Rewinding current transformer - Page 1
Forum section: electronics / projects

author: eslavko
author_rank: Contributor
timestamp: May 31, 2023, 06:57:33 am
content: Hello...I have a bunch of SCT-13 50A/1V split core current transformers. Now I'm in situation that I need few transformers with 50A/5A ratio. I assume that I can use SCT transformer core and remove windings and wound 10 loops (thick wire) for secondary. Will that work? I assume that core will see same magnetizing current so no risk for saturating. Did I miss something? (I need this for panel meter with standard 5A input)

author: jonpaul
author_rank: Super Contributor
timestamp: May 31, 2023, 07:31:12 am
content: Impossible to assist as no idea the part mentioned. Please include spec sheet, link or mfgj

author: TizianoHV
author_rank: Regular Contributor
timestamp: May 31, 2023, 07:38:39 am
content: In theory should work, but:-You should use some thick wire (normally CTs should be able to handle twice the nominal current, continuously) and I don't know if there's enough space.-A 5A analog instrument has high burden, that means that the poor little core could saturate causing errors."I assume that core will see same magnetizing current so no risk for saturating" - Remember, this is a current transformer. With directly shorted windings (without ammeter) you'll have minimum saturation. When connecting a load (ex. Ammeter with burden (internal resistance...)) the output voltage will increase and the core will get closer to saturation.Therefore: for it to work you must not exceed a certain Volts-Per-Turn threshold. Smaller cores have lower V/Turn.For example if you CT has 1000 turns and 1V nominal output (1mV/Turn) with 10 turns will have 10mV nominal output. By exceeding the nominal output voltage you'll get the core to gradually saturate, loosing accuracy. Usually 5A analog meters have 400mV burden, which is way over saturation.In conclusion, such a small core, will barely move the needle of a typical 5A analog meter.

author: Berni
author_rank: Super Contributor
timestamp: May 31, 2023, 07:55:09 am
content: 5A sounds like a lot of current from a current transformer.Unless it is a pretty large beefy transformer the measurement errors are going to be all over the place.The 5A panel ammeter you are wanting to use is likely actually a milivolt meter connected across a internal shunt resistor and amps painted into its scale. So you can probably open it up and replace the internal shunt resistor to turn it into a 50mA ammeter.

author: eslavko
author_rank: Contributor
timestamp: May 31, 2023, 10:50:17 am
content: The CT is like:https://www.ebay.com/itm/174480352469And the meter is:https://www.amazon.de/gp/product/B0BJ5XBPGX/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1I was thinking that primary winding (single loop) will magnetize the core and not secondary.And I have no idea of meter internal resistance.As my CT has primary range of 50A I do just assumption to make it 50A/5A.In reality the meter will be connected over 25A fuse so making range 25A/5A or 32A/5A should be better for meter, but much worse for CT if I understand correctly that not primary but secondary will force core into saturation.

author: eslavko
author_rank: Contributor
timestamp: May 31, 2023, 11:27:26 am
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: May 31, 2023, 11:51:06 am
content: Quote from: eslavko on May 31, 2023, 10:50:17 amThe 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.

author: eslavko
author_rank: Contributor
timestamp: May 31, 2023, 03:22:54 pm
content: 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.

author: TizianoHV
author_rank: Regular Contributor
timestamp: May 31, 2023, 04:01:19 pm
content: 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).

author: eslavko
author_rank: Contributor
timestamp: June 01, 2023, 07:01:47 am
content: Quote from: TizianoHV on May 31, 2023, 04:01:19 pmI 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.)