Magnetic Ampliers used for Variable Inductance for Audio Tuning? How to stay linear?

Discussion in 'The Projects Forum' started by Jason Moore, Mar 10, 2015.

  1. Jason Moore

    Thread Starter New Member

    Feb 12, 2015
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    I am trying to find an electrical way to vary inductance. Ideally I would like to be able to change that inductance from 5-12mH. Basically I would like to replace the inductor that I have in my circuit with a variable one that can change based upon feedback from the load. Purpose of the inductor is to match resonantly to the load. My load is capacitive with resistance. My source voltage comes out of my transformer at 300-400Vpeak between about 10kHz to 40 kHz. I thought a MagAmp might be a good candidate for the job but still working to fully understand its operation. Seems like I would have a tough time keeping my output linear. Any insight for solving this problem or ideas for another plan of attack?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    A synthetic opamp-like inductor might be the way to go, although 300-400Vp is quite a large range, but you might be able to overcome that using an output transformer.
    Can you tell us more about the requirements? Can the inductance change be made in steps or does it have to be continuous? Could you replace the setup with a fixed inductor and a varying capcitor? Are you sure you need 400V peak aka 800V peak-peak?
     
  3. tcmtech

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    Nov 4, 2013
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    So what are you listening to in the 10 - 40 KHz range that needs a 300- 400 volt signal? o_O
     
  4. kubeek

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    My guess is the goal is something similar to plotting a high power piezo transducer, but lets see what the OP comes up with.
     
  5. Papabravo

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    Feb 24, 2006
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  6. Jason Moore

    Thread Starter New Member

    Feb 12, 2015
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    Kubeek is right. Application is for high powered transducers. I am playing at discrete frequencies in the range of 10-40kHz. For each discrete frequency, I need a new matching inductor. Most, but not all of matching could be accomplished with a 5mH base which could vary up to 12mH. When the system is played loudly, it is possible to go in excess of 400Vpeak, but most of the time signals are played at 300-400Vpeak. The current running through the capacitive load with resistance would be about 1 to 1.5Amps peak. I had at first considered considered a scheme using "Artificial Voltage-Controlled Capacitance and Inductance using Voltage-Controlled Transductance" ~Abbadi & Jaradat But I would have to supply the DC rail voltages at the levels I am transmitting at. The roller inductor is something I considered (not completely of the table), but I am trying to stay away from motorized options if I can. Similar to that I saw a patent for a transformer like device that used an actuator to vary the air core distance. With the MagAmp/Transductor I was hoping to accomplish this same task, but with no moving parts. Part of my effort is to make things as small as possible and reliable as possible. Low power consumption is a secondary goal. The signal played out typically comes out in short bursts <100ms. I currently have, in one system, a scheme which adds in inductance via relay switches in small intervals. I am trying to find a way to improve upon that. I would like the system to have continuous feedback and inductance change in the hope that I will be able then able to generate swept signals and maintain a consistent output level. A combination of the two, with a continuous variable inductor for fine tuning would probably be acceptable. I have seen B-H curves showing the modes of operation for MagAmps transitioning from a linear region, over the knee and into saturation. Causing a smaller change in flux, lowers the impedance and raises the current allowed to pass. I am not sure how the output changes if you have a small change in flux relative to your potential flux density of the core and vary your DC Bias such that you do not drive the output into saturation. Would the output be affected and vary linearly with the DC Bias? Minimizing any signal distortion is paramount. I suppose it would be possible to design for the max inductance and then have a varying capacitor but I thought that would just be generating an excess of reactant power that I didn't need to.. Plus I don't know of a large varactor type device that could handle the voltage.
     
  7. kubeek

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    Sep 20, 2005
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    So is this a lab instrument or something that needs to be portable, or even going to be marketed?
     
  8. Jason Moore

    Thread Starter New Member

    Feb 12, 2015
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    Portable. Got 70 lbs of electronics in a case that two men have to carry that was designed in the 1970s. Trying to shrink that footprint down at least by half.
     
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