efficiency and higher EMI
  • high-frequency PWM (DC): more complex driver circuit, higher efficiency, lower EMI but increased wear on brushes

Regarding brush wear, this is what I infer from a high-level motor control presentation by ST Microelectronics, although no justification is given for the fact. If it wasn't for that, I would definitely go for the latter solution.

Any buzz noise is not a big issue since the machine is already super noisy (it's fully mechanical).

I don't plan to mass-produce or even sell the speed controller, so the cost is not an issue, as long as it is not prohibitively high. Same goes for construction complexity.

And now for the questions:

  • Are there any other options to consider?
  • Is the impact on brush wear actually relevant/correct?
  • What is the overall best choice for the intended purpose?

Finally, I would like to save some collateral discussion by saying that I know how to safely handle mains electricity (e.g. that I need opto-isolators and such), but any consideration is welcome :)

Hi,

Such a motor is not standard at all (even if it is vintage).

Can you post a picture?

Regards.

Hi.

Such a motor is not standard at all (even if it is vintage).

Every mains powered tool was a universal motor, most powered kitchen utensils were universal motors. Vacuum cleaners were and in a lot of cases still are universal motors.

Most popular speed control was SCR control, problem is you will not get better efficiency with a speed control.

They rely heavily on their internal shaft mounted fan to keep them cool, so if you are going to run it for any length of time at low speed you will need to provide a cooling fan independent of the motor.

Google SCR speed control or universal motor speed control

Tom.... :)

TomGeorge: Google SCR speed control or universal motor speed control

Hi Tom, thanks for your reply. I already did a good deal of googling here, which I summarized in the op :)

I guess the "SCR speed control" thing falls under the "AC phase-angle partialization" scheme, although I have seen a TRIAC in use. It was the first approach I took before discovering the DC option. Therefore, I was seeking advice wrt my discoveries, especially regarding the brush wear part under DC control and any apparent advantage this scheme yields.

The machine can already be speed-controlled and the metal body does not overheat, only the pedal. The rheostat is a carbon pills one, which also poses fire hazards. One more reason for getting rid of it :D

I do feel ashamed :-[

I assume you have blown all the lint out of the foot control.

How are you intending to control the scr/triac when you get the controller built? You certainly don't need an Arduino for this project.

Paul

Standard triac circuit is what is used with universal motors. Many AC motors are not universal motors, note, induction or synchronous motors are ubiquitous (nearly all mains fans are induction, clocks are synchronous - neither of these can be triac controlled but need VFDs)

Paul_KD7HB: How are you intending to control the scr/triac when you get the controller built? You certainly don't need an Arduino for this project.

I definitely don't need an Arduino, but I happen to have one lying around :) I probably don't need a microcontroller period, but it makes things like soft-start and closed-loop control so much easier (I'm thinking forward now).

The microcontroller is used to perform a zero-crossing detection and fire the TRIAC at an appropriate phase-angle in the AC drive configuration. In the DC drive case (aka "DC chopper"), the micro generates the PWM train that commutates an IGBT + freewheeling diode in parallel with the motor.

This last configuration is what I am particularly interested in, since it appears to have many advantages. However, I read that it might wear brushes faster than the AC counterpart, although there was no justification for such a claim.

MarkT: Standard triac circuit is what is used with universal motors. Many AC motors are not universal motors, note, induction or synchronous motors are ubiquitous (nearly all mains fans are induction, clocks are synchronous - neither of these can be triac controlled but need VFDs)

Hi MarkT, I am 99% positive that it is an universal motor. TRIAC drive was the first improvement over the rheostat, afaik, but now there is also the DC drive option and possibly others I am not yet aware of. And that was one of my questions.

If it has brushes and is very noisy it is a universal motor.

Both universal and induction are commonplace, and are completely different - just making the point for people who come across this thread in the future.

PartialTruth: In the DC drive case (aka "DC chopper"), the micro generates the PWM train that commutates an IGBT + freewheeling diode in parallel with the motor.

This last configuration is what I am particularly interested in, since it appears to have many advantages. However, I read that it might wear brushes faster than the AC counterpart, although there was no justification for such a claim.

It's for the same reason that relays or switches are rated lower for DC current/voltage than AC current/voltage -- the arcing across the contacts is much stronger with DC. For an example, see https://www.youtube.com/watch?v=Zez2r1RPpWY