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Current transformer question [AC]

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LAN_403

syntroniks

Wed May 13 2009, 12:24AM

Registered Member #1530 Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32

Well that settles that, now I have to see if some cores I have here would make the cut.

65 square millimeters of cross-sectional area and ferroxcube's 3E6 material (Al ~9000nH) is the first thing I reach for, but something tells me that it will not perform. They were bought for GDT use.

Would I need to hack apart some cheap/free AC adapter to get a core that wouldn't saturate?

Looking back on this post it seems that the only real option is to go with a nice large laminated core. Think I could get away with an EI as opposed to toroid? I'm also not looking to buy a transformer since I will probably want to wind my own in the future. I wouldn't mind purchasing/scavenging cores though

Dr. Slack

Wed May 13 2009, 06:56AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Richie wrote:

It's lack of distributed air-gap gives rise to rapid saturation as each cycle of current begins to flow. Once the ferrite is saturated, any further increase in output would be due to "rogowski coil" action I guess.

No, a current transformer is an "ideal transformer" as defined in the wiki. That means saturation is avoided by having the volts/turn low enough for the cross section of the core, even with an ideally infinite permeability, not by having a low permeability core material, whether with air gaps or what. *Any* un-gapped core will do, as long as it is has the right number of turns. 3E6 will do, but the lower peak flux density compared to iron means you will need more turns to stay out of saturation. As transformer inefficiency and heating is essentially irrelevant in this application, the only penalty of that is winding time.

So how do we know the voltage to design a current transformer? Easy, the secondary current flows through the burden resistor. If the secondary current is 0.1A and the burden is 100ohms as in the example below, then the secondary voltage is 10v. If you are running a bridge rectifier before the burden resistor, as Steve and Richie point out is the right thing to do, then the secondary voltage needs to be > 2*Vdiode to make sure the bridge turns on, but it doesn't need to be much higher, 10v sounds ideal.

The transformer turns should be designed for the cross section and frequency as if it were an ordinary transformer with 10v on that winding. If you have got to the point where you have already chosen the core and wound the coils, and you need to increase the handling a little more, then reducing the burden resistor reduces the output voltage and so also the core flux. If you are unsure of the calculations, then post the Bsat of that material, and the primary current you want to use.

Note that with a "conventional" transformer driven from a voltage source, the choice of voltaeg determines saturation, and any current can be drawn, subject to heating limitations. However, with a current transformer with a fixed burden resistor, the output voltage increases as the curent increases. This means that saturation will occur at some current, proper operation only occurs below that limit.

So what happens when a CT saturates? The load in the primary couldn't care less, the impedance of the CT drops and current continues to flow. However, the core flux stops increasing, and the meter in the secondary simply sits at the limit determined by Bsat and the number of turns. No smoke happens, just an inaccuracy.

A very good place to start is to find a mains transformer whicn isn't completely potted (a toroidal with a hole in it is perfect), thread a primary of a single turn of heavy wire through the core, and use the existing "mains" winding as your many-turn secondary. Or if you can find a MOT, punch the shunts out (important), use the three turn heater as the primary, and you have a choice of secondaries to take the output from. Fire it up and see what you get. A disadvanateg is that you don't know the ratio exactly, but it's very quick.

klugesmith

Wed May 13 2009, 07:10AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dr. Slack's post apppeared while I was composing this offline, so some of the following may be redundant.

Good practical point by Richie again. I hadn't known the B_sat for those ferrites is -so- much less than that of steel. On top of that, we need more volt-seconds because the secondary voltage (into diode bridge) will be nearly a square wave. For your core area, and output voltage of +- 1.6V, I figure you would need about 350 turns to keep Bmax below 300 mT!

But neither an air gap, nor reduced permeability material, would help here. This is a -transformer- and depends on secondary ampere-turns to buck the primary ampere-turns, except for just enough to magnetize the core to required flux level. That flux level depends on voltage x time integral, not current. (Rated primary current would saturate the core of an ordinary CT if secondary winding were omitted or burden R too high).

Sounds like you are prepared to dabble in transformer design.
You might start with a salvaged transfomer of suitable size and remove one winding, leaving the other winding [with turns count in the appropriate range] in place to be the secondary of your CT. Or buy a CT at surplus store or ebay!

-Rich

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