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Question about the power rating of a transformer

  1. Jul 12, 2017 #1
    In some aerospace applications you need high voltages in the range of kilovolts. This is commonly supplied by transformers. However, transformers because of their iron cores are heavy. But in aerospace being lightweight is a key requirement.

    In the thread "Question about power capacity of electrical motors", I noted that high current density wires could improve the power to weight ratio of electric motors.

    The same question pertains to transformers: could high current density wires improve the power level rating of transformers in relation to their weight?

    Bob Clark
  2. jcsd
  3. Jul 12, 2017 #2


    Staff: Mentor

    Aircraft AC power systems often use 400 Hz because that makes the transformers smaller and lighter.

    Frequency is another degree of freedom you can adjust.

    Also remember that higher voltages means lower currents for the same power. Therefore ideas that improve the conductivity of wires is more important at low voltages than high voltages.
  4. Jul 12, 2017 #3

    jim hardy

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    Well, sure . It takes less core to encircle small wires than large ones.

    Back to basics:
    By raising frequency you lessen the amount of iron you need to achieve given dΦ/dt,
    as well as the number of turns of wire you need to wrap around that iron to achieve desired voltage..
    One can combine those two effects to arrive at a practical design.
    As anorlunda observed 400 hz aircraft electrical dates back to at least WW2.
    Next logical step was high frequency Switch Mode Power Supply .
    Look at the toroid in your PC power supply - better part of a kilowatt through something smaller than a plum?
    I remember my amazement at first one i ever saw about 1973.

    old jim
    Last edited: Jul 12, 2017
  5. Jul 15, 2017 #4
    How is high frequency AC obtained from battery DC?

    Bob Clark
  6. Jul 15, 2017 #5
    At the heart of it is an oscillator.
  7. Jul 15, 2017 #6

    jim hardy

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    Are you asking about WW2 airplanes ? A DC motor runs an AC generator.
  8. Jul 15, 2017 #7
    I'm really asking in the context of electric powered airplanes, which are intended to run on batteries. No gasoline engine.

    Bob Clark
  9. Jul 15, 2017 #8
  10. Jul 16, 2017 #9
    As an example, here's a motor speed controller for drones and RC planes. No inductors at all. It just switches DC on/off to a three-phase Brush-less DC motor. Not sure if that tech scales up for larger motors or not (I think it does, I read a Tesla blog where they compared BLDC to their induction motor, and it wasn't any slam dunk, they both have pros/cons).

  11. Jul 17, 2017 #10
    Let me add some clarity. We need thousands of volts for a certain aerospace application that is battery powered. This would normally be done by transformers, to ramp up the battery voltage but transformers have heavy ferrite cores whose weight is precluded in this application. (It needs to be highly weight optimized.)
    According to Jim Hardy we can reduce the size of the core by using high frequency AC. Ideally the transformer would even be air-cored with no ferrite core at all.

    Bob Clark
  12. Jul 17, 2017 #11

    Thanks for that. The oscillator being electronic would be in keeping with our application since modern electronics are normally lightweight.

    Bob Clark
    Last edited: Jul 17, 2017
  13. Jul 17, 2017 #12


    Staff: Mentor

    You don't have to invent it yourself. DC to AC conversion is called an "inverter." Plenty of them are available commecially.

    Small ones, like this cigarette lighter inverter could be scavenged for just the parts needed in a light weight drone. The schematic is below.

    But why have AC at all? Sticking with DC requires zero size or weight for an inverter.
  14. Jul 17, 2017 #13

    jim hardy

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    Hmm a radiation detector of some sort ?
    Low current ?
    For a look at a practical battery powered step up converter get a disposable film camera and look at the flash unit with its bean sized toroid.. Be careful taking it apart though, the flash capacitor is usually charged to a couple hundred volts.

    Victoreen used to be a friendly company. i've been to their factory and they were practical folks.
    If your project is radiation measurement it might be worth giving them a look.

    old jim
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