Re: [PSUBS-MAILIST] Magnetic coupler

Mark,

Eddy currents like you say are frequency based. I do not think we will see a very large frequency from our thruster. The rotational speed should be quite low. With the rotational speed low, the coupler eddy current losses shouldn't be much. If we ever get to 500 rpm .....I think we would be over doing it.

Motor manufacturers use laminated cores to reduce eddy currents, or composite iron substances.

I will do a lot of testing before ever truly constructing a thruster. Each test will be to verify one component at a time.

But your views are noted....and will be tested.

James Long
Owner/Designer
Lil Brother LLC (Instrument Division)

----- Original Message -----

From: Mark Roberts - Harbortronics

To: personal_submersibles@psubs.org

Sent: Monday, July 30, 2007 12:33 AM

Subject: [PSUBS-MAILIST] Magnetic coupler

Before you get too carried away with it, you might want to check the amount of eddy current losses you get. Moving magnetic fields induce a current in metal (anything conductive), creating a 'drag' force on the moving magnet. It's proportional to velocity, as well as magnetic field, so the higher the RPM, the more drag you are going to have. If the RPMs are low, like really low, then great. Unfortunately to get any 'power' through the coupling at low RPM, you would have to couple a proportionally higher torque, meaning bigger magnetic fields >> larger expensive magnets, and then more drag again... Unless you use a non-conductive coupling wall, I see lots of losses in your future :)

Motor manufacturers use special metal compositions to minimize eddy currents, and those metals are structually lousy.

Plastic or glass walls would do the job, but of course are going to have to be thick to handle any pressure. Magnetic force diminishes very rapidly with distance (wall thickness). Think about the tiny distances used in a motor between the magnets and the windings... if you want to achieve the same torque with a significantly larger gap, then you would need massively stronger magnetic fields, or massively greater diameter at the coupling. Unless you can do this, you will need to multiply the RPM at the coupler to acheive the same power.

Has this sort of coupler actually been used by anyone? I hate to be a sourpuss, but it seems doomed to me. I'd love to be proved wrong!

Mark

----- Original Message -----

From: Lil Brother LLC

To: personal_submersibles@psubs.org

Sent: Sunday, July 29, 2007 8:24 PM

Subject: Re: [PSUBS-MAILIST] Thruster ideas

Vance,

I'm going to put both replies in this message.

Yes, bldc motors are very efficient. Because the current is pulsed depending on the speed of the motor. The motor only goes as fast as the phases are changed. (there is a ratio depending on the bldc design...the number of poles in it)

I agree...there have been some modifications to improve the trolling motor to handle the depths. But they haven't been improved on the efficiency. It's the nature of the design. They could use PWM to help increase the efficiency....but it will only help a small amount.

Like I said....I'm not trying to sway anyone from using what they want. I just want to design something...that can be bought (yes...I would like to keep a pair of thrusters under 2000) within reason.

I will need a few test beds for prototypes.....I will need at least two subs in each depth category. I haven't broken any categories down yet.....so this is premature.

"How thin is thin for the mag coupling?"

This has yet to be determined.....I will have to perform some pull test to see how thick the material can get.

James Long
Owner/Designer
Lil Brother LLC (Instrument Division)