I know that one can reverse the direction of an ac motor by powering the different starter windings. If you power one starter winding, you create a magnetic bias, allowing the motor to rotate to that direction. Once it is spinning, the main winding will do the heavy lifting.

But what would happen if you were to power both starting windings? My guess is that when the motor is standing still, you would not create a magnetic bias, so the motor won't start to rotate. If the motor was running, I think nothing would change. You just remove the magnetic bias. The motor would keeps on running in the same direction at the same speed. (If the starting windings were connected using centrifugal switches, then it doesn't matter at all, as you would only be powering the main windings.)

I might be completely wrong about this, therefore I would like to know if my reasons is correct. I've attached an screenshot of the diagram from the installation manual of the motor (Somfy Ilmo 50 WT). I plan to power shutters using two non-zero crossing solid state relays. The software should prevent powering both phases at the same time, but I want to be sure as the motors are expensive. I want to know if I need to add protective circuitry (selecting mux).

The resistors in the diagram are probably used for a slow startup, and I think that they indicate the starter windings. (B and C are probably internally connected to the main winding as well).

Do you think it will "hurt" the motor if b is powered while c was already powered?

Somfy Ilmo WT, ac motor


1 Answer 1


enter image description here

Figure 1. The motor shows two windings and an overload switch.

Those aren't resistors. They are the forward and reverse motor windings.


simulate this circuit – Schematic created using CircuitLab

Figure 2. The resistor symbol is hollow, the inductor (winding) symbol is solid.

My guess is that they're using some sort of shaded-pole motor. Depending on the construction, with both windings energised the magnetic fields may cancel out eliminating the back EMF and resulting in a much higher than normal coil current.

I'd err on the side of caution. The switch or mechanical relay contact almost completely eliminates the likelihood of simultaneous coil energisation.

  • \$\begingroup\$ Ooh indeed. When the motor is standing still, both starter windings would continuously draw their startup current, if the motor cannot start rotating, resulting in an indefinite 2x power consumption, causing high heat generation. (This might not be a problem if the motor was already spinning.) Whilst the motor does have a temp sensor to power it of in case of overheating, it isn't made to run continuously, so it might cause damage over time. \$\endgroup\$ Mar 31, 2020 at 13:31

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