Physics Dept. Demonstration Manual

DEM45.0 - Rotating Pie Plate


To demonstrate the operation of an induction motor.

  • rotor
  • U-magnet (special)
  • pie plate on pivot and stand
  • four-pole rotating-field magnet


The four-pole electromagnet is connected with a capacitor in such a way that the alternating current phase angle 'leads' the next pole by 90 degrees, setting up a rotating magnetic field. The eddy currents induced in the suspended pie plate oppose the rotating field. The force created causes the pie plate to rotate.

A variation with the same results can be shown using an RD-1 rotor set vertically with a U-magnet clamped by its center bolt to the rotor head. The pie plate is suspended above the poles, giving the same result as before.