A question about the use and size of transformers?

Discussion in 'General Electronics Chat' started by Allenph, Jun 18, 2015.

  1. Allenph

    Thread Starter Member

    May 27, 2015
    76
    2
    I think I understand what a transformer is, but I have some questions.

    A transformer is two solenoids placed right next to each other so that the EMF field of one has an effect on the other when inputting AC. You can use different size solenoids to manipulate the output voltage, but I had some questions.

    If you plugged a transformer straight into the mains, it the solenoid you physically connected would be essentially the same thing as a short if I'm not mistaken. First of all, why doesn't this flip your breaker? Second, it seems that since the first solenoid is essentially a short, the opposing solenoid would be outputting a fraction or a multiple of the total short current through the first solenoid.

    Let me explain what I mean. Let's suppose you have a transformer. The main solenoid is physically connected to the American mains. Assuming there isn't a short (Which I think there would be.) the main solenoid would pull 20A regardless of turns.

    Assuming we have a ratio of 2 to 1, the output of the secondary coil would be 60V 60Hz AC at 20A.

    This doesn't seem particularly useful to me, so I know that I must be mistaken somewhere.

    In addition, everything I've read about talks about number of turns, not wire diameter, etc. So, how do you know what gauge of wire to use? This is really important because it dictates the size of transformer you have to use.

    One last question. Can transformers be used in series to produce different voltage nodes, or could you just add different nodes in the secondary solenoid to get multiple voltage nodes?

    Thanks in advanced.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    With the secondary open, the primary current is very small.

    This deals with what you are asking about.
     
  3. Allenph

    Thread Starter Member

    May 27, 2015
    76
    2
    Ahhh. That makes sense.

    Still doesn't explain the use of that. Will the circuit still just draw what it needs or will the full 20A just be forced on whatever is connected to the secondary solenoid?
     
    Last edited: Jun 18, 2015
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,511
    2,369
    Transformers operate with the same inrush as solenoids, very high until the inductive reactance comes into play, this is delayed also with the use of secondary rectification and capacitor smoothing until the capacitors charge.
    Very similar to a Induction motor where at turn on it represents a transformer with a shorted turn secondary.
    One of the reasons for solenoids to be DC type now rather than AC is that if the armature is prevented from moving over or someone operates the manual operator to force the spool over when under power it often causes a fuse blown or burn out of the coil.
    Max.
     
  5. Allenph

    Thread Starter Member

    May 27, 2015
    76
    2
    Ahh completely forgot about that principle.
    I actually didn't think about that. Each solenoid is in essence an inductor which resists change in current. I'm going to go out on a limb and run an unfounded theory...

    The primary solenoid is not a short because it resists changes in current...and an AC signal is constantly changing current direction proportionally.

    The secondary coil only draws what it needs from the main coil, because it is only altering the EMF of the primary solenoid in such a way to pull current.

    Kind of?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,511
    2,369
    A transformer primary current at switch on is limited to the actual resistance of the primary winding until inductive reactance limits the current, (in rush).
    The secondary current is reflected back to the primary, and level dependent on ratio.
    Max.
     
    Last edited: Jun 18, 2015
  7. WBahn

    Moderator

    Mar 31, 2012
    17,716
    4,788
    On what do you base this claim?

    This is at the heart of your difficulties. You seem to think that a circuit that is rated at 120V/20A will ALWAYS push 20A into whatever it is connected to.

    The battery in your car is capable of delivering hundreds of amps (many can deliver over 1000 A). If the only thing hooked up to a car battery were your cell phone charger would you expect a thousand amps of current to be forced into your charger? Why not?

    Now apply that reasoning to an AC outlet. What is that 20A rating really telling you?

    A transformer has a very high inductance. This inductance results in a very high impedance, which is the resistance to the flow of an AC current (somewhat simplistic, but that's the gist).

    So if your transformer primary has an inductance of 10 H, at 60 Hz it's impedance will be about 3.8 kΩ and the current in the unloaded primary would be about 30 mA.
     
Loading...