Kenmore 158: AC Motor Running on DC!

The sewing machine had a three-contact plug / terminal block that joins all the wiring:

Kenmore 158 - terminal block
Kenmore 158 – terminal block

For completeness, the matching socket (not shown) joins two cords:

  • AC line cord (two wire, not polarized, no ground)
  • Foot pedal

Extract the motor wiring from that block and connect it to a 50 V / 3 A bench supply, with the positive lead to the marked wire conductor:

Kenmore 158 AC motor - DC power
Kenmore 158 AC motor – DC power

Cranking the voltage upward from zero:

Kenmore Model 158 AC Motor on DC - RPM vs V
Kenmore Model 158 AC Motor on DC – RPM vs V

So that’s about 200 RPM/V, offset by 2800 RPM. Totally unloaded, of course.

The original data:

15 0.29 690 Barely turning
20 0.28 1380 Finger-stoppable
25 0.29 2350
30 0.29 3450
35 0.30 4450
40 0.29 5740
45 0.29 6780 Still finger-holdable at start
50 0.29 8000

I can hold the shaft stopped between my fingers up through 45 V, with 0.54 A locked-rotor current at 25 V. The motor doesn’t have a lot of torque, although it’s operating at less than half the normal RMS voltage.

I should take those numbers with the motor driving the sewing machine to get an idea of the actual current under a more-or-less normal load.

Reversing the power supply leads shows that the motor rotates only counterclockwise, which is exactly what you’d expect: both polarities of the normal AC sine wave must turn the motor in the same direction.