I am struggeling to understand the following:

If I consider a synchronous motor with only one polepair and apply dc-voltage to two of the three phases this results in a static magnetic field. The rotor, represented as a magnet, would aline like shown in drawing_1:


If I now try to rotate the rotor it "locks" in this one position.

If I repeat the same with a motor with two polepairs there should be these two positions (per mechanical revolution) in which the rotor locks, as shown in drawing_2: drawing_2

My questions about this are:

  1. Have I drawn the polepairs the belong together correctly?
  2. Are the shown positions in which the rotor would lock correct?
  3. Why wouldn't the rotor lock in two more positions? 90° from the positions as shown in a) and b) ?

(I took the pictures from this animation: https://www.renesas.com/sites/default/files/inline-images/fig3-a-bldc-monitor-en.gif )


2 Answers 2


I think that your motor's rotor would have four poles as well!

enter image description here

Figure 1. Modified image showing N-S-N-S arrangment on the rotor. With coils energised as shown the rotor should park rotated 45° anti-clockwise relative to the position shown.


You are not applying DC to the stator. You are applying it to magnetize the rotor to bring it in synchronization with the rotating magnetic field created by the three-phase ac stator.

From: How rotating magnetic field works in an Induction Motor?

enter image description here

The three-phase (as shown above) power is applied to three two-pole coils. This creates a magnetic field that rotates through 360° at synchronous speed.

Sychronous motor (or generator) starts as an induction machine. When it is close to synchronous speed, DC is applied to the rotor, which brings rotor into synchronization with the magnetic field of stator. This makes your 2 & 3 wrong.

Your drawing is wrong. Pole pairs are wrapped in the same direction to create a NS magnet. This makes 1 wrong.

From: LibreText 13.2: Synchronous Motors

enter image description here

4 poles / phase means 12 non-salient poles on stator. 4 poles on rotor.

From: EEE Made Easy: Aarmature Windings of Alternators enter image description here

  • $\begingroup$ Let's asume the windings are connected as star and I applied DC Voltage to two of the phases. Yes, DC, not AC. Would this Image show the correct magnet poles of the windings? i.ibb.co/3rw9tyW/1.png $\endgroup$
    – cor3kl
    May 16 at 19:28

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