Standard Wiring for Old Sewing Machines
Many Treadleonians mess about with old electric machines, as well as people powered. I had numerous requests for the standard wiring pattern to accommodate controller, lamp and motor. Usually, this is represented by the standard Singer three post electric hookup. Here is a diagram of how this wiring goes. If you do it wrong, one of two things will happen: you will blow a fuse, or you find that the motor races while the controller changes the intensity of the lamp!
Tthe main power lines from the plug go to the two outside posts. The lamp wires also go to the two outside posts. This means that the lamp is fully powered when the plug is in. The motor wires go to one outside post and the center post. The controller wires go to the other outside post and the center post.
There are variations. There may be an on/off switch in the circuit. Usually, this will break one of the two primary power leads. The lamp may have a separate on/off switch, usually on its base.
On many old machines, expecially where there was a conversion to electricity, or on Japanese import machines from the 1950's through 1970's, there are no posts. Rather, there is a separate plug unit and both the lamp and the motor are plugged into it. Electrically, the same wiring as described above is already accomplished inside the plug box. Interestingly, these pre-wired plug units are still commercially available from sewing machine parts supply houses.