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I work for a company that builds electric motors, within the last few years they've started building control boards from brush-less dc motors. On the incoming power we have an Inrush Current Limiter to limit the draw on start-up from capacitors charging. I evaluate returns on these motors and have seen these current limiters cracked or crumbling , but there's no evidence of a short on the board. I'm having a hard time finding information of failure modes/causes for these. They are either Ametherm or Cantherm. Hoping someone has seen this or might have some info and what(other than a short) would cause this failure. Thanks.

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    \$\begingroup\$ To talk about specific items, it would be nice to know which those specific items are, and how they are used (i.e. voltages and amps and waveforms etc.) \$\endgroup\$ – PlasmaHH Feb 11 '15 at 16:57
  • \$\begingroup\$ I'd contact the manufacturer. They are likely to know more that we do. \$\endgroup\$ – George Herold Feb 11 '15 at 17:01
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    \$\begingroup\$ You need to provide specific details about your "current limiter". Is this some form of thermistor, or an electronic device, or even just a big-ass resistor? Exact specs if possible, along with photos of a new device as well as photos of a failed device. \$\endgroup\$ – Dwayne Reid Feb 11 '15 at 17:39
  • \$\begingroup\$ My guess would be that the component is being over-stressed due to ambient temperature over limit, or the input current pulse is larger or lasts longer than is allowed or something like that. It would be very helpful to capture the current waveform on a variety of new units. Try to capture the worst-case waveform, then send it to the vendor and ask whether it is acceptable for the specific component. \$\endgroup\$ – mkeith Feb 11 '15 at 18:49
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The inrush current limiter that you're using is under-specified and experiences too large of a temperature swing in your circuit. It fails due to excessive thermal cycling.

As expensive as it may sound, I've yet to find any inrush current solution more resilient than discrete resistors, and a contactor that bypasses them once the 24V control supply has started up. No need for any other delays: just a 24V switching supply, a contactor, and three resistors. The resistors would typically be in metal enclosures and mounted to a metal plate bolted to the DIN rail or the panel. You'd need to size them so that they can safely dissipate the energy they'll produce during the charging transient.

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I looked up "Antherm" on the Internet, and it appears to be a heat flow control device, not necessarily a current limiter of significance. However, if the failing part is a thermistor, it probably has a positive temperature coefficient. As it heats up, the resistance increases, thus limiting the current flow. If this is the case, you need to replace it with one that has a higher power rating. The Antherm device is much more complex and requires a deeper analysis, which may be available on the vendors web site.

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