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Efficiency of Transformer, increasing output current

Discussion in 'Electronic Basics' started by [email protected], Apr 4, 2005.

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  1. Guest

    In a standard (low voltage eg 40V)transformer - same number of input
    turns as output turns, should the effiency go up with increasing the
    output current or down?
  2. PeteS

    PeteS Guest

    The efficiency will eventually go down for a number of reasons.

    1. Copper losses will increase due to P = I^2R. Increase the current
    and you increase the losses in the windings (notably by a square

    2. The input and output windings will heat up, thereby increasing their
    resistance (copper has a positive temperature coefficient). With a
    higher resistance, you exacerbate the problem in (1)

    3. Core and eddy current losses in the magnetic core will increase.
    Increased output current (which implies increased input current) will
    increase the magnetic flux density. As you increase it, the losses in
    the core will increase, up to magnetic saturation, where you can
    effectively get no more current (the maximum energy through a
    transformer is limited by the magnetics as well as the winding limits).

    That's a simple overview - there's a lot more to it, but those are the


  3. Agree with 1 and 2.
    It is not true that magnetizing losses increase with
    output current. They actually go down a little.
    This is because the increased IR drop in the primary
    reduces the amount of flux change necessary in the
    core to provide enough induced voltage to equal the
    applied primary voltage adjusted by the IR drop.

    For magnetization, you can think of the primary as a
    more or less pure inductor [1] in series with the primary
    resistance and in parallel with some real impedance
    representing the eddy current, hysteresis, (and radiated)
    losses. As the voltage across that inductor drops, so do
    its losses.

    [1. The inductor is usually non-linear, but its inductance
    is a monotonic function of current.]


  4. Also agree with 1 and 2. Item 1 is very real and makes a very tangible and
    signicant impact and should not be ignored. The influence of item 2 however
    is usually quite small for a normal transformer operated with a normal
    temperature range. Since most loss and thermal rise calculations are
    somewhat approximate anyway, item #2 can often be ignored.

    I suppose that is one way to think about it. I think of it a little
    differently. Ampere's Law would have you believe the magnetic flux density
    B is proportional to the number of turns times the current flowing through
    those turns in any given magnetic device. Since a transformer is a magnetic
    device, it would seem logical that as the output current increases the
    magnetic flux density in the core also increases. This would suggest at
    some current level the transformer's core would saturate.

    This is not the case however for a regular transformer (IE: one not a
    flyback transformer, they are different). What one must realize is that a
    transformer has two or more independent windings on a single core. Ampere's
    Law applies to the primary winding, and it also applies for the secondary.
    As the load current on the secondary increases you would tend to get more
    flux generated in the core. However, as the primary current increases to
    supply that secondary current, the primary winding also generates it's own
    flux. If you studiously apply the right hand rule, you will find that the
    flux will be in different directions for primary and secondary, and so they
    both serve to cancel each other out. As a consequence the flux density in
    the core is essentially independent of the load current of the transformer.
    In effect the maximum power output rating of a transformer is limited by the
    winding resistances, or by total thermal dissipation limits resulting in a
    given temperature rise. Practical transformers are thermal dissipation
    limited long before winding resistance limited. In theory a transformer
    made with superconducting windings could be made very small and output an
    outrageously huge amount of power (efficiently at that).

    Flyback transformers are different, and have properties more like inductors
    than do regular transformers. In the flyback transformer current does not
    normally flow simultaneously through primary and secondary windings. At any
    given time only one of them conducts. As a consequence you don't get the
    flux canceling effect mentioned above for regular transformers. If you keep
    increasing the load on a flyback transformer it will eventually saturate the

    As for the OP's original question, does the efficiency improve with
    increasing or decreasing load current... Obviously at zero output current
    the efficiency is zero since any transformer will waste some idle power
    primarily due to core hysteresis and eddy current loss. As you apply a
    heavier and heavier load the efficiency continues to improve, up to a point.
    At some point the I^2*R loss effect will start to dominate and thus the
    efficiency will start to decrease again. In practice, where this peak of
    efficiency occurs depends on the design of the transformer. Typical
    transformers will often be designed to have maximum efficiency somewhat near
    (though often a little below) their maximum rated continuous output current.
    The efficiency peak is relatively broad.
  5. Bob Eldred

    Bob Eldred Guest

    All transformers consume magnetizing current to energize the core. Some of
    this is reactive while some is resistive. The resistive portion involve
    copper and core losses, mostly core when the current is low. These losses
    occur whether there is a load on the transformer or not. If there is no load
    on the transformer, no output, there is still magnetizing loses so the
    efficiency is zero. Pout/Pin = 0/(small number) = 0. As you increase the
    load, the output becomes higher and the copper losses also become higher but
    the efficiency increases because there is now an output. The efficiency
    continues to increase with load until it reaches some maximum where the
    transformer is most efficient. Once that point is reached, the efficiency
    will decrease with increasing load because heating will exacerbate copper
    losses (higher resistance with temperature). Any transformer can give
    several times it rated output current for short durations. Heat is what
    limits it. The flux density in the core and the related magnetic losses are
    NOT a function of load. They are a function of the primary voltage and not
    load, by faraday's law. The secondary load current balances and is in the
    opposite direction of the primary current. The flux density stays the same.
    When the primary goes positive, current flows into the primary, by
    convention. The same polarity winding on the secondary also goes positive
    but the current flows out of that winding. The two currents times their
    number of turns (amp-turns) balance, one in the other out. This is reversed
    every half cycle. That balance means that there is no net flux density
    caused by the load.

    To sum it up, the efficiency is maximum when the load is at some rated value
    usually at or near the nameplate values and is zero with no load and becomes
    low again when the transformer is smoking.
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