Transformerless motor controller - is it safe? - Page 1
Forum section: electronics / projects

author: word1121
author_rank: Newbie
timestamp: November 27, 2022, 04:45:17 am
content: Hi all,I recently bought an imported "750w Sewing Machine Servo Motor" for a small milling machine. It plugs into a 110V outlet and has a lever for speed control that works via Hall sensor (like thishttps://youtu.be/iiIJCVngs1E?t=99).I didn't think much about the overall design (or how light the control box was) until I tried to automate the speed control. It seems like a simple 5V circuit: Red + black + analog 0-5V signal. Indeed, swapping out the sensor for a potentiometer works great.However, outputting 0-5V from a secondary controller... did not go well. While troubleshooting I eventually measured relative to ground and found Red=-163V, Black=-168V. So I finally took a look inside the control unit:I'm pretty much illiterate when it comes to power supplies, but from what I can tell the entire unit is basically using rectified mains directly. This seems scary to me. Is this type of design safe to use in a human-operated machine like a sewing or milling machine?

author: Psi
author_rank: Super Contributor
timestamp: November 27, 2022, 05:19:48 am
content: Quote from: word1121 on November 27, 2022, 04:45:17 amWell the sewing machine metal case is earthed. So it's pretty safe as long as it's hidden inside the case away from the user.But it does make working on it a bit scary.It's nothing that unusual, old CRT TV's often had the entire internal metal frame at 150-200V DC and surrounded by wood and plastic for safetyYou could run it on a isolation transformer if you want to make it a little safer while working on it.

author: Whales
author_rank: Super Contributor
timestamp: November 27, 2022, 07:08:17 am
content: Eek, yes this is a mains referenced 5V rail.For the original intended purpose: this is cheaper and probably fine. The hall effect sensor sounds like a way of ensuring safe contact away from people.For your purpose: this is horrible. You don't want to go near that OR anything connected to itin any waywhilst it's plugged in.Is the lever in that video a magnet? Perhaps try simulating it with a coil, that way you can automate whilst retaining full and safe isolation.

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2022, 08:59:27 am
content: Yep, that's a non-isolated mains live controller. Not uncommon if the control mechanism can be built with adequate insulation. e.g. the original control board of the minilathe in the video would have been similarly non-isolated, either a fairly dumb analog TRIAC phase angle controller or an analog MOSFET PWM one.(Oddly enough the TRIAC one is preferred for minilathes as although it has worse speed regulation and poorer low speed performance, its a lot more robust and doesn't blow at the drop of a hat if you stall the lathe motor!)Transferring analog control voltages across an isolation barrier is hard to do accurately, so is generally best avoided. The easiest option to gain safe low voltage isolated control of the above board would probably be to PWM the LED of a logic output optocoupler and filter (2nd or 3rd order) and buffer the output with a RRIO OPAMP, which would give you a signal compatible with its hall sensor input, and the opto output side and filter/buffer circuit could draw power from the same hall sensor connector.

author: word1121
author_rank: Newbie
timestamp: November 27, 2022, 05:47:59 pm
content: Thanks for the replies! Good to know the overall design isn't too crazy. I like the idea of using an optoisolator to communicate across the isolation barrier -- I actually have a spare I2C DAC module that would probably make this easy, but hadn't thought to put it on the control side.That being said, for fun projects like this where I'm learning as I go, I try to stay pretty far from things where mistakes can hurt/kill youIt might be worth the peace of mind to get a different motor that runs on 48V like the rest of the machine.

author: word1121
author_rank: Newbie
timestamp: November 27, 2022, 06:02:51 pm
content: QuoteYes! It's so simple I almost can't believe it works.

author: Whales
author_rank: Super Contributor
timestamp: November 27, 2022, 09:24:48 pm
content: Quote from: word1121 on November 27, 2022, 05:47:59 pmAlternatively you should be able to get motor controllers that have a proper transformer on them for safely isolating HV & LV.What type of motor is the sewing machine motor? It has 5 wires, so perhaps 3 phase AC induction? If so then a "VFD" might be what you're after.

author: word1121
author_rank: Newbie
timestamp: November 28, 2022, 10:54:23 pm
content: Yes I think it's 3-phase AC induction. I like that idea since I've already made a mounting bracket and some other parts to fit that specific motor. Thanks for the idea!