Driving a DC treadmill motor from AC - Page 1
Forum section: electronics / projects

author: daxliniere
author_rank: Regular Contributor
timestamp: August 29, 2022, 12:24:02 pm
content: Hi everyone,I'm looking at upgrading my lathe spindle motor and it seems like DC treadmill motors are a good cheap source of high torque across a range of speeds.The cost of a 180v 20A DC power supply seems to negate the cost benefits of the treadmill motor, so I was thinking of a workaround.Could I use an AC motor speed controller (SCR-based) and set it to a fixed voltage, then feed that into a high-current bridge rectifier to obtain the 180vDC? I figure I can then send that to a PWM controller (potentially upgraded with higher-rated mosfets) to obtain the final motor voltage.Would anyone have any thoughts on this?All the best,Dax.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2022, 12:33:51 pm
content: Figure out how the treadmill got 180V DC for the motor!Its possible the motor has adequate insulation to run off fast PWMed or phase angle controlled full wave rectified (but unsmoothed) mains but the peak voltage can reach over 357V (supply 230V +10%), without any allowance for transients and ringing, so its also quite possible its unsafe to run a specific treadmill motor without an isolated PSU between it and the mains supply.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2022, 05:37:11 pm
content: Quote from: daxliniere on August 29, 2022, 12:24:02 pmYes. It'll be fine with rectified 230VAC. The motor's frame will need to be earthed of course.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 30, 2022, 12:29:28 am
content: Take a look at a buck mode PFC. You'll want relatively smooth DC to minimize vibrations and reduce artifacting.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: August 30, 2022, 12:48:04 am
content: In the dark days, I was presented with a treadmill with a broken control board. We believe lightening hit the controller and no parts able to be sent, we had to do an ugly hack with stuff I had laying around.Option 1. Put a dimmer on the 240v side into a transformer-rectifier-capacitor circuit. Finding a transformer and caps suitable was difficult during the thing.Option 2. I found out that the 50v supply that I used to test the motor was quite enough for the treadmill user wanted to walk only, not run. I had a 50v PWM controller. Big 35mm knob drilled into the front panel...The treadmill motors are pretty torky. How big is the lathe? I wonder if a low voltage would be enough? What I've found in the past with this type of thing is you really need a speed controller, not a so much a power controller. With a lathe, when you start cutting you want the speed to remain fairly constant.

author: james_s
author_rank: Super Contributor
timestamp: August 30, 2022, 12:53:48 am
content: The electric weed whackers I've taken apart all had a permanent magnet motor with a simple diode in series to deliver half wave rectified power. A similar arrangement should work for a treadmill motor, for speed control a variac would work, even a light dimmer might suffice. The treadmill will have a PWM controller with closed loop monitoring of the speed, you could try to find one of the boards from a treadmill and use that.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: August 30, 2022, 01:10:11 am
content: Quote from: james_s on August 30, 2022, 12:53:48 amYeah. There are heaps of old treadmills around, all with stuffed controllers. Motors are usually fine. The corpses I've seen most often, it's the power control chips with big holes in their side.I bought a few number of cheap 'dead' treadmills off ebay to see what I could cannibalize.As for closed loop, one of the electronics mags here (SC?) had a revised 10AACmotor controller which had a ramp circuit to detect if the amps went up without the need for a speed sensor. I've got the kit unbuilt somewhere and have always been puzzled how well the controller works.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: August 30, 2022, 01:19:05 am
content: Just thinking more about it.A 240v to 120v stepdown transformer isn't that expensive.Rectify and filter the 120v and that's getting near the 180v DC without going over (?)Add a sewing machine pedal to the primary side.Might come cheaper but it wouldn't come any rougher.Put an emergency stop button on the lathe if it hasn't already got one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 30, 2022, 01:25:27 am
content: Quote from: james_s on August 30, 2022, 12:53:48 amVibration on a lathe would show up as artifacts on the finished part. Chopped mains would create a lot of vibration.A light dimmer and bridge rectifier would probably be good enough to test if the motor would work well enough, then you can work on a better drive circuit.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 30, 2022, 02:29:34 am
content: I considered adding a treadmill motor and home brew 4 quadrant to the vintage SB9A here.It still has the original 1939 120 V single phase DOL cap start and the tumbler reversing switch.The desired improvements would be stopping at position when cutting or threading to a shoulder,and lower speed for coil winding.Sometimes I have to pull the layshaft pulley by hand which is tedious.A problem with improved dynamics is that these lathes do not have positive lock on the spindle thread.The present drive can not unscrew the chuck. If the tumbler is accidentally thrown into reverse, the motor will continue to run forward. due to the centrifugal switch staying open.Considering all, I think I will try to find an equivalent spare motor in case that 83 year old start capacitor or its switch finally gives up.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2022, 10:51:01 pm
content: A bridge rectifier with a PWM circuit for the motor will work quite well.Don't worry about exceeding the voltage. The maximum speed rating of the brushes and bearings is more important. The 33% over speed for short periods won't be a big deal, but if it goes on for too long it could damage it.

