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

Transformer efficiency at low (and high) frequencies

  1. Dec 15, 2004 #1

    I've been thinking about why (elementary) transformer efficiency drops drastically at very low frequencies. I know hysteresis effects play a major role in reducing efficiency at high frequencies, but why low? I realise that as we reduce the frequency of the emf, we're making the circuit "more and more DC", but a freqency still exists, no? The transformer that I'm talking about is a simple two-solenoids-magnetically-linked-by-a-ferromagnetic-core single phase one (home-made). Any help would be greatly appreciated.
  2. jcsd
  3. Dec 15, 2004 #2
    >I realise that as we reduce the frequency of the emf, we're making the circuit "more and more DC"...

    You've basically answered your own question.
    The phenomenon of mutual induction (a.k.a. back EMF) between conductors in a transformer (or motor) winding is what limits the current. A very large utility distribution transformer will have 735 Kv on a primary winding of only a few ohms resistance. (0 ohms if they could make it). The equipment is designed to work efficiently on 60 Hz.
    Another example is an adjustable frequency motor drive. As the frequency is reduced, so is the voltage. Otherwise the current would be excessive and burn the motor out.
  4. Dec 16, 2004 #3
    What are the pros/cons of 60Hz and (what was it) 440Hz?
  5. Dec 16, 2004 #4
    Wow, that seems logical. Thank you so much. :biggrin:
  6. Dec 16, 2004 #5
    Almost all motors and power transformers use a core of laminated steel to conduct magnetism. The iron molecules line up according to the direction of the magnetic lines, which is decided by the electrical polarity. There's a slight hesitation of the molecules to follow the (constantly reversing) magnetism called hysteresis, which is energy loss. At 60 Hz the loss is small but increases with frequency.
    A 2 pole induction motor will spin at 3500 rpm at 60 Hz so 400 Hz is used to run special motors at higher speeds. For even higher frequencies into the MHz range, ferrite is used in place of iron.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook