[PSUBS-MAILIST] power converter

Wed Apr 27 01:57:26 EDT 2016

Fair enough Keith, I'm happy to concur :)

On Wed, Apr 27, 2016 at 1:48 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.
>
>
>
> Sent from my Verizon, Samsung Galaxy smartphone
>
> -------- 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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>>
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