Method for controlling AC voltage with current transformer?

Discussion in 'The Projects Forum' started by stef757, May 19, 2016.

  1. stef757

    Thread Starter New Member

    Apr 28, 2011
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    I have an application I am trying to reverse engineer, it involves a closed loop heating system of a nichrome type wire. The setup is as such:
    Transformer steps down 240/120ac to 12/24/30/36ac (secondary voltage depending on application)
    Secondary coil is connected to heating element
    A current transformer is coupled with one of the wires between the secondary and the heating element and is connected to the controller.
    A wire pair is connected to the terminals of the heating element (supposedly for reading the voltage) and connected to the controller.

    My understanding of how the control system works is the circuit is closed, the current is monitored through the current transformer. The heating element has a known relation of resistance to temperature, so by monitoring the current you can calculate the elements temperature. I am not sure what the exact function of the wire pair for monitoring the voltage.

    The big question I have has to do with the control of the current. It is my understanding that the controller can control the output going to the heating element more than just an on/off manner. This is for more precise control of the heating elements temperature. Is it possible to use the current transformer to choke the current driving the heating element?

    The current transformer is configured as a split-core current transformer, the power wire travelling through the core is either straight or wrapped 1-2 times depending on the magnitude of the current for the specific application.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,229
    No, the current transformer cannot directly control the current.
    It's purpose is to monitor the current only.

    Saturable reactors can be use to control AC current but they are large transformer type devices, and are largely obsolete with the advent of cheaper and smaller solid-state control devices.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,369
    What is the purpose of reverse engineering? To trouble shoot or reproduce?
    With resistive type heating loads it is generally more efficient and not so electrically noisy to use burst switching, rather than phase angle method.
    Just a note of safety on current transformers, they are typically used with some kind of load, if open circuit, extremely high voltages can exist.
    Max.
     
  4. stef757

    Thread Starter New Member

    Apr 28, 2011
    9
    0
    Hmm alright thanks for the input! Purpose is to reproduce.
    I was able to find out more info on the system. I found out that the controller is actually performing some type of switching on the primary feed to the transformer, which makes a lot more sense!

    Max, for burst switching, I'd imagine you can use some sort of a solid state relay to control the output duty cycle? I'm a (new) EE but I haven't done a lot with power electronics, but I sure love learning new things! I am not familiar with the specifics of either of these methods (phase angle or burst switching), could you kindly give me a comparison between the two? I'm guessing burst switching is a form of PWM, but I'd guess you have to time it to be in sync with the waveform so you don't start switching out of phase.

    I found this page on google and I'm guessing this is what you're talking about? Makes sense what youre saying about the noise, since you're not cutting the waveform mid cycle and not producing any sharp impulses.

    And yes, the current transformer will have to be in circuit with a resistor of known size so I can measure the current. I also realized that you would have to measure the voltage across the heating load in order to accurately measure the resistance, given that the voltage could either vary slightly due to variation or transmission losses. This also leads me to question how to take the load switching into account while measuring the real voltage across the resistor.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,518
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    Because of the long heating/cooling duty cycle in controlling a resistive/heating device, there is no need for fast control such as AC phase angle Triac control etc.
    For inductive devices that require rapid response of the control such as motors or light dimming etc, then this is more appropriate.
    Max.
     
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