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What do they do?

The aux contacts are normally open and the device is MR PQM 500 reactive power compensation system.

  • $\begingroup$ they look like power resistors to me... $\endgroup$ Dec 21, 2019 at 19:39
  • $\begingroup$ @niels, they're inductors. See my answer below. $\endgroup$
    – Transistor
    Dec 22, 2019 at 8:27
  • $\begingroup$ They surely are, but the manufacturer calls them resistors. $\endgroup$
    – storgt2
    Dec 23, 2019 at 5:38
  • $\begingroup$ bnj-usa.com/wp-content/uploads/2019/03/… $\endgroup$
    – storgt2
    Dec 23, 2019 at 5:39

1 Answer 1


The aux contacts are normally closed ...

This is unlikely. I suggest that they are normally open and close before the main contacts do.

You haven't provided a link for the MR PQM 500 system but I suspect that it is a capacitor bank with a controller which switches the contactors to switch in capacitance as required to bring the power factor closer to unity.

There is problem when switching in capacitors. If the mains voltage is close to peak at the instant the contacts close a very high current will flow with resultant arcing, etc. because the discharge capacitor appears like a short-circuit. To prevent this a special arrangement is employed:

  • The auxiliary contact block at the front is driven by the main contactor movement but the auxiliary contacts close slightly before the main ones.
  • The three coils on top are inductors which have enough inductance to limit the current into the discharged capacitor to a safe value. This is enough to bring the voltage up so that the main contacts don't see the big surge they otherwise would.
  • $\begingroup$ You are right, they are actually normally open. Thank you so much. Have a nice day. $\endgroup$
    – storgt2
    Dec 22, 2019 at 6:44

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