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Which part of a brushless motor must not touch water for it to function properly?

Hi

We’re working on a very small impeller (2.5 cm diameter), and the driving motor we’re using is a drone motor as we need the housing to be as compact as possible, Emax motor, provided in the link

The motor does not have an en embedded encoder or anything, it’s just stator and rotor.
In order not to get the motor burnt out or short-circuited by the water into failure, what parts exactly of a brushless motor, or in this case a drone brushless motor, that should not touch water???

Some say it’s the whole winding, others say that it’s only the connections, and we also found people telling us that the whole motor can work just fine without chemical seal…

Please help us out if you know anything about this, I have worked with the T-100 and found that it has chemical seals, but it’s not so easy to figure out which parts are important to seal…##

@ValyrianSeif
Brushless motors can run fully submerged fully exposed to water, no burn out no short-circuit. I prototype with bare brushless motors often. They will take quite a bit of maintenance to upkeep, strongly suggest to fresh water rinse them immediately after all dives, then rotate to insure as much water is drained as possible. Also suggested is to open them up time to time to check corrosion that will start to appear on the magnets. The primary issue you may face will be with the bearings themselves, as most are not stainless or a rust resistant stainless grade. If you start to see rust appear on the bearings, I have used PTFE Lubricant spray to rinse them off, then spin the axle shaft via a power drill can help clean them up a bit. Note lubricants can be argued to increase particulate wear, but once they start to rust it is just to extend the life before they will need replaced. Another note, you can also encapsulate the coils with epoxy yourself, some will argue it extends the life of the motors, others will argue that heat will cause osmosis to draw in capturing moisture, so something you can look into. There was even a university study that submerged a brushless motor into a fish tank and it ran for over 6 years continuous, it is the rust & corrosion once exposed to air out of motion that will be your biggest enemy. But keep them low cost and is a fine affordable solution and easily changed out.

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Thank you so much for this dense reply it will sure help us a lot. I also just wanted to ask if you happen to know, why is it then that BlueRobotics have so many Marine Epoxy added in the lower part of the T-100’s coil?

If you get 2-part epoxy resin, rather than the cheaper polyester resin, you can thin it up to 50/50 with denatured alcohol. This will make it flow like water over, around, and into the stator coils and coat the magnets, without adding a thick layer that might cause clearance issues.
If you search on Ebay, you can find replacement bearings made of ceramic, that won’t rust.

I never had any issues coating my motors/electronics this way. My motors are the exact opposite of yours, 190kv 70mm diameter, monster torque. But my ROV is the size of a household chest freezer.

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@ValyrianSeif
One issue of where you may encounter as to why the need for epoxy added to lower part is with the cables and connection point to the coils. Primarily thruster cables are braided, at the point of soldering to the motors water can penetrate the cable housing and leak following the wire space inside of the jacket. Epoxy coating this connection point can reduce this risk.
@Oddmar
Great tip on the 50/50 denatured alcohol, I hand painted the coils leaving the magnets exposed, believe I will give this a try myself.

Hi Mostafa, I have used the same size motors as the one you linked successfully. I haven’t run them in water long but so far they are working well. I did use marine epoxy to seal the bottom where the wires connect to the coils. You can also use some paint on conformal coating to seal the coils if you want further protection.

Lastly, I wanted to say that the cost of the motors you are looking at is really high compared to what I was buying. You should be able to find that same exact size motor on Amazon, Ebay, or Alibaba Express for about $8-$10/motor at low quantities.

For Example: https://www.ebay.com/itm/4pcs-2212-CW-CCW-Brushless-Motor-920kV-2-3S-7-12V-for-DJI-Phantom-free-shipping/253815628085?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649