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I'm thinking of using industrial motor contactors at the output of a 3-phase electrical generator. An example here : CONTACTOR DATASHEET

The only difference / problem is that the electrical frequency of that generator is in the kHz (1-3) range (high-speed generator).

All the infos available for these industrial contactors are given for 50 or 60Hz but no mention of a maximum (or minimum...) frequency.

Would it have no influence at all as long there's a zero voltage crossing every now and then? Should I derate the current given for 50/60Hz somewhat? If so, what would be the physical explanation behind that?

Thanks for the feedback,

Marc

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  • \$\begingroup\$ If your talking about the contacts, then frequency is not an issue. If your talking about the contactor coil (50/60 HZ), then you have a serious issue. \$\endgroup\$ – Sparky256 Feb 17 '18 at 21:16
  • \$\begingroup\$ Hi Sparky, I am talking about the contact indeed. \$\endgroup\$ – VoltsAndNuts Feb 19 '18 at 9:37
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Also consider the Skin Effect. Which causes conductors to have higher voltage drop at high frequencies compared to low frequencies. This will start to become a problem if your frequency is higher than what would be considered an audio frequency.

And there is also a point where you begin to loose the self extinguishing nature of an AC arc compared to a DC arc. This is due to the amount of time that the AC waveform spends at the zero crossing point becoming so small, that the air in the arc no longer has enough time to deionise. This means that a high frequency AC arc can be as difficult to quench as a DC arc.

In both cases you could probably manage by derating your contactors. But ideally we would need to know an approximate frequency. There will be a big difference if your system will be operating at say 3KHz Vs say 95KHz.

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  • \$\begingroup\$ +1 . Good call on arc suppression and skin depth considerations. \$\endgroup\$ – Marla Feb 18 '18 at 3:03
  • \$\begingroup\$ Hi Aredwood, I edited my original post, the frequency would be between 1-5kHz \$\endgroup\$ – VoltsAndNuts Feb 19 '18 at 9:38
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The contactor coil has a 50/60 HZ limit at its rated AC voltage, but the contacts do not care about sine-wave frequencies. They will work just fine from DC to 10's of KHZ. If arcing is an issue you can install 40 mm MOV's rated for 50% above the contactor voltage across the contacts.

They will cut way down on arcing when the contacts open under load.

Leakage current will be in the uA range.

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