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?