[PSUBS-MAILIST] Air compensating thrusters

Sat Jul 26 20:41:43 EDT 2014

Hi Vance,
I'm still plugging away on this one, looking at the pros & cons. 
If I wrote a white paper everyone that read it would end up more confused than when they started.
But this is a summary of what I've learned so far. Others may want to add or comment on this.
The main cause of electric motor failure is overheating. So there is a plus for oil compensation here
in that it transfers the heat to the casing. Most electric motors rely on air cooling which we can't 
achieve underwater. Brushless motors disipate heat to the casing easier but are normally geared.
Emile has a 2hp protruar brushless motor but said it was noisy.
The Minnkota motors must be a robust design that can handle the heat build up while submerged.
Although I am wondering if there is heat transferance up the tube from the thruster to the control box,
with air circulation caused by the rotating motor.
The air compensation may work with trolling motors designed for high temperatures, but fail with other
motors. Or fail if the props are too large & put too much strain on the motors causing them to overheat.
In oil compensating the oil can lift the brushes, but this has been countered by using heavier spring tension on the brushes.
The wear of the carbon brushes causes the oil to become more conductive & cause arcing.
I have read of recomendations to change the oil every 10-20 hrs.
Oil causes the motor to run more slowly & less economically.
Greg mentioned that his compensating oil (I think WD40) was causing seals & other plastics to harden.
I experimented with WD40 & found it destroyed some plastics in hours.
Carsten & Emile moved to a thin expensive silicone oil. Emile then moved to rim thrusters.
Commercial oil compensating units have a 4-5 psi overpressure. However Carsten says that his units without
overpressure other than being mounted above the motor, can leak when out of water. But this brings in the 
other part of the equation which is the motor seals.(Don't know much about these, have to learn)
Another thing that Carsten mentioned was that the cooling of the oil can suck in water past the seal. However
I would have thought the system would remain ambient & his oil reservoir would release oil back in to the motor,
also a system with overpressure would stop this.
He also said that you can get water ingress into an oil filled motor & it survives, but one drop of salt water in 
an air compensated motor can wreck it.
Which brings me to Hugh's air comp suggestion of using a pressure relieving regulator with back pressure relief
& setting it with an overpressure. I think this is a great idea & want to experiment with the stainless one he found
 I have pulled 2 other types of these relieving regulators to bits & they look a good option. I am just not sure how 
much flow the releiving function can handle & what pressure fluctuations there would be in the motor. Probably only a few psi. 
Karl's motors would have 500 psi at 1000 ft to force out through his thruster shaft seals on his ascent.
Ladies turned up, got to go.
Alan

________________________________
 From: via Personal_Submersibles <personal_submersibles at psubs.org>
To: personal_submersibles at psubs.org 
Sent: Sunday, July 27, 2014 2:08 AM
Subject: Re: [PSUBS-MAILIST] Air compensating thrusters

The voice of experience. I knew about the brush problems, having fought those carbonizing issues with Hymak oil compensated thrusters (Pisces and Aquarius). Perhaps someone will develop this further and generate a white paper for the psubs archives? Air compensation certainly works for Karl S. and could well be a viable and inexpensive alternative for those in the planning/building stages. 
Vance

-----Original Message-----
From: Alan James via Personal_Submersibles <personal_submersibles at psubs.org>
To: psubs.org <personal_submersibles at psubs.org>
Sent: Fri, Jul 25, 2014 6:35 pm
Subject: [PSUBS-MAILIST] Air compensating thrusters

Hi psubbers,
in an attempt to get more information on the compensating issue
I contacted fugu subs, who I remembered used air compensation.
http://www.fugusub.com/

They operate small semi dry ambients probably to a max of 100ft. 
One thiught is that they operate in the warm waters of Florida where you would think
oil compensation would be an advantage in cooling the motors.
This is their reply.....
Hello Alan,

I seldom respond to technical letters, but I thought it best to save you some headaches.  Over the last thirty years, we have tried several types of oils for filling motors with, as some "authorities" had recommended.  Here's what happens:  On motors with brushes, the centrifugal force and pressure of the oil passing between brushes and rotors causes the brush to "lift off" of the rotor, causing power loss.  The motor slows down dramatically from internal fluid frictions as well.  Another issue is that some of these oils can wreck the vinyl insulate on wires and possibly o-rings.  On brush motors, 
oil is a failed prospect.  Stick with an air system.  Good luck with your project!

Kind regards,

Russell J. Canfield

_______________________________________________
Personal_Submersibles mailing list Personal_Submersibles at psubs.org http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles 

_______________________________________________
Personal_Submersibles mailing list
Personal_Submersibles at psubs.org
http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20140726/06c53bb8/attachment-0001.html>