[RE-wrenches] Compressor motor/SW+5548

[bob at midnitesolar.com]

On 5/30/2016 1:48 PM, Jeremy Rodriguez wrote:
> Bob
> Can you elaborate a little on this extension cord idea either here or 
> off-line? Thank you


Sure.   The extension cord, in this case, is just added resistance. Some 
inductance but
mainly resistance.  The current from the starting load is reduced by the 
voltage being
dropped along the cable.  So instead of the SW sourcing quite as much 
current, it sees
less current but some power is dissipated along the extension cord. Way 
more power
at high surge of course since power (Watts) is current squared times 
cable resistance.
If the startup current doubles, then the power dissipated in the cord 
quadruples
during that surge time.

This less current drawn from the SW inverter allows the voltage at the 
motor to
drop as was mentioned and the Over Current Protection (OCP) inside the 
SW does
not get activated (hopefully) and the output voltage does not go to zero.

Since the motor startup is fairly short, there isn't much lost energy in 
the cord.

Also, since the cable is sort of long-ish, the power dissipated is 
fairly nicely spread out
along the cord and no one spot gets hot, except maybe the plug.

You could also just add longer real wire or a resistor.   Fun Ohm's law 
stuff.

boB





>
> Jeremy Rodriguez
> Solar Installation And Design Expert
> All Solar, Inc.
> 1463 M St
> Penrose Colorado 81240
>
> Sent by Jeremy's iPhone. Sorry for typos and shorthand!
>
> On May 30, 2016, at 1:15 PM, "bob at midnitesolar.com 
> <mailto:bob at midnitesolar.com>" <bob at midnitesolar.com 
> <mailto:bob at midnitesolar.com>> wrote:
>
>>
>> The (newer) SW5548 was harder on over current protection, mainly because
>>
>> it could put out more current than the SW4048...
>>
>> One trick to try is to place a long(ish) extension cord between the 
>> SW and the load to give it a wee bit of series resistance.
>>
>> Doing this would often reduce that OCP and help motors to start.
>>
>> boB
>>
>>
>> On 5/30/2016 11:03 AM, Dan Fink wrote:
>>> Does your air compressor have an unloader valve, or can you install 
>>> one?
>>> That should do the trick, sounds like you are just running into the 
>>> regular high surge current demand.
>>> Off the grid, compressors are in about the same nasty league as old 
>>> well pumps as far as surge demand.
>>>
>>>
>>>
>>>
>>> Dan Fink
>>> Adjunct Professor of Solar Energy Technology, Ecotech Institute
>>> IREC Certified Instructor™ for:
>>> ~ PV Installation Professional
>>> ~ Small Wind Installer
>>> Executive Director, Buckville Energy
>>> NABCEP Registered Continuing Education Providers™
>>> 970.672.4342 <tel:970.672.4342>
>>>
>>>
>>> On Mon, May 30, 2016 at 10:46 AM, Jeremy Rodriguez 
>>> <allsolarjeremy at msn.com> wrote:
>>>
>>>     Hello
>>>     I've got an SW+5548 that will barley start my air compressor.
>>>     Once it does run it shows 17 Amps on the inverters loads meter.
>>>     During start up it reads 60+ amps.
>>>     Do you think better capacitors would help the situation?
>>>     BTW I've never ran the compressor off grid prior.
>>>     I even tried #8 from the Breaker to the pressure switch, about
>>>     9ft, with no benefit. The compressor will only start with the
>>>     tank completely empty.
>>>
>>>     Jeremy Rodriguez
>>>     Solar Installation And Design Expert
>>>     All Solar, Inc.
>>>     1463 M St
>>>     Penrose Colorado 81240
>>>
>>>     Sent by Jeremy's iPhone. Sorry for typos and shorthand!
>>>

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