Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Variable Frequency Transformers

  1. May 30, 2012 #1
    How are described in the Industry those transformers able to operate with a variable frequency?
    These transformers will be used downstream of a Frequency Drive.
    Anyone knows?

    Any specific characteristics?

  2. jcsd
  3. May 30, 2012 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    Do you mean broadband transformers? Like the ones used in communication circuits?
  4. May 30, 2012 #3
    No I am not talking about communication. I am talking about Power Transformers, used to step up and step dowm voltage, with the special feature that the frequency (usually 50 or 60 Hz) will be variable within a range.
  5. May 30, 2012 #4


    User Avatar
    Gold Member

    Please be a little more specific and I think someone can answer your question.......
    like explain exactly what your source and frequency are into the primary side of the transformer....and what is the load on the secondary side. What type of variable frequency? 50 or 60 hz.....or are you talking 5,000?

    I'm not sure I get your question?
    Last edited: May 30, 2012
  6. May 30, 2012 #5


    User Avatar
    Gold Member

    Well, you ask how is it able to operate? Or how is the drive able to calculate the desired output frequency...

    I mean, the transformer will give an output for the range of frequencies, they will not all yield the same characteristics as the nameplate rated frequency obviously, but it will still operate at multiple frequencies*

    As for the desired transformer frequency, you could use a great deal of variables as a feedback and with some sort of reference automatically output a new frequency for a new loading scenario

    However, I've never seen a xfrmr operated from a drive? I guess I have yet to see the actual NEED for it, transformers aren't cheap, and for a variable frequency application I can't see as to why we couldn't use some cheaper technology.
    Last edited: May 30, 2012
  7. May 30, 2012 #6


    User Avatar
    Gold Member

    In general....if a VFD has a transformer....it's part of the drive and only gets the normal 50 or 60 hz from the factory.

    Voltage source to switchgear....then to transformer....then to full wave rectifier......then to inverter (variable frequency created)......then to load.
  8. May 30, 2012 #7

    Let me try to be more especific.

    I am working in specifications for a Power Transformer (5 MVA) to feed electrical motors SUBSEA, for different applications.
    Due to the voltage losses, the idea (already in the market) to elevate voltages comes along with the motors rated voltage.
    But the speed of the motors needs to be variable, then the VFD comes to play.
    The idea is to feed more than one motor with the same drive, but stepping up the voltage topside, also changing the frequency.
    So the Transfomer will face an AC waveform with variable frequency, not fixed to 0 or 60 Hz.
    What is the name of those kind of Transformers?
  9. May 30, 2012 #8


    User Avatar
    Gold Member

    i've never heard of that particular application before, but you're using it as a sort of boost transformer.

    but i mean, if you're controlling motors, why can't you just change the frequency with the drive? and if you need a higher** voltage for the motors, why doesn't the transformer just feed that whole system? I don't see why still you would need the drive powering a transformer, than a transformer powering the motors. I think you could achieve what you wanted with just the drive powering the motors couldn't you
  10. May 30, 2012 #9
    FOIWATER that is the key. we will feed the whole system with the transformer, downstream of the drive.
    The Million Dollar question is how do you name this kind of variable frequency transformers?
    Not all the manufacturers are able to make these units.
    Just Bust up Transformers for a range of variable frequency?
  11. May 30, 2012 #10


    User Avatar
    Gold Member

    I can't help but agree with this statement.

    Also....as soon as you start drastically changing frequencies....you are messing with the properties of the transformer....with is basically a couple inductor coils....which have the impedance of JωL.......adjust that ω back an forth and I can't see your system working correct.

    Are you stepping down the voltage right near your motors? I assume you are since you want to avoid voltage drop.

    Also, why cant you run high voltage to a VFD drive transformer set up near your motors?
  12. May 30, 2012 #11


    User Avatar
    Gold Member

    any transformer will work on multiple frequencies though??

    It doesn't take a 'certain type' of transformer to accomplish this?

    A change in frequency leads to a change in primary inductive reactance which leads a new current being drawn into the primary either more or less current depending on an increase or decrease in frequency respectively.

    So here I would like to say, since less current is drawn, the field becomes weaker to induce voltage into the secondary.

    But the equation Ns/Np = Vs/Vp almost makes it look like the output voltage is not frequency dependent, which kind of contradicts Ip/Is = Vs/Vp ... haha

    I have to let someone else answer you ! I need a transformer lesson big time
    Last edited: May 30, 2012
  13. May 30, 2012 #12

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    There's no frequency term in inductance.

    In principle if you build a transformer with sufficient inductance it'll work at any frequency.

    What you run into is practical limits of the iron core at low and high frequencies.

    First - inductance is flux linkages per ampere. So a lot of inductance implies a lot of flux.
    Iron can only carry in the range of 1 or two webers per square meter
    so a highly inductive transformer implies a physically large core.

    Fortunately, there is a frequency term in inductive reactance
    ohms = 2pi X freq X inductance
    so at reasonably high frequency a practical transformer needs only modest inductance hence a reasonable sized core.
    In early days of AC power they used 20 hz but soon realized 60 hz allowed them to use smaller iron cores.

    At higher frequencies you run into the next problem - heating inside the iron core.
    The magnetic flux induces current in the iron core which heats it by simple ohmic loss.
    To combat that they make the cores out of thin sheets, laminations, to break up the induced current into long flat loops. That makes the core current take a longer path, through more of the iron which lowers its amplitude hence less heating.
    The higher the frequency the thinner the laminations need to be. That's because the induced currents want to flow in circles , so they make the laminations thinner than the diameter of the circle. Higher frequency = smaller circle so thinner lamination.

    I have seen power transformers that are rated from 50 to 440 hertz operation.
    They have thinner laminations than plain 60 hz transformers, so they can go to clear up to 440 hz.
    They have thicker cores, ie more of those thin iron laminations, than would be needed for 440 hz operation . That's so they'll have enough inductance to go clear down to 50 hz.

    that's the very basic fundamental idea.

    i think what confuses people about transformers is the idea of flux
    and voltage being derivative of flux.
    Further we are introduced to them as if all they ever handle is sinewaves .
    In reality the sinewave is a special case.
    Transformers should be taught as inductors in close proximity.

    It is a useful thought experiment to pass a triangle wave current through a itransformer primary , instead of a sinewave current, and observe voltage induced in secondary.

    Read up on volt-second integral and volts-per-hertz protection , it'll help you work the flux in your head.

    old jim

  14. Jun 7, 2012 #13
    So far I have found these specials transformers are used almost on Off Shore Aplications, Siemens, GE and Scheneider have some experience already.
    They are using a Topside Transfomer (Boost up) and a Subsea Transformer (Boost down)
    Drives still are Topside.
    See atatched picture of one unit (Topside)

    According to some manufacturers the size of these "special" transformers is 10 to 15% bigger than standard ones.

    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook