[RE-wrenches] Battery Bank to Inverter Wiring

[John]

That is why for years we have been twisting those leads around each other.
I was told it was to cancel out the opposing fields on the wires, but for
whatever the correct technical reason is,  we have always twisted those
heavy wires.         John V.

 

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Jarmo.Venalainen at schneider-electric.com
Sent: Saturday, 27 June 2015 5:45 a.m.
To: RE-wrenches
Subject: [RE-wrenches] Battery Bank to Inverter Wiring

 

Hi: 

>From time to time over the years I've come across systems where the routing
of DC cables between the batteries and the inverter has been the cause of
issues. 

I'm not referring to wire thickness or quality of terminations.  For the
purposes of this discussion, just assume that wire thickness and
terminations are perfect. 

What I am referring to is the routing of the positive and negative battery
cables.  In particular, the loop area within the + and - cables as shown in
the image below, 


The problem I've seen in systems with a large loop in the setup is that the
inverter does not provide good surge power and can even go into low voltage
shutdown during large surges. 

Recently this happened again and I wanted to get a better feel for it, so I
did some math.   

For a cable length of about 12', the loop is an inductor which has a value
of inductance of about 1 uH for side by side cables and as much as 6 uH for
cables about 1 foot apart.   

This inductance is greatly multiplied by any ferrous metal in the loop and
can easily be in the range of 10's to 100's of uH.  Examples being cables
which run in steel conduits or along the steel frame of a motor home. 

Inductance causes a voltage drop proportional to the rate at which the
current is changing.  To get an idea of how large that rate can be for
typical inverters, I did surge tests with a 5kW inverter and found that the
rate of change of current can be as high as 100A per milli-second or 100,000
Amps/second. 

Given that, the voltage drop of the wire inductance is then , Vdrop = (rate
of change of current) x (inductance), 

Vdrop for 1 uH = (100,000 A/s) x (0.000001 H) = 0.1V 
Vdrop for 10 uH = (100,000 A/s) x (0.000001 H) = 1.0V 
Vdrop for 100 uH = (100,000 A/s) x (0.000001 H) = 10.0V    clearly this is a
problem. 

Have any of the wrenches had systems with this issue?  If so, how often. 


JARMO 


 
____________________________________________________________________________
_________ 

Jarmo Venalainen  |   Schneider Electric   |  Xantrex Brand  |   CANADA  |
Sales Application Engineer 
Phone: +604-422-2528  |   Tech Support: 800-670-0707  |   Mobile:
+604-505-0291 
Email:  <mailto:jarmo.venalainen at schneider-electric.com>
jarmo.venalainen at schneider-electric.com  |   Site: <http://www.xantrex.com/>
www.Xantrex.com  |   Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 


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