author: daxliniere
author_rank: Regular Contributor
timestamp: August 31, 2022, 08:42:16 pm
content: Thanks everyone for your input.If the output is choppy, I can always add some capacitance. 75v isn't all that high, so proper rated caps are easily added.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2022, 09:23:45 pm
content: Quote from: daxliniere on August 31, 2022, 08:42:16 pmWhere did you get 75V from? The peak mains voltage is 325V.A 3.6kW motor should have enough inertia the ripple won't matter, if a rectifier and TRIAC/SCR driver.Also note you won't be able to run it at full power of a UK mains socket.

author: daxliniere
author_rank: Regular Contributor
timestamp: September 01, 2022, 01:18:33 pm
content: Hey Zero, duh! Yes, I'm confusing the industrial servo motor I got the other day which I also posted about. 🤦Yes, you're right 180vdc @ 20A (4HP) would require 15A mains. I have that in my workshop, but you've actually highlighted that the motor I proposed is very likely to be overkill.

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2022, 06:01:40 pm
content: Quote from: daxliniere on September 01, 2022, 01:18:33 pm180V at 20A would require a little more than 15A mains, to make up for any losses in the converter.I'd just use a 20A TRIAC controller & bridge rectifier, on a fast 15A breaker. It would need to be started by slowly increasing the power on the controller, otherwise it would trip the breaker. The reason for suggersting a 20A controller & rectifier is to provide a generous enough hearoom, as semiconductors are quite fragile. A tachometer would also be a good idea, so you can avoid overspeeding it for too long.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 01, 2022, 11:34:24 pm
content: What size of lathe is it?What type of motor does it have now?What problem do you have that needs "high torque across a range of speeds"If existing motor is induction motor, you should compare its speed vs torque (droop) with the the proposed treadmill motor without speed feedback.Permag treadmill motor will have speed droop approximating a shunt DC motor which might not be as good as induction motor if open loop.If you use a simple phase controller in front of a rectifier that might have droop too.If you add speed control, would you use armature feedback or add a tacho?Also the induction motor has inherent foldback torque on overload ( for example if your parting tool jams in a deep groove).If a bigger motor stalls, there is extra load on the toolpost which might distort/damage it or the saddle.To handle that you would need fast acting foldback current limit on the permag motor.The above are some of the issues that led me to naysay it on my 9 inch South Bend. I thought it would be quite a project to get working properly along with new pulleys and mounts and controls etc. And as I mentioned in earlier post, adding dynamic braking is a problem with a screwed spindle mount.However you might find on internet, details by someone who demonstrates a successful conversion.Try one of the UK hobby machinist forums.

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2022, 09:02:01 pm
content: Here's an example of phase control and PWM. Obviously with the PWM circuit, the 16V part needs to be treated with the same precautions as the mains.

author: daxliniere
author_rank: Regular Contributor
timestamp: March 15, 2025, 08:28:47 pm
content: I just discovered that my notifications for thread replies were disabled for some reason! Thank you everyone for taking the time to respond and sorry for not responding, myself.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2025, 08:53:32 pm
content: You may find this interesting:https://andysmachines.weebly.com/variable-speed-controls.htmlIt details the speed control circuit used by many Chinese mini-lathes and mini-mills for their DC motor. One key difference from suggestions above is that it uses two SCRs in a controlled bridge rectifier rather than a TRIAC and a separate bridge rectifier. This eases control as the current flow alternates between the SCRs giving each plenty of time for commutation to occur before it has to be retriggered.

author: daxliniere
author_rank: Regular Contributor
timestamp: March 15, 2025, 08:58:40 pm
content: Thank you, Ian, that's great.