Phase Lead Questions

[Saattvik24]

I didn't have access to coilcraft inductors so I decided to make my own variable inductor using a ferrite transformer core from a PC SMPS and a 3D printed structure and screw to adjust the position of the ferrite core. The inductance is tunable from approximately 20uH to 100uH.


This is my current setup. I'm measuring the primary current using a Tektronix current probe rated for 100A.


The yellow waveform is the Collector-Emitter Voltage of one of the IGBTs and the blue waveform is the primary current. I didn't use an isolated voltage probe while taking the measurements as for testing I'm powering the inverter using a lab bench power supply set to 66VDC.

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In the video I go from min to max and again to min inductance but still the primary current doesn't get in phase with the collector-emitter voltage. Is it because my inductor is bad? The inductor wire leads are too long? Or because its not shielded?

[Unrealeous]

This may or may not be helpful but I thought i'd give you some feedback after doing a lot of phase lead tuning over the last few weeks - that at first didn't seem to go very well.

I bought cheap some inductors from aliexpress - https://www.aliexpress.com/item/1005005004587604.html  You get a stack of them.   They come with 50 turns standard, so I rewound some to do 25, 37, 50, 65, 75 etc..

I was working out expected inductance using this calculator:
https://www.66pacific.com/calculators/coil-inductance-calculator.aspx

Here's what I found.

1.  At 25 turns, the inductance is calculated at 8.6 uH.  At 75 turns its 77.6 uh.   This is similar to what you are expecting - and I was getting much bigger phase movement in scope traces using this range of inductance than what you are showing in your video.  (Although my system is running around 138Khz - which might explain it the variance).
2.  I couldn't get nice scopes (ie reduce the voltage spikes) until i redesigned my bridge and removed the long wires from my large capacitor banks.
3.  Most importantly, it was a bit of a pain for me to fit these inductors into the UD2.7c board.  They are oversize but you can make them fit on a UD2.7c board by cutting excess pins and bending the two that count. Not ideal but you get what you pay for.   To make the testing easier, I added about two x 2 inches of wire  with easy connectors so I could swap out inductors quickly for testing - but my scopes were terrible doing it this way.  Not sure if it was noise or the extra wire length was doing something bad.  The only good tests came when they were soldered in place.

Hope that helps.

Here's a picture of all the inductors....

[davekni]

I'm measuring the primary current using a Tektronix current probe rated for 100A.

That probe is rated for 100kHz bandwidth.  Delay within probe is likely as much or more than phase lead adjustment range.  Easier and more accurate to measure voltage across 51-ohm CT burden resistor on UD2.7 ECB.  (Or make another CT secondary core/winding with its own burden resistor.)

The yellow waveform is the Collector-Emitter Voltage of one of the IGBTs and the blue waveform is the primary current.

Video is a bit too fuzzy to decipher time/division setting on scope.  What is time/div (or what is coil operating frequency)?

Is it because my inductor is bad?

Inductor itself looks great.

The inductor wire leads are too long?

Leads appear to be twisted, so likely not a problem.

Or because its not shielded?

Yes, that may be a problem.  Depends on proximity to TC primary coil.  A large unshielded inductor makes a good pickup for magnetic field.  One easy way to tell if this is a problem:  While scoping phase as you did, change inductor orientation (while leaving inductance constant).  Will be most sensitive to orientation at maximum inductance (core inside coil).  If scope display changes (frequency and/or phase) as inductor is reoriented, shielding is needed.  (Or move inductor farther from TC primary.)

BTW, nice variable inductor implementation.  I've made DIY variable inductors, but using friction-fit, not with nice screw adjustment.

[Saattvik24]

That probe is rated for 100kHz bandwidth.  Delay within probe is likely as much or more than phase lead adjustment range.  Easier and more accurate to measure voltage across 51-ohm CT burden resistor on UD2.7 ECB.  (Or make another CT secondary core/winding with its own burden resistor.)

I got a differential probe and measured the voltage across the 51ohm resistor and got the following waveforms. I tried adjusting the phase lead but the voltage spikes didn't reduce that much. Note: The spikes and noise in the current waveform were due to some interference in the differential probe. They were not present when I was using the current probe.

Video is a bit too fuzzy to decipher time/division setting on scope.  What is time/div (or what is coil operating frequency)?

time/division is set to 5us. Primary Circuit resonant frequency is approximately 142kHz at the moment.

BTW, nice variable inductor implementation.  I've made DIY variable inductors, but using friction-fit, not with nice screw adjustment.

Thanks Still can be improved much more.

[davekni]

I got a differential probe and measured the voltage across the 51ohm resistor and got the following waveforms. I tried adjusting the phase lead but the voltage spikes didn't reduce that much. Note: The spikes and noise in the current waveform were due to some interference in the differential probe. They were not present when I was using the current probe.

Phase lead looks reasonable.  Spikes are likely not enough to cause problems.  I'd guess spikes are due to H-bridge inductance.  Even good H-bridge construction still has some inductance, from interconnect, IGBT internal leads, and capacitor inductance.
Current waveform spikes may be at least in part due to inter-winding capacitance of CT.  Probably not an issue.  UD2.7 has a small capacitor on CT feedback intended to filter out such spikes.
Current probe's limited bandwidth would not pass such high-frequency spikes.  No way to know if they would be there or not with a higher bandwidth current probe.