AC Motor Speed Controller and NEMA 34 Motors

Saturday, I went and got some parts for the AC motor speed controller I was going to make for a friend, who supplied me with the stepper motor brackets for the MF 70. When I went to the electronics store, which belong to friends of mine, I came some across huge NEMA 34 stepper motors, and immediately bought 3 of them ($155 total, they were more expensive with S&H from China).


48V – 5.6A – 4 Nm – 2415g

Since I have no suitable power supply (48V – 1000W) or driver boards (7.8A) for them, they will sitting on my shelf for a while. Then I’ll convert my BF20L to a CNC mill.

In the meantime, I started working on the AC motor speed controller, which I found here. The PCB is wrong, so I made my own on Proteus Isis and Ares. The values in he schematic are what I used at first. Here‘s the PCB in PSD format so you can print it on Press’n’Peel at 100% size. Here‘s also the original BMP.



AC Motor SCH

The schematic copied from the aforementioned site.

AC Motor PCB

My revised PCB.

3D Boards

Board shape simulated in 3D.

Later on, I copied the PCB prints (I made two boards) on a small piece of Press’n’Peel sheet on my laser printer, ironed the sheet on the copper clad for a while,  peel it off when it was cool, etched in Fe(III)Cl, and scrubbed the board with steel-wool under water when it was all done.

PCB Work

Completed PCB’s after scrubbing.

After splitting the boards and drilling the holes, I soldered the parts and completed the board. No cables for the pot yet.

Complete Card Front

Completed card with all parts soldered.

The card did not work it was supposed to. The motor was turning weak at some point and that was all. So I changed some parts as rohart suggested in the comments of the same page. I changed C4 to 22nF and R3 to 33K. I didn’t scrap R2 (when I did, it fried the pot at some point), but changed it with a 47K, and replaced the pot with a 50K one. Then it was running pretty smoothly.

All I have is a 15W Orange light bulb and a 19W Ebmpapst cooling fan to test it, until my friend’s 450W motor arrives. I think it will need much better cooling for such a motor. Here’s a video of both. I think I’ll get better results with R2 changed to 33K and the pot changed to 100K. Turn up the volume to hear the motor change speed.

Update: The card was almost ok with my 125W scroll saw, but I had to change C4 back to 100 nF to be able to control my grinder / belt sander, which has a 250W motor on it. I guess it all depends on the motor’s wattage. None of the parts got hot, the TRIAC wasn’t even warm.

I’ll soon try it with my friend’s 450W mini-lathe motor when it arrives.

32 thoughts on “AC Motor Speed Controller and NEMA 34 Motors

  1. Jess

    Thanks! Also I have a few other questions if you don’t mind..

    What is component AV1 on your Pcb?
    Did the 100 k pot end up being the best choice? I want to control a 250w motor similar to your grinder

    1. Technoshaman Post author

      That’s actually RV1, the pot. Weird PCB font. The pot value was trial and error, and yes, 100K was ok, and C4 was back to 100 nF. The circuit could not handle a 750W universal motor by the way. Thanks for visiting 🙂 I updated this page, because I realized I had to add a warning…

  2. Jess

    Thanks for the great info! Last question- did you use the 220 microH inductor listed on the original schematic? The one on the board looks larger.

  3. Jess

    Is the black and blue image of the PCB shown the one you transferred to the copper PCB? If not, I’m wondering if you would be willing to send/post the image so I could use it?

  4. Jess

    We’re you running yours on 220V or 110V? I want to use mine on 110 and wanted to make sure before I try it out

  5. Jess

    I have this hooked up to ,y motor but Im getting no resolution as far as speed goes. Nothing happens for the first half turn of the pot then it slows a little bit and then cuts out. I left R2 out and it’s a 1/3 hp running on 110. Do you have any ideas why this may be occurring?

    1. Technoshaman Post author

      Yeah, I got that, too. I’m no AC expert, so I can’t talk much on the theory. Try decreasing the value of the pot, and increase C4 to 100nF, if you used 22nF (or use 100nF if you used 100nF). You can also play around with R2. The comments of the original article have some hints.

  6. Panjwani

    I intend to replace potentiometer/P1 220K with single pole 4-5 step/way rotary switch for my old ceiling fan. What are the required voltages of capacitors c1-c4. Which Diac will match with Triac BTA10 or BTB10. Can you arrange to enlist part list with required voltages for capacitors and wattage for resistors.
    Please suggest changes.

    1. Technoshaman Post author

      I’m no expert in AC electronics, so I refrain from commenting on theory. I just went for trial and error. If you want more details, please refer to the main site I talk about in the post.

  7. christopher

    i intend to replace 220uH with 100uH or 300uH will that have effect of the circuit if there is please explain the changes.

  8. marlo

    hi sir can you tell me the wattage of you resistor and what capacitors did you use 😀 to control 750w motor thanks 😀

    1. Technoshaman Post author

      Hi Mario,

      The resistors were all 1W, and the capacitors were changed depending on the motor. My text, and the link where I got the schematic (especially the comments section), has more info on the values. Take care, and be careful 🙂

  9. Ferry

    Dear sir, thanks for the correction of that particular circuit. But would you write down all of the right component for controlling the speed of bench grinder (250W,220V) ? and what type of the capacitor on that schema?


    1. Technoshaman Post author

      Dear Ferry,

      I changed C4 back to 100nF for that motor.
      R3 is 33K. Try 47K or 33K for R2, and try 50K or 100K for the pot. I fried the pot a couple of times, so you may need a powerful one (but a friend a had no problems). All caps are metal film, and the big ones are 250V minimum. All resistors are 1W. The other values are the same as the schematic, unless stated otherwise. I’m not good with AC, so please don’t depend on my knowledge for it, do more research, and be very careful.

      1. Ferry

        Thanks sir for your reply … how about the inductor? 220uH/0.5W or 220uH/3W? which one is good?

  10. jappar

    Can you tell me what the highest limit for dis circuit? Does it work for large AC current motor (5kv) ? What suppose to modify? Need ur reply soon & thx in advance

    1. Technoshaman Post author

      No, it would blow up under such large loads (I blew up the inductor once, and the pot another time). You should use a quality AC Inverter Drive Speed Controller, like the ones Simenens makes.

      1. Robert

        Thank you for the reply but in the 3D print it shows L1 then a component with Şebeke label then motor out put is that component a fuse? it does not say in the schematic PCB layout or in the 3D print

        1. Technoshaman Post author

          No worries 🙂 That’s actually a terminal block for the cables to the AC line, the PCB program’s 3D output is not that detailed. If required, a fuse could be connected in series with the AC line.

  11. Technoshaman Post author

    I used another terminal block for L1, since the footprint (hole distances and sizes) was suitable (and there was no actual 3D model or PCB footprint for it). L1 is the inductor, if you refer to the schematic.


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