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Old Nov 06, 2013, 07:47 AM
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reversing a two wire brushless

I'm heading up a student rov competition team and we are required to use "sealed" motors. i.e. not open to the water.
To avoid having to build custom enclosures and trying to avoid the bulk and expense of bilge pump motors, we were considering modifying small brushless submersible pumps which can be had for a few bucks by adding props.
They claim to be brushless but are of the two wire variety. The question is are these motors reversible?
We need need control in both directions.

Any insight would be appreciated.
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Old Nov 06, 2013, 01:48 PM
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Your title "reversing a two wire brushless " is not clear.

A bare brushless motor has three wires, it cannot have two wires. If it is truly a two wire brushless motor it has to have a built in ESC (or some kind of electronic controller). In that case you would have to hack the motor to reverse it.

If it is a bare, two wire motor it is something else, maybe a brushed motor.

There are many two wire AC motors, is it one of these?

In any case there are many motors that can run under water. In fact just about any motor will run under water. The fuel pumps in all new cars are in the gas tank and run submerged in the gasoline. All deep well water pumps run submerged in water. Many people break in their new brushed motors by running them submerged. And so on.
The small water pumps used in cars for the windshield washer have very good seals.
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Old Nov 06, 2013, 04:23 PM
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well, aren't pc fans brushless? They only have two wires and they don't reverse with a simple switch of polarity. At least the ones I have don't

These are the pumps I'm considering... note the two wires.
http://www.ebay.com/itm/190868716630...84.m1423.l2649
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Old Nov 06, 2013, 04:54 PM
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Not sure exactly what you are describing.
But in my small experiences with Pond / Aquarium pumps these are often simple coils within sealed enclosures while their impellers are magnet based.
Simple 60hz AC thru the coil does the spinning.
Just need to feed 120v 60 hz AC to your model
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Old Nov 06, 2013, 05:42 PM
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Quote:
Originally Posted by scubersteve View Post
well, aren't pc fans brushless? They only have two wires and they don't reverse with a simple switch of polarity. At least the ones I have don't

These are the pumps I'm considering... note the two wires.
http://www.ebay.com/itm/190868716630...84.m1423.l2649
PC fans are not reversible because they include the motor controller (ESC) as part of the fan (a little PCB stuck to one side of the motor).

Those pumps do not have a standard motor in them, the rotor is the pump impeller and the coils are sealed away, would be hard to modify for a prop. The one I have (a little larger) operates on the same principle as a BLDC motor (electronic commutation), but I dont see how you could modify it for a prop as the impeller is the rotor (has magnets in it) and you need the whole housing to keep it in place (there is no shaft inside you can mount anything to).

A normal BL motor will actually work fine under water, only the connection point would need to be protected, the motor windings themselves are already insulated by enamel coating. The bearings would require frequent lubrication but other than that they would work (at least for a while). How reliable do you need this to be?
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Old Nov 06, 2013, 07:45 PM
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I see... I was unaware of the internal design of these pumps.
I am using open brushless motors on two of my own rov designs without a problem, but the rules of this competition specifies sealed motors.
Anyway, thanks for the info.
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Old Nov 07, 2013, 09:18 AM
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Well, I would seal a brushless outrunner motor then. If you worry about the water pressure breaking through the sealing, fill the enclosure with petroleum. As it is incompressible, there will always be nearly the same pressure at the inside and outside. Only very small amounts of water will leak in, and petroleum is a better lubricant than water and a better coolant than air.
As you will use very low kV motors with limited RPM, friction losses should be tolerable.
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