When we introduce an iron star inside the stator of a 4-pole 3-phase AC motor and then generate the rotating magnetic field, the star being made of a ferromagnetic material is magnetized and starts to rotate at synchronous speed (1800 rpm). On the otherside, when we swap the star with an iron disk it doesn't rotate at synchronous speed. Instead it rotates at a lower speed (aprox. 1200 rpm) and we start to see slip.

Picture of Problem.

My question is: Why does this happens? The iron disk being made of a ferromagnetic material shouldn't start to become magnetized and to rotate at synchronous speed? Are there two magnetic fields in the iron disk: the one created by the magnetized material and the one created by induced currents on it?

  • 4
    \$\begingroup\$ "and the one created by induced currents on it?" No. That describes an induction motor but what you have on the left is like a synchronous reluctance motor which requires favoured flux paths. Circular disc = every position is equal is equal, no favoured path. Nothing is induced anywhere. And even if you had an induction motor you can't just stick in a solid metal disc for the rotor. You need to look at how the currents are induced and how the directions of current and magnetic field produce the Lorentz force to spin the rotor. \$\endgroup\$
    – DKNguyen
    Commented Sep 11, 2021 at 1:55
  • 1
    \$\begingroup\$ The star has features that can align with the rotating magnetic field so it sort of "locks in" to the rotating field and follows it. The disk has no such features. But eddy currents develop in the disk, and this causes it to act like a poor induction motor rotor. You can look up "why do induction motors have slip?" For more information on that. \$\endgroup\$
    – user57037
    Commented Sep 11, 2021 at 16:36

1 Answer 1


The iron disk behaves like the rotor of an induction motor. Induction motors have AC currents induced in the rotor resulting in a rotating magnetic field. However they require slip for currents to be induced. An iron disk is a high resistance rotor, so the motor has a lot of slip, 600 RPM is 33% slip vs. 2% or 3% for a good induction motor. That is a pretty crude answer, but it should convey the general idea.

The star rotor makes a synchronous reluctance motor as described in a comment.

Both motors are likely to have rather poor performance, compared to properly designed motors.


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