[PSUBS-MAILIST] power converter

Wed Apr 27 07:55:55 EDT 2016

Thanks' guys.Is it possible to have the motors rewound so they work at a lower voltage?Hank

    On Tuesday, April 26, 2016 9:48 PM, k6fee via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Most inverters these days are 90 - 98% efficient at converting DC to AC. You would only need a 100,000 mfd cap to handle smoothing the DC ripple to acceptable levels to 50 amps and they rarely go south (fingers crossed) in my experience.
With that said having 120vdc direct from a battery bank is the preferred  option.
Keith T.

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-------- Original message --------From: Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org> Date: 4/26/16 7:45 PM (GMT-08:00) To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> Subject: Re: [PSUBS-MAILIST] power converter 
Hi guys,Getting it to AC should be more straightforward in terms of being able to get a standard product, but still needs a pretty big/expensive inverter.  The DC rectifier could work but again for the currents involved the capacitors required for a smooth output would have to be huge, and it's another significant exercise/stuff to go wrong.  Plus it's probably all quite inefficient (I wouldn't be surprised if you lost 20% of the electrical energy just through the conversion).
Cheers,Steve

On Wed, Apr 27, 2016 at 12:18 PM, k6fee via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Another option is to use an off the shelf 48v to 120v ac inverter then use a bridge rectifier to take it back to DC. To get away from a cooling fan the heat sink could be mounted to the hull to dissipate the heat and make it quiet.
Keith T.

Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org> Date: 4/26/16 6:30 PM (GMT-08:00) To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> Subject: Re: [PSUBS-MAILIST] power converter 
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
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