Electronics > Projects, Designs, and Technical Stuff

Rewinding current transformer

<< < (2/2)

eslavko:
I just did more googling and found teardown of that meter.

Seems that inside is already current transformer 5A/2.5mA.

So Assuming that I think it's safe to use my split core CT and wind few turns of (as thick as possible) wire as inside the meter is just wire loop without burdens. No warranty void.

On the other way I can remove internal CT in meter and attach my external CT without burden resistor voiding waranty.  The meter has option to set transfer ratio of CT.

Gyro:

--- Quote from: eslavko on May 31, 2023, 10:50:17 am ---The CT is like:
https://www.ebay.com/itm/174480352469
...

--- End quote ---

The listing indicates that the transformer has a 1V output. This implies that it already has a load resistor on the potted secondary winding, which needs to be taken into account.

eslavko:
If I go to rewind the CT, I know that I need to remove all but core.
If I go to void warranty and open meter to replace CTs then I need to remove load resistor from my CT. It's easy accessible as is not potted.

TizianoHV:
A)Since it's a digital meter the 5A mod could work but you should keep the 5A wires as short as possible, accuracy could be bad and if your "50A line" is far away from the meter could be an issue.

B)Modifying the instrument would be the cleanest and best way, but you would have to find and replace its internal resistor (since they are selected for 5mA CTs). After changing those resistors it may need recalibration.


Have you already bought it ? There are other similar meters with similar prices that support SCT-13 CTs.



Z?) without touching too much the pcb you could modify the internal 5A CT by winding a few tens of "thin" magnet wire thus reducing its primary current from 5A to a more manageable level, (this would also be less accurate).

eslavko:

--- Quote from: TizianoHV on May 31, 2023, 04:01:19 pm ---B)Modifying the instrument would be the cleanest and best way, but you would have to find and replace its internal resistor (since they are selected for 5mA CTs). After changing those resistors it may need recalibration.

Z?) without touching too much the pcb you could modify the internal 5A CT by winding a few tens of "thin" magnet wire thus reducing its primary current from 5A to a more manageable level, (this would also be less accurate).

--- End quote ---

I have meter ordered and waiting delivery. AFAIK the meter have option in menu to select CT ratio. From the photo I see internal CT is 5A/2.5mA so the ratio is 1/200. I have CT 50A/1V, but in datasheet I see that it's 1/1000 ratio, with internal burden, and I can remove that burden as it's easy reachable, and current adjustment can be done in meter.

Now about accuracy. Magnitude can be verified and adjusted. But I'm afraid that phase cannot be adjusted. So I think my options are:

1. Add external CT with 10 or 20 turns of thick wire and connect to unmodified meter. With 20 turns I got range from 0 to 25A with better magnitude precision, (25A fuse, and I don't care for short overcurrent precission). I assume that two CT's in series will produce more phase differences.

2. Keep my CT as is but remove burden, and in internal CT make 50 turns of thin wire. Should have same current range, but unknown phase diff.

3. Replace internal CT with my just removing burden from my CT. Original have 1/200 ratio and mine 1/1000 ratio. So without changing internal burden I got 25A range. And as there is only one CT I assume the phase will be better.

I don't know how much phase differ in two CT's in series, but I need as precise as can, as meter will operate near zerro power. (adjusting grid tie inverter to not supply power to grid.)

Navigation

[0] Message Index

[*] Previous page