Upgrade current transformer for current source - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: December 20, 2018, 08:57:43 pm
content: Many years ago I needed a 60Hz, 3-phase AC current source for some testing I needed to perform. I cobbled together a working source using 3 x audio amplifiers. Nominal output current was about 3 Arms. The load required a fairly high compliance voltage (about 12Vrms from what I recall). I now have a need for a similar source with more current 10Arms (per phase), but with much lower compliance voltage.The CT's I used were just something I found laying around and were by no means optimized for the situation, they were just handy and free so I used them. I'm now hoping to replace the CT to allow about 3x the output current and 1/3 the voltage. Magnetics is not my strong point.Can anyone lead me in the right direction in terms of a suitable CT to replace the existing one?Digi-Key offerings appear to be of the current sense style with X amps : 1 or less, so nothing that looks vaguely appropriate.

author: duak
author_rank: Super Contributor
timestamp: December 21, 2018, 06:30:59 am
content: From what I understand about CTs, they're optimized to work low down on the B-H curve, with lower flux levels than, say, a power transformer. The cores are oversized for the power they handle. As one of my teachers used to say - here's a fabulous concept: the magnetic flux induced into the core by the primary current is essentially removed by the secondary current. For a given primary current, the highest secondary current occurs with the lowest secondary load impedance; ideally a short circuit. Since voltage is a function of the flux and the frequency, little net flux means voltages are also low.I've worked with AC current sources but never thought about using a transformer to change the V vs I relationship but don't see any particular reason why it won't work. Since you want a 3:1 reduction in V and a 3X increase in I a 3:1 stepdown transformer would be needed. I can't think of any off the shelf transformers with this ratio but one with four identical windings could be wired to give the desired ratio. I don't think that CT's offer any particular advantage over, say, a good audio transformer or a power transformer designed for 50 - 400 Hz operation and operated well below its nominal operating voltage.You mentioned a compliance voltage. When you're using this gizmo, do you expect it to develop significant voltage when operating?Cheers,

author: capt bullshot
author_rank: Super Contributor
timestamp: December 21, 2018, 07:19:45 am
content: You could try to find real CT's as these (sorry, it's a German link, I don't know of anything comparable in the US):https://mbs-ag.com/produktprogramm/stromwandler-fuer-niederspannung/and use them in reverse.A 25A : 5A model could do your job, just put one wire through the conductor hole, use heavy gauge and keep it short.You could also use a larger ratio one (e.g. 60A : 5A) with e.g. four turns of wire through the core makes a 5A : 15A (reverse) model for you. Works fine as long as your compliance voltage is small enough.

author: jbb
author_rank: Super Contributor
timestamp: December 21, 2018, 07:33:26 am
content: An appropriate power transformer would probably do quite well provided that the parameters are reasonable.Most 50 Hz transformers are designed with high flux density - near saturation. If you run it at less than half the rated voltage (appropriately scaled for frequency), it should be fairly linear and the magnetising current should be low.A toroidal transformer could be quite good here; the shape of the core leads to good distribution of magnetic flux in the core (may help with linearity) and the shape of the windings leads to low leakage inductance (will help reduce impact of magnetising current).For say 10A @ 3V output, you could probably use a 115V to 15V @ 10A toroidal transformer. It would be heavy and expensive but probably quite effective. It would run at much less than normal flux density so probably nice and linear. You’d need to have a look at the impact of magnetising inductance and leakage inductance to determine what accuracy you might expect.

author: Jester
author_rank: Frequent Contributor
timestamp: December 21, 2018, 05:11:40 pm
content: Thanks to everyone, that was easier than I thought, I was able to get 13Arms, using two loops through a 100:5 CT that I had laying around.Only problem is the 100:5 CT I have is pretty big. If I were to try a wind my own transformer, can anyone suggest a suitable core and windings?Looks like 10:1 is about right.

author: capt bullshot
author_rank: Super Contributor
timestamp: December 21, 2018, 05:26:09 pm
content: Size (and weight) of the core depends on your burden resistor.The small it is (and the smaller the compliance voltage is), the small gets the necessary core. In general, I'd look for cores from common mode EMI supressor chokes, they usually have a material with a high AL (uH per n^2) value, you'll want a high value here (typical > 30uH per n^2). This works if the load is a near short circuit with a core of maybe 15mm height and 40mm outer diameter. For higher impedance loads, you'll need larger cores, maybe cores from toroidal mains transformers, starting from 10VA rating (of the transformer).Winding would be similar to your 100:5 CT, 10:1, or 20:2 turns.

author: Jester
author_rank: Frequent Contributor
timestamp: December 21, 2018, 06:00:22 pm
content: Quote from: capt bullshot on December 21, 2018, 05:26:09 pmI appreciate your guidance, I'm looking on Digi-Key website under Ferrite cores perhaps I should be looking elsewhere? The highest AL core they show is 21-22uH, with Ae of 102-108 mm^2https://www.digikey.com/product-detail/en/epcos-tdk/B64290L0616X038/495-76562-ND/3913522Do you think this would work?

author: capt bullshot
author_rank: Super Contributor
timestamp: December 22, 2018, 09:46:44 am
content: 20uH is probably the lowest end that might work, this core looks a bit small (mechanical). Better look for higher values, but these aren't ferrites. It's rather nanocrystalline stuff like that:https://www.vacuumschmelze.com/en/products/cores-components/applications/cores/vitroperm-standard-types-encapsulated.htmlFor lower Al, I'd try more turns, but this reduces compliance voltage and may be unusable.

author: Jester
author_rank: Frequent Contributor
timestamp: December 22, 2018, 09:51:41 pm
content: Thanks, I might still try a wind it myself CT, but for now I lucked out. I visited the local surplus shop and found some CT's that just might do the job.These are smaller, I was able to get 10Arms into a short. I will need to do more testing, to see if I can generate sufficient compliance voltage into say 1I picked up a box of them for $5 / piece, so I can double or triple them up if need be.