transformer inrush [RE-wrenches]

Mon Jan 5 20:03:52 PST 2004

Hey Kirk and Mark,

I would re-wire what the electricians did and not have the transformer
in there 24/7.  Not that big a deal, just another sub-panel and such.
Also, consider wiring in a 240vac, 30 amp, 2P2T power relay on the pump.
This would leave the pump on the utility and allow it to become a
"critical load" only in the event of a utility power failure.

Bob-O turned me on to this in the past.  You also put in a second
pressure switch on the pump and set both its low pressure cut-in set
point and its high pressure cut out voltage lower than the pressure
switch/settings currently on the pump.  Use this pressure switch to
supply the 120vac/primary side of the transformer who's output is wire
to the pump through the 2P2T transfer relay.

Perhaps pull all the 240vac out of the mains panel, give them their own
sub-panel, and jumper across the two hot lugs in the mains panel to give
120 of the same leg to all breakers.  Also post notice on the panel
cover inside and out saying that it is "120vac only".  The same
transformer can be used to supply the 240 in the above mentioned
scenario.

If the effluent pump is also a 240vac load, I have used another relay to
keep both the well pump and the effluent pump from running at the same
time (over current).  The best way to go with this is to have the
effluent pump not come on until the well pump is satisfied (another 2P2T
relay).  While most septic systems have approximate 200 gallons of
reserve when the E-pump comes on, and it is unlikely that the well pump
will continue to run that long without allowing the E-pump to come on,
over-filling the tank should not be an issue.  However being a worrying
solar bozo I have utilized the output from the "tank alert" alarm signal
(code on most tank systems) to keep the well pump from overfilling the
septic/holding tank.

I have done a lot of whole house systems to date and have only done one
dual inverter system ever.  Up here in northern Wisconsin we usually
have a budget issue.  We just have to make it work, often with
mod-square and often with a few power relays.  These relays are very
reliable and fairly inexpensive.

Kurt Nelson
SOLutions

-----Original Message-----
From: Mark Edmunds [mailto:mark.edmunds at xantrex.com] 
Sent: Monday, January 05, 2004 5:05 PM
To: 'RE-wrenches at topica.com'
Subject: RE: transformer inrush [RE-wrenches]

Hi Kirk,

I think the autotransformer should actually help keep the surge current
down, due to its added impedance. Do you have any way of measuring the
peak
surge current demand from the load? I suspect one of the pumps starting
up
while there is a high load may be going over the limit.

Fluke has a new line of clamp meters (The 330 Series) and most of them
have
a "motor inrush" feature, which is quite handy for checking on this sort
of
thing. I don't know how accurate this is but it will give a good
indication
of surge current.
http://www.fluke.com/products/home.asp?SID=5&AGID=3&PID=30405 

Cheers,

Mark Edmunds
Xantrex Technology Inc.

-----Original Message-----
From: Kirk Herander, VSE [mailto:kirk at vtsolar.com]
Sent: Monday, January 05, 2004 9:26 AM
To: RE-wrenches at topica.com
Subject: transformer inrush [RE-wrenches]

Hello,

I have a SW4048 inverter used in a UPS mode to backup some household
critical loads - a septic and well pump and 2 fridges amongst others.
The
system is always feeding the critical loads with grid ac through the
inverters transfer switch, supposedly rated at 60 amps. The surge
potential
of the inverter is rated at 78 amps. The 120 vac output is fed into a
120/240 autotransformer, so 120/240 vac is supplied to the critical load
panel. My preference was to have only the 240 vac loads fed from the
transformer, but the electrician didn't set things up that way.

Twice in the last week the customer has called telling me that the
inverter
has shut down due to an overcurrent error. It might be possible that
continuos plus/or surge currents from the critical loads could exceed 78
amps, but I'm wondering if the transformer could be adding to the
problem.
If the primary of the transformer is constantly energized(core
magnetized),
there should not be a high transformer-related inrush current, when a
motorized load turns on, adding to the surge requirement and creating
the
overcurrent error, correct? Would it be helpful to just step up the
voltage
for the sake of the 240 vac loads(water and septic pumps)?

Kirk

Vermont Solar Engineering
North American Board of Certified Energy Practitioners Certified Solar
PV
InstallerT
Xantrex Certified Dealer Charter Member
Kirk Herander
PO Box 697
Burlington, VT. 05402
www.vermontsolar.com
www.backeastsolar.com
800.286.1252 - 802.863.1202
fax 802.863.7908

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