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Old 10-01-2010, 23:06   #1
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Installing BSS VMS - Alternating Current Question.....

Up to the install on the BSS VMS, which basically monitors voltage, current, tanks levels, bilge pump cycles etc. Replaces a link 10 that I had.
MY question is about installing the AC current transformer. It is supposed to be installed on the Line, before the loads. OK. I understand that, but I have a 120/240 v system. I am NOT a electrical engineer, but seriously want to understand what is happening here. Guess I am a bit 'short' on the theory aspect of it.
Here are some of the snippets from the install manual.



AC Current
In most cases the AC Current Transformer should be located on the main
AC feed before any other devices. See Current Flow in 120/240 Volt AC Systems - Resources - Blue Sea Systems
for more information on AC Current Transformer location. The location does
not affect state of charge (SOC) calculations.



I have read the resource on current flow in 240 volt systems, and find myself at odds at what I THOUGHT I KNEW about 240 v systems. And how to install this.
My understanding was that current flow was via the L1 or L2 or both, to the appliance, then back via neutral. That is of course how it is in a 120v system, and so, I understood it to just be twice that... L1+L2=240v. But the resource on the BSS site at Current Flow in 120/240 Volt AC Systems - Resources - Blue Sea Systems indicates something else.
So my question is twofold. And PLEASE, do not suggest that I get a qualified marine electrician to do this. I am not interested in that. I want to do it myself, and learn in the process, and perhaps other can learn as well.

Do I use 2 transformers, one on L1, one on L2 and connect them to the same place on the back of the unit? I have no 240 volt loads, it is split into 2 120 volt systems, with the neutral split after the main breaker into 2 separate Neutral buses. 1 for inverter loads, the other for the non inverter loads.

And the other is, how does current really flow in a 240v system? is it via the L1 then out via the L2 as the BSS doc suggests? and if so, why is there a neutral in that case? Just seems counter intuitive to me.

Thanks,
Bob
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Old 11-01-2010, 06:53   #2
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Bob,

Remember that in a "properly" loaded 120/240V system, which means that the 120V L2 loads are equal to the 120V L1 loads, then the net current in the mains neutral line will be zero. Why? Because the voltages (and the currents) of L1 & L2 are 180 out of phase. So when the current in L1 is at its positive peak, the current in L2 is at its negative peak, and they cancel each other out. When the current in L1 is at 90, the current in L2 is at 270 - always exactly opposite.

Does that make sense to you? It's hard to envision without graphs, but good if it does.

Now for the fun part! You can almost never have perfect loading between the two! So even if you have "matched" loads on each, there's always going to be something a little out of whack which will unbalance things..

But seriously - any time you have a 120V load, you're "unbalancing" the system, and you'll end up with net non-zero current flowing through the neutral. But if you happen to have those "magic" matched 120V loads on L1 and on L2, you'll still have current flowing through each branch's neutral lead, but when they meet up at the AC panel's neutral buss bar, the net current on the mains neutral back to the shorepower/genset will be zero because they're exactly out of phase and cancel each other out. All your net current flow is between L1 and L2.

The only reason you'd need a third current transformer is if you have any 240V loads - like a heavy duty watermaker, a scuba compressor, large clothes dryer, etc...

The "fun" part mentioned above is where you begin to consider what's known as the power factor (pf). There's a decent writeup over at Wikipedia on it. Remember back in advanced math in high school when you learned that the square root of -1, usually called i, was an imaginary number? Well, that "imaginary" is actually very real, and comes into play when we're messing with alternating current - and inductors and capacitors. Inductors resist changes in current, and capacitors resist changes in voltage, so they're in essence "opposites", and you'll see when talking about power factors the terms inductive loads and reactive loads, and loads with a bit of each. But this stuff isn't needed to have a passing knowledge good enough to wire an AC system - just follow the ABYC recommendations and you'll be fine. If you want to dig in and actually understand the theory behind the recommendations - even better!

So, yes - install the two current transformers on L1 and L2 as indicated. Since you don't have any 240V appliances, you don't need the third one. Remember that the xformers are polarity sensitive. When the electrician installed mine, one was backwards, so my panel's amp meter was measuring the difference between L1 and L2 rather than the sum - it took me a second to figure it out when the amps displayed on the meter dropped when I switched on more AC loads!

I hope I haven't misstated anything here - it's been almost 30 years since I learned AC theory as an EE undergrad...
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Old 11-01-2010, 10:37   #3
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Thanks Jon.
AC current is sure hard on the brain! Funny too, cause I thought I understood it just fine until I go to rewire the boat and a whole host of things I thought I knew, now I don't emerge.
I took a builders school after getting out of the army and electrical was heavy in it. And have rewired homes as well. The theory was never that important to me, just the practicality of it. But on a boat it is a entirely different story.

I asked Peter Kennedy Peter Kennedy Yacht Services who I purchased the equipment from, and the BSS VMS system eveidently cannot support both load readouts without some additional equipment. Or he says put it on the neutral line to read both sources in 120v mode.
I have a email out to BSS tech support as well to see what they have to say about it.
Recently Blue sea dropped the price significantly on this unit, which to me says is was just not selling well. I will have to wait and see how it does. The only thing I have to compare it to is the link 10, on both prior boats. I was going to go with the victron unit, but thought the tank readout, bilge cycles ect that this one does would be nice to have. Now thinking it was a mistake. Time will tell.
Thanks for your thoughts, and the electricity theory. I need that.
Bob
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Old 11-01-2010, 11:35   #4
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I downloaded the installation manual and took a peak. Looks like Peter Kennedy knows his stuff - the unit can only measure one "leg"... You could setup a transfer switch-like capability to switch from one neutral to the other, but that could cause problems if not done right. Wait and see what BSS recommends...
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