Half Speed Universal Motor

[Homa]

I do some heavy duty sewing on occasion and recently viewed a video that shows how to swap out the motor between two Singer sewing machines and increase the power of the recipient machine. The swap includes some wiring, with a switch, that cuts the speed of the new motor to about half its normal maximum rpm, apparently without degrading the motor’s torque. The speed of the motor is controlled by a resistive (or electronic in my case) foot pedal. The motor is a very basic, brushed universal AC electric motor. Here’s the video if you want to watch it.

I have been able to obtain the necessary motor for the upgrade. This will increase the power of my machine from 0.70 amps to 1.00 amps, a pretty good performance boost. What I’ve not been able to figure out is how Singer implemented the speed reduction feature. The author of the video claims that the motor she used has a third winding. I’ve looked at the parts lists for the machines involved and I doubt this. I’m pretty sure the “special” motor is just the standard 1.00 amp motor with an extra wire connection. I was not able to obtain a whole “donor” machine, nor can I find the special wiring involved.

Way down in the comments of the video, there is some discussion of how the slowing feature was achieved. Two posts are significant:

I had a bit of a breakthrough, with a little help. The element soldered to the switch on my 620 is a diode, not a resistor or a capacitor. This is verifiable by measuring that its resistance differs depending on the positions of the multimeter's probes - infinite resistance in one direction, and some measured value in another.
The diode, when activated by the switch, cuts away half the sine wave, thus reducing the effective voltage supplied to the motor by one half.
And…
How sure are you that the element you have on your switch is a capacitor, not a diode? I had a chat with someone who understands electronics a bit better than I. He thought that the diode is actually a very smart design by Singer, because it's a poor man's PWM. The diode cutting half of the sine off is the same as PWM with duty cycle 50%. This is better for the motor torque because the motor receives higher voltage during the pulses (compared to regulating with the pedal alone). So, full pedal with the diode the motor would be receiving full 120V pulses, but the average voltage would be 60V, so the motor would be spinning slower, but with more torque than if feeding it 60V continuously.
I have since picked up a Singer 737, it has a slow/fast switch, so I was hoping to see the three wire motor inside. Nope, it was the same deal as with my 620, the switch simply inlines a diode, and the motor is a 2-wire affair.

One poster even included a photo of the back of the three way switch (high,off,low), which I have attached.

My question is: Can anyone identify the component shown on the back of switch and, could you recommend a diode that would create the electrical conditions described for the half speed operation?

[Zero999]

Yes, a diode can be used to reduce the speed of a universal motor. In my experience with a vacuum cleaner motor, the motor still runs a little faster, than half speed.

The component on the back of the switch appears to be a diode. The switch doesn't appear to be rated for mains. I'd recommend using something more robust.

[Homa]

It has occurred to me that even if I figure out what the correct diode is and how to wire it into the system, I may still have an issue. The foot pedal I am using is electronic. The original foot pedal was a carbon stack device. Wouldn't the electronics of the pedal get hosed up if I cut the current from 120VAC to effectively 120V pulsed DC? The electronic foot pedal is a two wire device, so I don't think there is an easy way to isolate its power.

[Gyro]

You could achieve more low speed torque with a traditional Triac based phase angle controller than a resistive controller, but at much greater risk of overheating the motor (due to more power with less ventilation), and probably more audible noise too.

An off the wall suggestion - can you get a smaller diameter motor pulley? That would give you lower machine speed and greater torque for a given motor speed, and at greatest efficiency.

[Zero999]

It has occurred to me that even if I figure out what the correct diode is and how to wire it into the system, I may still have an issue. The foot pedal I am using is electronic. The original foot pedal was a carbon stack device. Wouldn't the electronics of the pedal get hosed up if I cut the current from 120VAC to effectively 120V pulsed DC? The electronic foot pedal is a two wire device, so I don't think there is an easy way to isolate its power.

The type of diode isn't critical. It just needs to be rated to the mains voltage and a few times the motor's current rating, to take into account the start-up current.

It's also possible the electronic foot pedal will work perfectly with the diode.

[amyk]

When I read the title my first thought was "put a diode in series"...

I believe this is also common in the small multitools like Dremels and such.