This aquarium pump has a non-magnetic metallic disk between the poles of an electromagnet. When 240V AC is connected, the alternating magnetic field induces eddy currents in the disc.

But how does this cause it rotate? It has a single electromagnet composed of two coils in series, with no obvious asymmetry to produce a net torque. Yet it always rotates in the same direction, even if manually started in the opposite direction.

A motorised aquarium pump

Inside the motor, showing the electromagnet and yoke

  • 2
    \$\begingroup\$ You should add the edit as an answer. \$\endgroup\$ Commented Dec 2, 2022 at 22:48
  • \$\begingroup\$ @TimWilliams I'll do that \$\endgroup\$
    – William
    Commented Dec 2, 2022 at 22:50
  • \$\begingroup\$ I'm very curious why this unusual motor design was chosen over a more conventional one. Or is this actually common for single-phase motors? I don't do much work with motors other than three-phase ones. \$\endgroup\$
    – Hearth
    Commented Dec 2, 2022 at 23:16

1 Answer 1


The poles are shaded

Removing the electromagnet entirely reveals that the poles are shaded. This delays the magnetic field of the shaded portion with respect to the unshaded portion, so that on each half cycle of the current the net magnetic field moves from between from the unshaded portions to the shaded portions. Eddy currents induced in the disc oppose the movement of the magnetic field, causing the disc to rotate.

The electromagnet showing shaded poles


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