I am using a 350A DC, normally open vacuum contactor that experiences a failure while the contactor is closed (coil energized by 24VDC) for an industrial 1000VDC application. The manifests as the contact set being "stuck" in the closed state even after the 24VDC is removed from the coil's terminals. It is unclear to me why the spring mechanism would fail to release after removing the control voltage.

After some failure analysis, it seems that if one taps the solenoid casing with a soft hammer, the spring will release and the contactor will return to its normally open state. If I reapply 24VDC to the coil, the contactor will close and, if I remove the 24VDC from the coil, the contactor will open.

I have attached a snippet from the datasheet for this device below: enter image description here

Has anyone seen this phenomenon before? Any insight into what can be happening electrically to cause the coil to get "stuck" closed would be appreciated.

Edit 1; I've added a photo of an example failed contact that was opened up to view the damage. Please note the pitting, but no obvious signs of melting on the copper conductor used to switch the voltage. The pitting was smooth to the touch and doesn't seem like it would have kept the contactor together against the force of the spring trying to release it. enter image description here


2 Answers 2


Almost surely the contacts sticking or welding. This is not uncommon in relays of all types when the contacts are subjected to high peak currents upon closing or there is excessive arcing. So it would come down to the load (including the layout of the conductors, due to inductance effects) and probably the relay manufacturer would be best able to advise on the suitability of their product for the load. 350A @ 1000VDC is actually a pretty fearsome load to have to switch mechanically (not so trivial electronically either).

I can think of one other issue that might be having an effect- the relay coil contains internal suppression. If you add an external flyback diode you will slow the opening of the relay and exacerbate the arcing. You should switch the relay with an open-collector or open-drain transistor (or a mechanical relay contact) that has sufficient SOA and voltage breakdown capability to handle 60V or more).

  • \$\begingroup\$ At first I suspected this was likely the issue as I have seen contactors on 480V and 600V 3-phase applications get welded together due to excessive short circuit current situations. I opened up the contactors and noticed some light pitting which is smooth to the touch. Please bear in mind that these contactors are designed with the expectation of pitting and damage over the life span. What seems strange is why they would remain closed like this despite no obvious melting of the conductor. Edit; I was also going to add that the failure occurs while the contactor is still closed. \$\endgroup\$
    – rbarb88
    Apr 29, 2022 at 17:18
  • \$\begingroup\$ @rbarb88 How can you distinguish between a failure that occurred at closure (and is therefore latent) and one that occurs during during the opening attempt? \$\endgroup\$ Apr 29, 2022 at 17:49

You have a Gigavac HX21 or HX200.

The contacts are welding themselves shut because you wore them out.

Read the application notes. First, it has instructions for use of snubber diodes.

Second, the electrical life rating is based on a resistive load with 27 microhenry maximum inductance in circuit. (In other words, NOT A MOTOR). They suggest testing the contactor in your circuit.

Third, the power switching lifecycles are based on current flow from A2+ to A1- so if you are running current backwards the lifecycles will be shorter. Note that the "inductive kick" from interrupting a motor would be backwards current.

However, if you look at the interrupt lifecycles in the diagram you posted, you can see that "350A" is not even on the diagram. The diagram is a single-"log" graph, meaning the vertical axis is exponential - that is why the subdivisions are ramped like that.

If we grab the Photoshop clone stamp tool and extrapolate the diagram out to 350A, and extend the blue line (in red), we get...

enter image description here

Oh, snap. That is indicating a life-cycle of 33 cycles when interrupting 350A. Even if we are optimistic about that dog-leg in the blue line, we can only hope for about 100 cycles.

In other words, this contactor is not rated to interrupt 350A on a regular basis.

Generally, these dainty electric vehicle contactors are expected to never operate under current. There is supposed to be silicon between these and the motor, and the silicon shuts off current flow before the contactor opens. It is expected to interrupt rated current once or twice in its entire life cycle when magic smoke is actively billowing out from under the hood as the silicon burns.

If you need to interrupt full current regularly, it's time to look at railroad-grade stuff. Talk to Larry's Truck and Electric about getting a (32V or 75V coil) traction current contactor off a locomotive. These have very substantial air or magnetic blowouts, the former needing air to actuate the contactor.

DC ratings are typically 1/10 of AC ratings. I have also seen 2000VAC-rated 3-phase contactors used with all 3 phases wired in series for 600VDC, but that is definitely an "off-label" use.

As always, high voltage DC is dangerous and scary stuff, and needs to be respected far more than most people do.

  • \$\begingroup\$ Thank you for your response. One thing to note is that I am not opening the circuit at full rated 350A load. Max load current is about 100A to 120A to the DC bus of a VFD. Max load of the contactors I've seen break is about 85A DC, but typical break is when current is less than 10A because I manage VFD output current toward 0 prior to opening them. \$\endgroup\$
    – rbarb88
    May 2, 2022 at 15:34
  • \$\begingroup\$ @rbarb88 OK, well that makes more sense because I would expect more destruction at 350A. Having worked a lot with contactors switching in the ranges you are talking about, it's clear to me that these are cheap EV contactors which are really made for daily interruption at less than 1 amp (i.e. presuming the EV firmware has shut off all load via silicon). I don't see how they can expect to daily interrupt your kind of current (at that voltage!) without having proper blowout, arc chute and accessibility to access contactor tips so you can dress them down with a file or change them. \$\endgroup\$ May 4, 2022 at 20:04

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