Maintaining Constant RMS Current in an AC L-R Circuit

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Discussion Overview

The discussion revolves around maintaining a constant RMS current in an AC L-R circuit, specifically for a device that requires a stable magnetic field strength while varying the frequency of the AC current. Participants explore potential solutions, including the use of capacitors and tank circuits, while addressing the challenges posed by high frequencies and component specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses the need for a method to maintain a constant RMS current through a solenoid while varying the frequency of the AC current from 1 to several thousand Hz.
  • Another participant questions the feasibility of using a current mirror in an AC context.
  • A participant proposes linking multiple L-C tank circuits with different resonances to manage the current, while expressing concern about the durability of transistors at high AC voltages.
  • There is a suggestion to size a capacitor to provide the required current over a wide frequency range, with the possibility of shunting excess current to avoid exceeding peak levels.
  • One participant requests specific details about the current requirements, frequency range, and coil impedance to provide more targeted advice.
  • Another participant clarifies that the current will not exceed 2 Amps, with a frequency range of 1 to 5000 Hz, and provides details about the coil's inductance and resistance.

Areas of Agreement / Disagreement

Participants are exploring various approaches to the problem, but there is no consensus on a specific solution or method. Multiple competing ideas are presented, and the discussion remains unresolved.

Contextual Notes

Participants mention the need for dynamic or discrete capacitance adjustments and the potential limitations of components under high voltage conditions. There are also uncertainties regarding the exact specifications of the coil and the implications of varying frequencies on performance.

enigma
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I'm working with a device which is basically an AC L-R circuit.

The device I have hooked up to the voltage supply which I was informed was a 'current regulator' is only maintaining a constant voltage. This results in a trickle of current to flow through at high frequencies.

I need to find a way to keep a constant RMS current running through the solenoid (I need a relatively constant P-P magnetic field strength) while the frequency of the AC current varies from one to several thousand Hz. Discrete increments may be OK, but a dynamically varying capacitance would be optimal.

I know they have these devices, because the power companies have capacitor banks attached to power poles. What I don't know is what they are actually called, whether they are dynamic or discrete, how much they cost, and if it's easier to buy them or make one.

Any information would be helpful.

-thanks
 
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Hmm.. I find myself wondering if a current mirror can operate in AC... let me think about this.

- Warren
 
My preliminary thoughts are to somehow link up a bunch of L-C tank circuits with different resonances hooked up to (in sequence) inverters, an RC with low resistances, and then transistors which would turn on and off capacitors for the main LRC.

EDIT: I am concerned with this one that the transistors will get cooked if operated in 100V AC

Another option might be to simply size a capacitor to provide at least the required current at the voltage limit over a wide enough operating band (with the very low frequencies clipped, obviously). Then somehow shunt the current if it exceeds the peak level. This won't result in a clean sine wave, but I think I can deal with that since the material we're testing will be reaching saturation levels before the field hits full strength anyway.
 
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*bump*

Anyone have any comments or ideas here?
 
Can you give me some specifics to work with? How much current do you need? What frequency? How much impedance will the coil have at its worst frequency?

- Warren
 
The current will not be more than 2 Amps, and will most likely be quite a bit less.
Frequency will range between 1 and 5000Hz
The current coil's inductance is in the 10 milliHenry range, although that may change a bit (no more than by 10^(+-1)).
The resistance of the coil is ~9 ohms, but again that may change if we wind a new one.
 
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