[PSUBS-MAILIST] power converter

Tue Apr 26 21:30:11 EDT 2016

Hi Hank,
I've dabbled a little bit in (small!) DC converters and my guess is that a
converter that will do the current you're after will
be expensive/large/complex and probably generate a significant amount of
heat.  Also seems like an obscure set of operating conditions that will be
hard to find off the shelf.

You've probably thought about it, but splitting the battery bank
and rewiring part of it to supply 120VDC is probably the easiest low-tech
solution if you have enough cells to put in series and achieve the higher
voltage.  You would get some nice redundancy there too if you
have 2 battery banks controlling different thruster sets.

As an aside, I suspect 48VDC is pretty lethal, but 120VDC even more so.

I just saw Alan's email come through - running it off lower voltage is
definitely an option, but depends on the sort of motor.  Ie. a brushed
motor speed is proportional to the voltage applied (so running at 48VDC
would give you less than half speed) - no idea what the Perry ones would
be, but being older they may well be brushed.  Brushless motor speed is
proportional to frequency applied by the controller.  In both cases torque
is proportional to current which is proportional to heat generated.  So the
greater the current in your wires, the more heat needs to be dissipated,
and if it's not then something will burn out - probably the thin layer of
insulation on the motor windings.  The trouble is dissipating the heat from
the inner windings (it has to pass through the outer windings, which are
producing their own heat) - but like Alan said, if you've got it oil
compensated that will help a lot with heat dissipation.  The thermal
conductivity of oil is ~6x that of air (and water ~24x air), although the
heat transfer is not quite as simple as that.

I'm also pretty sure that the motor will have an inherent resistance that
will be designed into it appropriate to the nominal voltage and current,
and related to the size of the wires (ie. as Hank pointed out, thinner
wires for high voltage as less current is required).  So if you apply
less voltage to the same resistance, it will draw less current - ie. I
don't think you will be able to get extra current into the motor at lower
voltage.  Not quite that simple, but you get the idea.

(that was mostly dredged up from memories of a university subject I did on
electric motors a long time ago, feel free to correct me!)

Cheers,
Steve

On Wed, Apr 27, 2016 at 10:18 AM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Thanks" Kieth,
> I started to that but I have no idea if one system is better than another.
> Hank
>
>
> On Tuesday, April 26, 2016 6:11 PM, k6fee via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>
> Hank,
>
> You want a dc-dc converter, just Google it.
>
> Keith T.
>
>
>
> Sent from my Verizon, Samsung Galaxy smartphone
>
> -------- Original message --------
> From: hank pronk via Personal_Submersibles <
> personal_submersibles at psubs.org>
> Date: 4/26/16 4:53 PM (GMT-08:00)
> To: Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> Subject: [PSUBS-MAILIST] power converter
>
> HELP,,,
> Turns  out my Perry thrusters are not 36V but 120 Vdc.  I can only manage
> 48V dc from Gamma's battery bank.  Can I step the voltage up with something.
> Hank
>
> _______________________________________________
> 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/20160427/a5ee2b17/attachment-0001.html>