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cancelling inductance on a long wire pair

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radiotech

Sun Aug 22 2010, 03:14AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"I am trying to measure the voltage and phase against the current."

Is this the E ,I and theta of the tank circuit only? If so how are you getting the tank current?

"It is hard for the software algorithm to know if a voltage value is to the right or left of 0 degrees " "I need to 90 capacitor phase shift"

Just hook a RC network between the voltage divider and the analog input to your processor when you know exactly what frequency the tank is tuned to.

How about posting the algorythm and top-down flow chart for your program?

klugesmith

Sun Aug 22 2010, 03:15AM

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

IamSmooth wrote ...

Radiotech, ... I need to 90 capacitor phase shift; it is the additional shift caused by the wire and resistors that is undesirable. It is hard for the software algorithm to know if a voltage value is to the right or left of 0 degrees. When it was 90 degrees, I knew to go up or down.

Sorry to butt in again. Are you trying to auto-tune for resonance as the load varies? Perhaps by seeking coincident phase between the inverter drive and the tank capacitor voltage or current?
Perhaps a current transformer or Rogowski coil would serve as a pickup.
The CT with resistive burden would give a voltage in phase with tank current.
The RC would give a voltage in phase with tank capacitor voltage (if sinusoidal).
But your original design and Radiotech's already do that, eh?
Mind if I ask at what range of tank voltage or current you operate?

IamSmooth

Sun Aug 22 2010, 03:39AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

tank voltage can be up to 600v rms
tank current is up to 2000A

I am comparing the capacitor voltage against the LC current. At resonance I should see the capacitor voltage shifted 90 degrees from the tank current. The problem I found was the phase of the measured capacitor voltage was not exactly 90 degrees off at resonance, and I could vary this small amount by changing the length of the probing wire or the resistance that I added to the wire. The resistance was added to protect the clamping diodes from the current.

My algorithm does work and deals with the small error. My question for this post was simply for a theoretical understanding of what is going on and how I could ideally get rid of this small amount of error.

klugesmith

Sun Aug 22 2010, 03:48AM

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

Sorry, I'm missing something.
To the extent that tank capacitor is an ideal capacitor, isn't its voltage
90 degrees away from the tank current, for -all- frequencies?

IamSmooth

Sun Aug 22 2010, 03:54AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

yes. the tank capacitor voltage is 90 from the capacitor current. I am comparing the capacitor voltage to the current of the entire LC system. Depending on Lx and Cx, the capacitor voltage can have a varying phase difference from the net RLC system current. I guess I didn't make clear which voltage and current I am comparing.

klugesmith

Sun Aug 22 2010, 04:30AM

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

Maybe we should take this to PM or email -- I was doing well at respecting a self-imposed posting limit, until today.

Is this your work? Very nice!

Are we talking about a tank circuit with work coil, capacitor, and a coupling transformer in series?
How is the tank current not the same everywhere in the loop, and 90 degrees out of phase with the voltage across the ideal capacitor? (actual capacitors and their interconnections include R and L which may be significant).
How do you sense the tank current?
Are you including the primary side of coupling transformer in the "overall system current"?
Perhaps our misunderstanding lies there.

IamSmooth

Sun Aug 22 2010, 05:02AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Klugesmith wrote ...

How do you sense the tank current?
Are you including the primary side of coupling transformer in the "overall system current"?
Perhaps our misunderstanding lies there.

The LC current is sensed through the coupling transformer. I guess if I had a resistor in series I could just measure the voltage across the resistor.

The LC current is the same everywhere in the loop, and the capacitor voltage lags the current by 90 if Lx and Cx are equal; otherwise, it is somewhere between 0 and 180. My problem is after a run of wire with 50k of resistance at the end, an error is introduced as the wire has some amount of L, R and C.

I'm done for the night. I do appreciate the help. Again, my algorithm takes this into consideration and it is not hard to track how close I am to the true resonant frequency.

Russell Haley

Mon Aug 30 2010, 11:19PM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

Because your twisted pair sense cable is less than half a wavelength, it behaves as a capacitor at the switching frequency. I suspect that the phase shift occurs because that capacitance has to be charged through a 56k resistor.

Insulate your cable for 1kV and move the resistor to the logic end of the cable. This way, the line capacitance is charged from the low impedance tank capacitor instead of through a 56k resistor.

See if that fixes it.

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