In reviewing the following wiring schematics for a vacuum contactor, I can't wrap my head around why things are the way they are.

Vacuum Contactor Wiring Diagrams Source > Downloads > Catalogues

Wiring Diagram #1, Size 4 Contactor

My thought-train for the circuit is:

  1. START is pressed, powers the circuit.
  2. AUX is not powered so it shorts the resistor/capacitor
  3. Coil M receives the fully rectified line voltage.
  4. Sometime later, AUX closes and the resistor/capacitor are in series with the rectifier.

What is the purpose of this resistor and capacitor? Is this just to reduce the voltage on coil M? M would require a specific VA to keep it energized, so wouldn't dropping the voltage simply increase the current required?

Bonus: What would be the purpose of substituting the resistor/capacitor for a resistor and potentiometer in series? Tune the voltage at the rectifier?

Wiring Diagram #2, Size 5 and 6 Starter

This circuit doesn't include the resistor/capacitor pair, but it has two coils in series. This didn't make sense to me so I researched some and found this excerpt:

The operating coil has a "figure-eight" shape and is really two coils in series, with a connection to their common point. [...] When adjusted correctly, this contact allows a relatively high current through the pickup winding, and as the controller closes, the contact inserts the holding winding, which reduces the coil current to a low value [...]

That makes general sense. One coil for high current draw, one coil for holding current draw. What doesn't quite make sense is, again, how putting them in series reduces the total draw. Don't they each need their required VA? Or, does the holding winding essentially knock-out the pickup winding? I think I might be overthinking it as I rarely see coils in series, always paralleled.

Lastly, with both of these coils being fed off full-bridge rectifiers with no smoothing capacitors, do they ever risk opening as the DC voltage bounces to 0 V?

  • \$\begingroup\$ is this a school assignment? \$\endgroup\$
    – jsotola
    Jun 26, 2019 at 5:45
  • \$\begingroup\$ @jsotola, no. Work assignment. The circuit these questions came from is confidential but it's a hybrid of the two above. I need to make some changes to it. \$\endgroup\$
    – user199402
    Jun 26, 2019 at 14:45

1 Answer 1


What is the purpose of this resistor and capacitor

After full activation, C raises impedance with low loss, R lowers Q to avoid LC resonance.

potentiometer in series?

  • too lossy (hot) for holding current.

Don't they each need their required VA?

  • No. S is rated for Start, R when added to S reduces to holding power for Run operation. They dont’ have to be identical coil impedance.

do they ever risk opening as the DC voltage bounces to 0 V

  • No. these would shading pole type relays like those that are AC/DC
  • \$\begingroup\$ I’m from the Peg too. \$\endgroup\$ Jun 26, 2019 at 6:48
  • \$\begingroup\$ In each of the scenarios, the impedance is just being increased after pickup, right? And the purpose of this increase is to reduce power draw? \$\endgroup\$
    – user199402
    Jun 26, 2019 at 15:09
  • \$\begingroup\$ Yes 2pi f L + 1/2pifC + R = Z (f). It assumes Aux contact occurs at same time or after shortly. \$\endgroup\$ Jun 26, 2019 at 15:17
  • \$\begingroup\$ OK, making more sense. My initial assumption was that the power requirement of the coil would change dynamically but I guess that's not the case. Some work is done to move between the inrush and holding specifications. Side note: very cool that you're from around here. What a small world the internet makes. \$\endgroup\$
    – user199402
    Jun 26, 2019 at 15:35
  • \$\begingroup\$ Back EMF rises with velocity then stops so the Run/Start Current Ratios depending on these impedance ratio. Yes small world. and my "son-in-law" is an EE Prof at U of T and his Dad taught me some courses at U of M in early '70's \$\endgroup\$ Jun 26, 2019 at 15:41

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