Author Topic: Phase lead change caused by Current Transformer?  (Read 190 times)

Offline GrantV

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Phase lead change caused by Current Transformer?
« on: February 03, 2021, 10:59:28 AM »
Hi there,

I caught this piece of information in the translated manual from the cloudleopard drsstc driver: (which does not have a seperate phase lead input or adjustment)

For the feedback transformer, please use a double magnetic ring transformer. The phase shift of the single magnetic ring transformer can damage the entire bridge.

As I will be running at less than 300Apk, I was planning on using standard 300:1 current transformers from Triad Magnetics (CST206-3A)* CST206_3A.pdf, but the note above now makes me think that I may need to revert and rather wind my own cascaded CT pairs.

Any thoughts?

« Last Edit: February 03, 2021, 04:30:22 PM by GrantV »

Offline klugesmith

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Re: Phase lead change caused by Current Transformer?
« Reply #1 on: February 03, 2021, 04:23:43 PM »
This non-coiler doesn't see how a single stage CT could have more phase shift than a cascaded pair.   I assume that the ideal for this application is no phase shift.  The winding volts-per-turn (per Hz) must be kept low enough that cores don't approach saturation.

Unless the secondary winding capacitance or resistance become significant in the 1:N^2 transformer. 

Your CT datasheet link doesn't show frequency response numbers. It does say max current is 70 A RMS; might that be for something other than heating?   Maybe this forum has posts where people have characterized those popular CT's. Beware of using a milliohm current-sensing resistor as phase reference -- they can have enough ESL to have large phase shift at TC frequencies.

[edit] Here's a question for experienced coilers. If frequency is fairly well known, and CT or other sensing element brings unwanted phase shift, how about a passive compensation network?  Ought to be very simple, if we are well below any resonant frequency of the sensor.
« Last Edit: February 03, 2021, 04:38:14 PM by klugesmith »

Offline davekni

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Re: Phase lead change caused by Current Transformer?
« Reply #2 on: February 03, 2021, 08:54:04 PM »
I suspect their real concern is CT output current being too high.  Phase shift wouldn't damage the controller, but excess current would.  Do you know how much current the feedback inputs are designed to handle?

If there is a phase shift concern, it is likely with CT winding capacitance and leakage inductance causing phase lag.  Doubt that is an issue at 300:1, but could be with 2000:1 single-stage CTs.
David Knierim

Offline GrantV

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Re: Phase lead change caused by Current Transformer?
« Reply #3 on: February 04, 2021, 09:33:57 AM »
I suspect their real concern is CT output current being too high.  Phase shift wouldn't damage the controller, but excess current would.  Do you know how much current the feedback inputs are designed to handle?

If there is a phase shift concern, it is likely with CT winding capacitance and leakage inductance causing phase lag.  Doubt that is an issue at 300:1, but could be with 2000:1 single-stage CTs.

Hi there. The manual (such that is is lol) shows a max of 900mA for the feedback circuit. As my IGBT's are unlikely to handle much more than 200-250Apk, that will keep my CT output below the 900mA specified and is why I figured it would be ok to use that specific CT.

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Re: Phase lead change caused by Current Transformer?
« Reply #3 on: February 04, 2021, 09:33:57 AM »

 


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