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We are testing solar water pumps which have the 3phase inverter for the BLDC in the down hole pump. The boards (one power board, one controller board) are submersed in the lubricant oil that is used for the motor. This oil is a synthetic (PAO) oil, but do not know if there are additives. (we do not make the electronics or the pump)

The BLDC controller and the Power board (with three half H-bridges) all have aluminum electrolytic capacitors. I am particularly concerned about an increase in the ESR of the bootstrap capacitor used on the high side gate drive on the N-channel FETs.

We have pumped water for over thirty days, but have had some failures,

  1. High and low side FET shorted on Half H bridge

  2. Motor failure (still under investigation)

  3. Controller failure (still under investigation)

There is a brief note in the specifications indicating that capacitors should not be exposed oil(it appears all the manufactures copied the same notes from somewhere)

Confirm the environment does not have any of the following conditions... Where capacitors are exposed to oil or an atmosphere that is filled with particles of oil.

So submersing them in oil doesn't sound like a great idea, but the details are a bit light on the expected failure mechanism (like oil penetrating the electrolyte changing ESR or spoiling the anode oxide)

What type of failure would be expected when operating an aluminum electrolytic capacitor(wet) in synthetic oil.

I have requested information from four different manufacturers of aluminum electrolytic capacitors , but have got no response.

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    \$\begingroup\$ are those FETs adequately heatsinked? why did they fail? \$\endgroup\$ – analogsystemsrf Jun 7 at 3:00
  • \$\begingroup\$ could be some de-watering additive in the oil scavenging all the moisture from the electrolyte. \$\endgroup\$ – Jasen Jun 7 at 3:08
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    \$\begingroup\$ Can you remove a capacitor from one of the failed units and make some measurements on it to compare with a new one? \$\endgroup\$ – pericynthion Jun 7 at 5:23
  • \$\begingroup\$ The FETs are mounted on a metal based pcb. The failures are a short from Drain to Source. I don't think these failures are related to the caps. I removed three of the bootstrap capacitors and a Bulk DC cap which was across the bridge and they all were with in spec (assuming +/- 20 %) The long term reliability may be impacted by the rubber gasket that seals the electrolyte. It is possible the oil will react with the rubber and eventually cause it to fail. \$\endgroup\$ – LaserSmoke Jun 10 at 17:50

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