In the manual of a remote controlled power adapter (one of those that you plug into a power socket and switch on/off via the 433MHz remote control) it says that you should not use it with devices that have a motor capacitor (e.g. a refrigerator).

What is the reason for that?

The remote control receiver has some maximum allowed specifications (e.g. 3680W).

Is the reason that a (motor) capacitor may leads to temporarily exceeding these specs on start-up and/or power-down?

  • 1
    \$\begingroup\$ We don't know with what components your device is built with so it is not possible to know why the manual mentions not to use loads with motor capacitors. \$\endgroup\$
    – Justme
    Aug 20, 2022 at 17:35
  • \$\begingroup\$ @Justme You are correct, but I thought there may be some common knowledge about this that I missed. \$\endgroup\$
    – RhinoDevel
    Aug 21, 2022 at 8:00
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    \$\begingroup\$ There might be but we don't know which common knowledge would apply. The switching element could be a mechanical relay that is not rated to the surge current. It might also be a solid state relay that can't handle the current or it might have problems turning on or off if it uses sensing of zero current or zero voltage and has incompatible load. The manual might also have a translation error and it may not convey the intended wording about capacitive or inductive loads properly. \$\endgroup\$
    – Justme
    Aug 21, 2022 at 8:47
  • \$\begingroup\$ This question should not have been closed. It does not ask about "how to use a device" That is just not there at all. (It asks the physics, ie the design of electrical motors. It does not ask for help designing an electrical motor, but that is not a requirement for this site) \$\endgroup\$
    – david
    Sep 8, 2022 at 2:59

1 Answer 1


The inrush current of an AC motor is the current required to energize the magnetic field. There would be a large inrush current even without a capacitor. 3-phase motors have the same inrush current, but no capacitor.

Single phase AC motors come in four kinds:

  1. Can't self start. Not typically used in the home.

  2. Commutated. Historically, very small, low power, runs on DC or AC.

  3. Permanent magnet. Modern magnets make it possible to have useful power: your washing machine may have a permanent magnet synchronous motor. You can power that if it doesn't exceed your power rating.

  4. Has capacitor. Traditional for large domestic things like refrigerators and floor polishers. Large inrush current, poor power factor.

So firstly the restriction means that you can't use the power plug with any motor large enough to matter, and secondly means that you can't use the power plug with any motor that requires inrush current to energize the magnetic field. The commutated motor requires inrush current, but it's a small motor, running a drill or blender.


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