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Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Just made another CT for my latest DRSSTC build , its the usual 33:1:33 dual core setup to get 1000:1. It will be loaded with 10Ohm to start with , giving me a 5v signal @ 500Amp and I am wondering: Just how accurate can this arrangement be expected to be in this context? Is it ok for the cores to be tightly bound together like this , or should I be leaving some space between them?
The cores Ive used are cores I have also used for GDTs , and they produce a nice output at my target frequencies.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Typically the CTs are not the source of inaccuracy but the loading resistor itself. If you are worried about the absolute accuracy, please take into account that 33x33=1089, and not 1000 even. There is nothing particularly magical about that winding arrangement, it was my first guess and worked ok so i listed it in the schematics. I often use higher ratios for larger primary currents.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Excellent, thanks Steve. I'm not too concerned about absolute accuracy, I just wanted to know if its likely to be even vaguely close! Within 15-20% is fine by me at this point.
Registered Member #1642
Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96
Dylan,
A little helpful tip on using CT's and maintaining reasonable accuracy: CT's need a very low burden resistance. That is, the higher the burden resistance, the quicker the CT saturates and loses it's accuracy. I have found that if you keep the resistance below 10 ohms (even better--below 2 ohms!) the better the accuracy. I have tested tens of thousands of CT's over the years and: The higher the burden; the more iron you need. The best burden, of coarse, is zero ohms. This, however, is not a practical value for those of us that would like to use a voltage drop to define the current; so for the size CT's you're showing in the photo something in the neighborhood of 5~6 ohms is quit acceptable.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
Great, thanks for the info. I will be running with 5ohm later for a peak of 1000Amp @ 5v , so thats all good. For now I am running 10ohm whilst I setup and test things, to keep the peak down to 500Amp.
Registered Member #1024
Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95
The Math: CT ratio = 33*33 = 1089 Target max peak reading Voltage = 5v Target max peak reading current = 250A CT out current at target = 250/1089 = 0.229568 Required load (R=V/I) = 5/0.229568 = 21.78 Ohm
So , yep 20 is close , but as mentioned earlier in the thread the lower your load R the more accurate the CT can be, and keeping in mind you are only setting the Maximum here (5V) and you then tap off this for your actual overcurrent trip, you are probably better off starting from the beginning with a 5 Ohm load for approx 1000A @ 5v .
Registered Member #575
Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263
Dylan wrote ...
The Math: CT ratio = 33*33 = 1089 Target max peak reading Voltage = 5v Target max peak reading current = 250A CT out current at target = 250/1089 = 0.229568 Required load (R=V/I) = 5/0.229568 = 21.78 Ohm
So , yep 20 is close , but as mentioned earlier in the thread the lower your load R the more accurate the CT can be, and keeping in mind you are only setting the Maximum here (5V) and you then tap off this for your actual overcurrent trip, you are probably better off starting from the beginning with a 5 Ohm load for approx 1000A @ 5v .
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Overcurrent detect current transformer accuracy...
