Current transformer

The physical implimentation of both current and voltage transformers is a compromise determined by material characteristics and cost. The important thing is that the final physical iteration performs repeatably as required, within a practical budget, with a full understanding of it's limitations under non-optimal conditions.

No transformer is actually useful under 'ideal' short circuit or 'ideal' open circuit conditions.

RL

Irrelevant. The flux is proportional to the voltage drop, both ON THE PRIMARY and on the secondary. Your intent is to have zero voltage drop on the primary, SO all the current transformer design criteria are to make that flux (and d(flux)/dt) as small as practical.

To make a CT design truly ideal, it's frequently useful to use op amps to make a zero-ohm (or negative resistance) load for the secondary. Low resistance load resistors are not just a compromise, they are a USEFUL compromise. Don't think they're irrelevant.

Just realize that IxR drop on the copper does not contribute to volt-seconds on the core, (B) , which is why drawing current on a winding of an unregulated voltage transformer, like a wall wart, will actually lower the flux density.

boB K7IQ

Okay. I have on hand a Triad CSE187-L. You can look up the specs, if you like, but here are the pertinent ones:

.1 to 30 amps at 50 to 400 Hz Np/Ns = 1:500 (has one turn on the sense, so the secondary is 500 turns) Suggested burden resistor: 60 ohms Sense DCR: 21 ohms

I measured the core center leg dimensions as .2" x .2"

The secondary winding resistance and the burden resistor are in series.

30A/500 through 81 ohms gives a secondary voltage of 4.86V.

My calculations say the flux density is just under 17 kiloGauss at 50 Hz.

I don't think instrumentation current transformers run the flux that high because of linearity issues.

Also, if an op-amp is used as you suggest, the secondary voltage (as far as flux is concerned) will reduce the flux density to about 6 kiloGauss at 50 Hz and 30A. A vast improvement, but not necessarily ideal.

John S

Yes, and the secondary voltage as well.

"John S"

** That means the core is heavily saturated.

The makers specs say the unit is not usable beyond 20 amps at 50Hz.

** Gobbledegook.

Say what you mean and stop being a smartarse.

.... Phil

Yeah, it's what I would consider high and into saturation for most materials.

No, they say it is 70% useable. Based on the plot, it only 80% useable at 60 Hz.

Sound advice, Phil. You should pay attention to your own advice.

** Utter, smartarse bullshit.

All you are any good for.

... Phil

It doesn't matter to me what you think I am good for.

Does it matter to you that I feel the same about you?

I have had a lot of experience using that part in our Programmable = Overload=20 Device, or POD, which we developed in 2003 and still use. It is true = that=20 the linearity falls off above 20 amps, but we have tested it and use it = at=20 currents up to 60 amps. The design is somewhat flawed because the = secondary=20 goes through a FW silicon rectifier bridge before the sense resistor and =

filter capacitor, so that the PIC can read the DC level, but even so, we =

have obtained *usable* output much higher than the rated 20-30 amps. To = do=20 so, we have implemented a look-up table that deals with the linearity=20 issues.

For my most recent project I am using a Talema AC-1200 PCB mounted CT = which=20 is rated at 200 amps nominal and 500 amps maximum. This design reads the =

voltage on the sense resistor directly, and I have tested it with 1 ohm = and=20

10 ohm loads, which are less than the recommended 100 ohms 4W. I found = good=20 linearity (better than 1%) over the range of 0.5 amps to 200 amps with a = 10=20 ohm load, and even better linearity from 1 amp to almost 1000 amps using = a 1=20 ohm load. The specifications can be found here:=20

I'm sure the Triad part will perform about as well, although the Talema = part=20 is a toroid which is probably better.

I have implemented the option to select between two load resistors in = this=20 new design, so I can choose what is best for the range of current being=20 measured. We need to handle up to 500-800 amps for short pulses, and = also=20 have the ability to measure currents as low as 0.5 amps to about 1%=20 accuracy. This device seems to do the job, and it's only about $15. This =

project will be the first to use an iron-core CT, so we may find some = issues=20 with waveform distortion. Our other devices (much higher currents, up to =

100,000 amps) use Rogowski coils, which have proven satisfactory for = more=20 than 30 years. And for calibration purposes, we use a shunt, which has = been=20 proven reliable and accurate (at 60 Hz).

Paul=20

the core

THE

amps

Yes, now think it through clearly. I did not say zero but nearly zero.

?-)

on the

*before*

core

secondary

Ummm, well maybe the extremely few that are designed to be operated that way. It is very few.

?-)

thus

milli-Ohm.=20

someone

the

secondary

the

Since when?

?-)

Since vkj's response to Sylvia.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

only, thus

milli-Ohm.=20

someone

so the

secondary

an

in the

Nope. Go read the thread again, you inserted Rogowski coils (for the second time)

someone

What the hell is wrong with you? You're behaving more like a little old lady than most little old ladies.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

only, thus

milli-Ohm.=20

As someone

so the

secondary

becomes an

longer in the

the

No you are being weird. check the thread. vkj returned to normal = current transformers.

Copied from above:

only, thus

milli-Ohm.=20

As someone

so the

Get the point yet?

?-)

thus

milli-Ohm.

someone

secondary

the

Are you going to discuss electronics, or get all whiney and netcoppey about who said what when?

Whiney, probably.

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

Actually, my *current* transformer is the one I'm using now.

My *potential* transformer is the one I might use in the future.

Paul=20