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Transformer question; safety

Discussion in 'Electronic Basics' started by ChrisG, Apr 5, 2005.

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  1. ChrisG

    ChrisG Guest

    I'm trying to get a better intuitive understanding of transformers.

    One text I have explains that an increase of current in the secondary
    causes an increase in back emf (of the secondary) which acts to reduce
    the back emf of the primary, which increases the current in the primary.

    Since power supplies frequently fuse the primary side, it seems that
    the secondary side can cause excessive currents in the primary. But
    what is the limit to which the primary current can be increased? In
    other words, what happens when the secondary is shorted?

    I have seen comments regarding isolation transformers - that they
    limit the current that can be delivered, and thus are safer. If output
    power is close to input power where does the energy go? Heat?
    Impedence from the coils?

    I have also seen things saying that a small voltage secondary is less
    dangerous (than high voltage), but isn't that low voltage secondary
    capable of delivering high current, which is usually described as more
    dangerous than voltage?
     
  2. That is a good way to understand it. Another way
    is to solve a simple circuit containing at least two
    inductors with a high coupling coefficient. Just a
    voltage source across the primary and a resistor
    across the secondary is enough for that purpose.
    The fuse should blow for the reasons given in
    that text you have. The limit is affected by the
    circuit resistances or impedances in series with
    the primary and secondary.
    An isolation transformer is safer because it
    reduces the chances of a person completing
    a circuit carrying the 5-10 mA normally
    required for a dangerous shock (in healty
    people). It does not limit the current to a
    degree effecting safety.
    Voltage sufficient to electrocute with no more
    help that a person completing a circuit is usually
    considered more dangerous than current which
    requires a good conductor (not a person) to flow
    and whose main danger is a burn from the short
    created by that conductor.
     
  3. Jim Gregory

    Jim Gregory Guest

    Chris,
    The fuse is there to protect the integrity of the *supply* feeding the
    transformer and everything else - from non-behaving equipment connected to
    the mains distribution network.
    The fact that, in this case, it will rupture (blow) when a shorting or an
    excessive current fault affects the secondary, and therefore the primary, is
    beneficial to and noticed by the user when it halts further powering!
    It is not unusual to have another, anti-surge, fuse protecting a secondary.

    In this country, half-voltage temporary site or safety transformers handling
    about 2kVA deliver 115V at their isolated secondary. They often have a
    line-to-neutral leakage-sensing, fast-acting RCD resettable trip added.
    And I have seen versions with a balanced secondary, ie, centre-tap to
    ground/earth, giving only a 57-V potential if one line is accidentally
    touched.
    Jim
     
  4. Bob Eldred

    Bob Eldred Guest

    I never hear the term "back emf" applied to a transformer only rotating
    machines such as motors. Please explain what back emf is. And, if it is
    real, how do you measure it?

    Yes increasing the secondary current increases the primary current. The
    maximum short circuit current is dependant on the resistance of both
    windings. The secondary resistance is reflected back into the primary by the
    turns ratio squared. The total resistance "seen" by the primary voltage
    determines the primary current under shorted conditions. This current causes
    the resistance of the windings to heat and will usually destroy the
    transformer if on too long. Any transformer can take overloads for a short
    duration determined by heating.

    Some small transformers and motors are deliberately wound with a high
    resistance and high inductance primary winding. These are called "impedance
    protected" and cannot draw enough current to self destruct even when the
    secondary is shorted or rotor stalled in the case of a motor.

    Some transformers are protected by internal, non-replaceable fuses. These
    will blow keeping the transformer from overheating and causing a fire when
    over loaded or shorted. Many isolation and modern power transformers do
    this. The rule is that they must not blow the line circuit breaker when
    shorted but must "self destruct" without fire. Furthermore, the fuse must be
    internal so it cannot be tampered with.

    It takes voltage to "push" current through the resistance of a person. High
    current capability alone won't do it. That voltage is what makes "high
    voltage" dangerous not current.
    Bob
     
  5. ChrisG

    ChrisG Guest

    Wow, you guys are a great source of information.

    The actual term used was 'counter EMF' (this is from a HeathKit AC manual)
    which I believe another way of refering to inductive reactance.


    So with regard to safety, while it is true that a step down secondary will
    have
    a higher current capability, the voltage is lower and the 'load' resistance
    is unchanged
    ( the large 200 lb salty-water bag variety of resistor) so the current will
    actually be less
    thru 'the load', by Ohms law. Makes sense.

    Thanks all.
     
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