FM80 wiring - cancelling inductive currents?

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FM80 wiring - cancelling inductive currents?

Postby JeffinthePeace on Sun Feb 10, 2019 9:34 pm

Hi - I’m just wondering how critical this is?
I do not have my cables running through the same knock-outs.
Any thoughts on this?
Cheers
Jeff

From the FM80 installation manual:
 Run positive and negative cables side by side.
~ Keep cables together (e.g., using a tie-wrap) as much as possible to allow the inductive currents
to cancel.
~ Ensure paired cables pass through the same knockout and conduit fittings.
JeffinthePeace
Forum Virtuoso
 
Posts: 80
Joined: Mon Oct 01, 2018 8:20 pm
Location: Peace Country, British Columbia
My RE system: PSX-240 Auto Transformer.
VFX3524 Inverter/Charger.
FM80 Charge Controller.

6 (total) Trina PV panels: 365W, 39.3Vmp, 48.0Voc, 9.3A Imp, 9.77A Isc
—> PV set up in 3 pairs. Each pair in series.
Midnight Sun Combiner Box.

Mate (Original) and Hub4.
TriMetric 2020 Battery Monitor.

2 battery banks, each:
1065 Ah Yuasa 85T-21 FLA battery bank (10 yrs old).
—> Battery set up in 6 boxes x 4V cells each = 24V system.
—> Bank 1 = 1065 Ah
—> Bank 2 = 1065 Ah
—> Bank 1 & Bank 2 connected via Blue Sea 4-Position Battery Selector Switch.

10kW 120/240VAC Kubota/Stamford diesel generator.

No Utility Grid available.

Re: FM80 wiring - cancelling inductive currents?

Postby raysun on Mon Feb 11, 2019 10:34 am

Like a lot of practical electrical engineering on critical limits, the answer has a large component of "it depends".

I assume the guidance in the documentation is to minimize the effect of induced voltage (EMF) and companion induced currents. When current flows through a wire, a magnetic field is created. When the current stops flowing, the magnetic field collapses, creating an EMF (voltage) across the conductor with resulting current flow. How much EMF and current is induced is a function of the strength of the magnetic field and how fast it collapses.

A case might be solar harvest on a day when the sun is high in the sky, and fast moving heavy clouds pass in front of it. The charge controller is feeding high current to the battery in full sun, a sudden change in solar energy caused by a passing cloud would cause a sudden change in current flowing to the battery. The sudden collapse of the magnetic field will induce an EMF and current. A great enough rate of change can induce enough EMF to cause damage in some extreme cases.

A more extreme case might be the charge controller(s) producing high current and then the battery disconnect breaker is opened. This could create enough EMF to arc across the breaker contacts and burn them.

The guidance with keeping the + and - conductors tightly bound is an attempt to limit the induced EMF and current by having the two collapsing magnetic fields cancel each other's inductive effects to the extent possible.

Is your system going to burn to the ground if the conductors don't pass through the same knockout? Not likely unless you are running a good number of panels and controllers.

Will the minor "insults" of induced EMF shorten the service life of the equipment? Perhaps, or perhaps not, but it's safe to say it will not improve the service life.

Personally, it would have been more convenient to ignore this guidance when routing the cables, but yielded to "better safe than sorry" and rerouted them.

Ya pays yer money and takes yer chances.
raysun
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Posts: 334
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, MX80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 4Sx3P)

Re: FM80 wiring - cancelling inductive currents?

Postby JeffinthePeace on Mon Feb 11, 2019 4:31 pm

Right. The issue is the wire I’m using for connecting the FM80 to the shunt and the breakers.
I made a terrible choice! I believe it’s AWG4 copper comprised of about 1 billion strands. It is very, very stiff. Almost unworkable.
I think when I install the second FM80 in the spring, I will get extra runs and replace what I have in place. Clean it up a little.
Recommendations on wire type for short runs from the FM80 to the panel (literally just a few feet) would be appreciated.
90V 2.2kW coming in from this array.
Cheers
J
JeffinthePeace
Forum Virtuoso
 
Posts: 80
Joined: Mon Oct 01, 2018 8:20 pm
Location: Peace Country, British Columbia
My RE system: PSX-240 Auto Transformer.
VFX3524 Inverter/Charger.
FM80 Charge Controller.

6 (total) Trina PV panels: 365W, 39.3Vmp, 48.0Voc, 9.3A Imp, 9.77A Isc
—> PV set up in 3 pairs. Each pair in series.
Midnight Sun Combiner Box.

Mate (Original) and Hub4.
TriMetric 2020 Battery Monitor.

2 battery banks, each:
1065 Ah Yuasa 85T-21 FLA battery bank (10 yrs old).
—> Battery set up in 6 boxes x 4V cells each = 24V system.
—> Bank 1 = 1065 Ah
—> Bank 2 = 1065 Ah
—> Bank 1 & Bank 2 connected via Blue Sea 4-Position Battery Selector Switch.

10kW 120/240VAC Kubota/Stamford diesel generator.

No Utility Grid available.

Re: FM80 wiring - cancelling inductive currents?

Postby raysun on Mon Feb 11, 2019 5:09 pm

Here in the U.S. 4 AWG is about right for the 80 Amp breaker used for the charge controller disconnect. (The circuit breaker is designed to protect the wire, not the equipment, so 80 Amp capacity - "ampacity" in the wire is appropriate.)

I have seen folks (who shall remain nameless in order to preserve a shred of credibility) use 6 AWG and live to tell the tale. I believe however, they are replacing it with 4 AWG in a current upgrade project.

Are you certain 4 AWG was used between the CC and the shunt connection? It sounds more like 2/0 gauge - that heavy "welding wire" stuff. The wire between the battery and the panel should be able to carry the current rated on the battery disconnect. 2/0 ampacity is 175A.

My pictures won't post :(

The red and black cables between battery and shunt. 2/0 AWG - about as thick as my thumb.

4 AWG - about the thickness of a pencil.
raysun
Forum Guru
 
Posts: 334
Joined: Tue Jul 26, 2016 5:57 am
My RE system: Flexpower One: FX3048T, MX80, MATE3s, FlexNetDC
Outback 200NC batteries (8 @ 48v)
Outback IBR3 battery enclosure
Suniva 330 watt panels (12 - 4Sx3P)

Re: FM80 wiring - cancelling inductive currents?

Postby JeffinthePeace on Mon Feb 11, 2019 7:03 pm

Ray - clearly I’m going to have to double-check my wiring. ie. gauges
The original installer used the right stuff I believe...
I’ll verify gauges at some point and get back to you.
Cheers
J
JeffinthePeace
Forum Virtuoso
 
Posts: 80
Joined: Mon Oct 01, 2018 8:20 pm
Location: Peace Country, British Columbia
My RE system: PSX-240 Auto Transformer.
VFX3524 Inverter/Charger.
FM80 Charge Controller.

6 (total) Trina PV panels: 365W, 39.3Vmp, 48.0Voc, 9.3A Imp, 9.77A Isc
—> PV set up in 3 pairs. Each pair in series.
Midnight Sun Combiner Box.

Mate (Original) and Hub4.
TriMetric 2020 Battery Monitor.

2 battery banks, each:
1065 Ah Yuasa 85T-21 FLA battery bank (10 yrs old).
—> Battery set up in 6 boxes x 4V cells each = 24V system.
—> Bank 1 = 1065 Ah
—> Bank 2 = 1065 Ah
—> Bank 1 & Bank 2 connected via Blue Sea 4-Position Battery Selector Switch.

10kW 120/240VAC Kubota/Stamford diesel generator.

No Utility Grid available.


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