Kenmore 158: Rectified AC Current

Because the universal-wound AC motor runs on DC, it will also run on full-wave rectified AC (top trace). The current waveform (bottom, 200 mA/div) never hits zero:

Rectified AC - 200 mA div - 875 RPM
Rectified AC – 200 mA div – 875 RPM

Note that the current lags the voltage, as you’d expect from an inductive load.

The average current at 120 VAC rectified is about 600 mA, a bit over the current at 50 V that I measured from the DC supply while driving the sewing machine. The locked-rotor torque averages 1 A, although it’s pretty hard to hold the handwheel at full voltage.

The key advantage of rectified AC: an ordinary MOSFET can control the motor current.

Given the motor’s sensitivity to current limiting, there’s not much point in measuring the current; unlike LED brightness, the speed isn’t proportional to the current. The MOSFET must act more like the carbon pile rheostat, burning whatever voltage the motor doesn’t need to run at the selected speed, with the RPM setpoint determining the gate voltage in a closed loop.

You can detect a stall by watching the motor RPM: when that drops too far below the setpoint, it’s stalled.

The gotcha will be keeping the MOSFET within its the safe operating area at both ends of the voltage range, due to the nearly constant current at any applied voltage:

  • High voltage + high current hits the maximum pulsed power limit of IDSVDS
  • Low voltage + high current hits the minimum possible voltage of IDSRDS

I think the relatively low current and power levels will simplify that mess; offering up a sacrificial MOSFET for measurement may be in order.

On the whole, it’s looking more do-able than I thought.