Brushless motors - running in water

I need some thruster motors (not main drive) and I've been looking at brushless motors because they seem to be a better bet than brushed motors (simpler,open construction, good bearings etc.)

I had some small outrunner motors from a doomed aircraft project. I have kept this in water for a week, sprayed it with WD40 and it still works!

Next thing is to try it underwater. It looks to have promise so far. Still problems about difficulty in reversing it (reversing speed controllers??) and it is rather high revving.

Am I mad and has anyone else looked at this?

Well I've just been trying it under water and it doesn't work - just runs for a fraction of a second. Take it out of the water and it runs up fine.

I wonder if it is something to do with it being sensorless ??(They need a back EMF to trigger the speed control.)

The trials continue but maybe I will have to go the route you suggest, Richard - "sacrificial brushed motors" + a large can of WD40!

The problem then is that it is just one more maintenance task (check for water etc.). It would be nice to have something I could almost forget.

Hello David,

electronically commutated (brushless) DC motors running directly in water are a very good approach for a reliable and nearly maintenance-free propulsion system, I can confirm this by my own experience (my S195/U16 has been running since 1995 without interrupts and without maintenance for the propulsion train/ motor. More than 400 hours in operation and more than 200 miles run). But I think it would be useful to try it with a sensor motor instead of a sensorless design. Sensor motors do not expose a strange behaviour and are easy to handle like a normal brush motor.
But I fear there are no suitable commercial solutions available at the moment. You must also consider that the motor has to be prepared to operate in water. You need rust-proof bearings, must seal your iron package and - very important - the rotor magnets if they are made of rare earth alloys (despite of their nickel coatings the may corrode rapidly if the coating has got some fine cracks). Also electrical wiring/ terminals must be isolated from water.

Best regards
Hermann

Thanks a lot for the very useful comments, Hermann.

Yes there is a lot going for a purpose built "wettable" brushless motor as a propulsion motor.

Trouble is I was looking more for a cheap and cheerful motor to drive a couple of thrusters (i.e non-essential).

The brushless motors looked promising - simple construction, very free draining, chunky bearings. The water during the one week soaking did indicate some "rusting." Maybe the sort of corrosion you describe.

But if "off the shelf" don't work at all I will have to examine some cheap and near disposable brushed motors.

David

HI Hermann
A brushless motor with hall effect sensors for picking up the rotation of the motor do offer more controll at low speeds. But i came across a design for a brushless speed controller which was based on synchronous rectification and the control of the unit is super at slow RPM and start up of motor is very smooth, i don't think you could tell any difference between hall effect sensor's and this speed controller design the guy know what he was doing with the software
John

John,
Would you be willing to share? I am collecting information on a future build and would like to move into brushless motors. If you prefer you can PM me. Thank you!
Peace,
Tom

http://www.elektor.com/magazines/2006/february/brushless-motor-controller.58087.lynkx

HI Tom
find at the top of this page down load site for files for brushless speed controller. The design of the speed controller is for just one direction but i was going to rework it to have forward and reverse one of the problems with a brushless speed controller is if you wonted to go forward then reverse you would have to make sure that the motor was stopped before you changed direction this circuit seemed ideal to do this
All the best John

John,
Thank you.
May I pm you with an electronic question? I want to create an battery operated torpedo in 1/72 scale that limits run time to 10 - 15 seconds.
Peace,
Tom

HI Tom,
Yes you can PM me when ever you won't
John

Hi John,

indeed sensorless designs of brushless DC motor controllers have improved. Nevertheless for myself I prefer a sensor design because of its ultimate performance at very low speed and clear torque development just from start. Even when stalled the torque will act right in the direction you want without any cogging. The behaviour is just equal to a brush motor. And the speed range varies continously from zero to full speed (1RPS, revolution per second, or even less is possible, whether it makes sense or not).
The control electronics for a sensor design is just simple, you only need pickup sensor decoding logic (can even be a wired hardware logic), a pulse width modulator for soeed variation and switching transistors for the stator coils (e.g. three MOSFET half bridges for the delta three phase stator system). I've built three different motors, the first in 1994, an axial flux (disc runner), 8 pole sensor controlled motor, a similar 6 pole motor of smaller size and the 14 pole motor of my class 212A model (the last in continous service since 2002). I must confess that this had been a lot of work (because the mechanics and the electronics are complete of own design and self made) but it was worth to do.

Kind regards
Hermann

HI Hermann
Yes i can see why you went for sensor based brushless speed controller, I went looking on hear to see if i could find any pictures of your sub would like to see pictures of brushless motor if you have any, as i find things like that interesting, I to like designing electronics every thing in that big sub of mine was scratch build but i have been doing a lot of up grading of the electronics. Hermann i also do model planes one of my big plane pulls 3400watts using 10 lipo cells of course this is also brushless motor the vertical climb in the plane is unbelievable it is not some thing to learn to fly with


Thought i would mension this Hermann have been testing have been testing antenna for under water on 27MHZ seem to work very well
All the best John

I see from Model Boat Mayhem that this kit is now being offered with a brushless motor running in water.

Very interesting!

http://www.ebay.co.uk/itm/RC-Sub-USS-SSN-21-Seawolf-Attack-Submarine-kit-set-Free-Rolling-water-pump-Set-/280827756600?pt=Radio_Control_Vehicles&hash=item4162a38038

Anyone able to comment on this or even better still, tried out this kit?

David

I've now done just that! Thanks for the suggestion,Richard.

The KV number seems higher than needed. I would think that 1,000KV is high enough, unless you want a porpoise-like submarine action.
I use e-Flite brushless motors.

http://www.e-fliterc.com/PowerSystems/Motors/Brushless.aspx

And Castle Creations Hydra Watercooled 120 ESC.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRJL4
Giovanni

...it is indeed interesting although I have a feeling of a "déja vu" experience...

In fact that is just a hermetical design as I had designed and applied for my submarine models (the class 206A and the 212A) and the SEAWOLF of my son. And it proved to be reliable over the years. So you can forget about leakage throug the shaft seal as there is no shaft seal at all. Also the pump ha savoided sliding seals as it is a roller pump. I think that is a reasonable design.

But I am not shure whether that motor is durable (long term) waterproof. It has the appearence of an ordinary brushless motor and the magnets and stator package maybe only protected from water by some thin resin. Due to my own experience with wet running electronically commutated DC motors (nowadays called brushless motors)since 1994, the rare earth magnets must be protected very carefully from water as they have the tendency of rapid corrosion! They are coated by a thin nickel layer but this is very thin and may get cracks or even chips so the magnet material can get contact to water. I suppose the roller bearings ar eat least made of stainless steeel and the windings are well moulded.

Kind regards
Hermann

I too have my doubts about the longevity of a motor operating in these circumstances. I still remember the rusty water and "bits" in the water after few days immersion.

By the way, Hermann, did I confirm with you that 3 of us are visiting the Dortmund model show this year? It would be good to meet up.

David

A reply from Donald.

(It is really now for some one to try this and report back - Volunteers?)

Hi,
We have modified brushless motor can be waterproof ,Do you have not routine maintenance of the motor ,If not will be corrosion .
Donald


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