multiple current sense coils combined for one digital meter?

Thread Starter

sdowney717

Joined Jul 18, 2012
711
I have a single AC digital meter which measure current and voltage.
I have one wire passing through the sense coil.
I have 2 more wires all on the same AC phase I would like to include those wires into the digital meter's current display.
It would be easier to just add more sense coils than run longer wires back to the original single sense coil.

I have 2 more sense coils made for these meters.
Can you join sense coils together for a single digital meter?
Would they be joined in series? or parallel.
Would the coils need to be matched as for as handedness since they are coils themselves of fine copper wire or just connect the sense coil wires to each other?

Or is this not going to work.
 

strantor

Joined Oct 3, 2010
6,817
Please confirm:

You have 3 wires per leg of an AC supply. These 3 wires are in parallel to share the load. So assuming single phase supply, you have 6 current carrying wires (plus ground), or for a 3 phase supply you have 9 current carrying wires (plus ground).

You have an ammeter with a current transformer that only reads one of the wires but you would like it to read all three. You are looking for a way to do that.
 

LesJones

Joined Jan 8, 2017
4,223
If my understanding of the question is correct the TS has wires to three separate loads connected to the same phase but only one of the wires passes through the CT and he wants to read the summ of the currents through the three wires. If this is correct then he could just tap the two extra wires into that phase after the CT rather than before the CT. He could also just thread the extar two wires through the CT with the existing wire and connect them back to the phase on the supply side of the CT where they are already connected. It would be better if the TS supplied a schematic rather than trying to describe it. To combine the output of three CTs each would have to have it's own load resistor and the voltage across the load resistors per amp of primary current in the CTs would have to be the same. The three outputs from the CTs (And their load resistors.) would be connected in series. (And phased correctly.) This would only work if the meter displaying the current was being used on the voltage range. If it was being used on the current range and connected directly to the CT secondary then I don't think combining the outputs would not be possible. (If all three CTs were identical then it my work by connecting the secondaries in parallel. Again they would have to be phased correctly.) NOTE. THE SECONDARY OF A CT MUST NOT BE LEFT OPEN CIRCUIT WHEN THERE IS CURRENT TROUGH THE PRIMARY AS THERE WILL THEN BE A HIGH VOLTAGE ACROSS THE SECONDARY.

Les.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
711
Hi, ok my situation.
I have my boat AC panel
I have the CT sitting on the incoming wire to the panel, works fine. Measures current going to each branch breaker.
However not all the AC power available is coming into this panel. Before the panel, is running a power relay DPDT which switches my power source from the generator and the shore power for the converter-charger.

So the AC power in is split in two. And I can not measure the AC power powering the relay and the converter-charger.

For multiple reason, I have split the converter - charger off from the main AC panel.
One is the gen has an auto start on demand feature, it uses 12vdc to sense when someone closes a switch and the gen will start automatically.
The converter-charger would always be on, so the gen would always run if it was attached as a branch of the main AC panel.

When I plug in my boat to shore power, that DPDT relay is energized and the AC shore power flows to the converter-charger. If AC shore power goes away, relay de-energizes and connects my generator to the converter-charger. on the generator, I tap power off for the DPDT relay before it's own CT sense circuit, so it does not just start and run all the time.

Another reason not to have converter-charger in the main AC panel, I have an inverter which when selected, runs AC power to the panel. If the converter-charger was on a main panel branch breaker, it would be a dumb power loop, sucking battery power to run inverter, which then charges the batteries.

The digital panel style meter is always connected hardwired and displays AC volts and AC amps continuously.
 
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Thread Starter

sdowney717

Joined Jul 18, 2012
711
The LCD digital panel AC meter works fine, was bought on ebay. It comes with a CT. I happen to have 2 more CT made for this meter.
There was no CT resistor, the CT wires simply attaches to the LCD panel meter.
So are you saying the CT would need to be in series, all 3 together? And about the coiling of the wire inside the CT, would they need to be like facing the same direction?
I assume these CT work by generating a voltage? Higher voltage, panel meter displays higher amps? And volts are additive in series.

I can run the other 2 wires through the original single CT, by splicing and getting longer wires, will cost me more money, it is cheaper and seems easier to just hook up more CT's to the one panel meter.

What will happen if 2 CT are connected in series and one has its two wires reversed ?
I dont think I can tell by looking at the CT coil which wire is which for the coil winding direction of the wire wrapping round the CT.
 
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LesJones

Joined Jan 8, 2017
4,223
As the CTs don't have their own load resistors the secondaries wil need to be connected in parallel. (The three CTs must be identical.) Start by adding one CT. With no load on the wire through the added CT put some load on the original circuit. Then switch on some load to the wire going through the added CT. If the meter reading goes up then it is connected the right way. If it goes down you need to reverse the wires from the secondary of the added CT. (Or thread the wire through the CT in the opposite direction.) Repeat this with the third CT. DO NOT BE TEMPTED TO SWAP OVER THE WIRES WITH THE POWER STILL ON.) Check that you get sensible readings as I have never tried this idea but I think it should work.

Les.
 
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