I have a HVAC control panel that controls 2 split unit and 2 exhaust fan via Fire Alarm Control Panel (FACP). FACP sends start-up signal to all the contactors (coil 380V see joined figure) that work at the same time, the first 3 contactors work fine but the contactor number 4 makes noise knowing that all the contactors are new. When I remove the contactor number 4 and start the contactors the first 2 contactors work fine but the contactor number 3 makes noise, it looks like the last contactor in the chain always makes noise. I searched a lot in the internet for the origine of this problem and did not find any solutions

Any suggestions to solve this problem?enter image description here

  • \$\begingroup\$ Maybe all of them make the noise but #4 is nearest your ear. Then #3 is etc.. Is this a new design? Did you design it? Is it something that is in production? \$\endgroup\$
    – Andy aka
    Feb 14 at 9:34
  • \$\begingroup\$ Drop voltage on wires may cause it. May be "star" connection than contactors feed by separate wires may help. \$\endgroup\$
    – user263983
    Feb 14 at 11:02
  • \$\begingroup\$ This is an electrical engineering site, so just to clarify, when you say "make noise" you mean EMI and not audible noise, or...? See the noise tag description. \$\endgroup\$
    – Lundin
    Feb 14 at 12:55

2 Answers 2


Noisy AC-coil contactors can be caused by coil voltage that is out of tolerance (high or low).

Coil voltage that is out of tolerance can be caused by poor power quality or poor wiring combined with voltage drop caused by the contactors themselves switching loads.

Measure the '380VAC' with and without loads switched on and compare with when the noise appears.

  • \$\begingroup\$ "out of tolerance" means lower than minimum from spec? Or may be higher than maximum? \$\endgroup\$
    – user263983
    Feb 14 at 10:58
  • \$\begingroup\$ "high or low", so either way. \$\endgroup\$ Feb 14 at 11:20

The high starting current and consequent voltage dip, caused by simultaneous start of four motors, could lead to contactor chatter.

Sequential motor start could prevent that from happening.

enter image description here

Contactor auxiliary contacts would need to be wired as shown.


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