Inductive Kick from transformers

Discussion in 'General Electronics Chat' started by jubaitca, Jan 30, 2013.

  1. jubaitca

    Thread Starter New Member

    Jan 30, 2013
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    I have just familiarized with the notion of inductive kick from inductors due to switching out a path to ground.

    However, I am not entirely sure about the effect we would see from transformers because in effect its primarily an inductor. With respect to transformers with the following sequence of events:
    1) Voltage source connected to primary and load connected on the secondary (i.e. normal operation).
    2) Load on secondary is opened or removed (similar setup as an open circuit test for transformers).

    Would there be an inductive kick from the transformer causing a voltage spike on the secondary open circuit terminals even though there is some minimal current flowing through the primary terminals? If so, how large of a spike will it be relative to the existing secondary voltage? Will it continue to increase until an arc is formed across the secondary open circuit terminals? Is there a possibility of the core saturating?

    My guess is there will be a voltage spike due to the change in current, but I can't be sure since there is a path to ground for the current to flow from the inductor.

    Any help on this matter would be much appreciated.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Transformers will have only a small inductive kick on the secondary when it is suddenly opened, which is due to the small leakage inductance of the transformer, and the energy stored in that inductance due to the secondary current.

    It will not saturate the core since that is determined by the primary voltage and frequency (magnetizing current).

    If you suddenly open the primary then the primary inductance can cause a spike due to the magnetizing current being interrupted.
     
  3. jubaitca

    Thread Starter New Member

    Jan 30, 2013
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    Thanks for replying. I am trying to draw similarities to how fluorescent bulbs work using a ballast that contains a transformer and the starter switch. Would there be enough voltage from the leakage inductance to fire up the filaments in the tube?
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    Fluorescent tubes use the filaments to pre-heat the mercury vapor. The inductive kick when the starter contact opens provides enough voltage to establish ionization (conduction) through the mercury vapor. The simple setup for a starter type ballast system has the following..... Line, ballast, filament, starter, filament, neutral in that order. On initial power being applied, the starter is closed. Current flows through the ballast, filaments, starter in order to pre-heat the mercury vapor. After a time delay in the starter, it opens. The sudden interruption of current creates a high voltage spike from the inductance of the ballast.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I think the fluorescent ballast is an inductor, not a transformer. The ballast is to limit the current, not transform the voltage.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    I agree. The inductive reactance limits the arc current when the light is on. The same inductance provides the voltage spike that establishes the arc to begin with. Otherwise, a simple resistance could be used to limit current.
     
  7. jubaitca

    Thread Starter New Member

    Jan 30, 2013
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    Thanks, it makes sense now. Some of the older ballasts had transformers along with reactors. But, from what I hear, the transformer would have had little to do in causing the voltage spike.
     
  8. jubaitca

    Thread Starter New Member

    Jan 30, 2013
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    Sorry to bring this up again, but how would this scenario differ from a current transformer (CT)?

    I know for a fact that the instant the secondary of a CT is opened, the voltage will surge really high when the primary is connected to a current source. It is actually a safety precaution to not open the secondary of a CT while leaving the primary energized (or connected to a current source).

    Would it not be the same leakage inductance as in the Power Transformer (PT) that will cause the inductive kick? Or is there something more to the CT?

    I understand that the CT and PT are exactly the same except in the way they are operated. CT has a fixed current with a varying voltage while a PT has a fixed voltage with varying current.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    The reason it's different is the difference between a voltage transformer and a current transformer.

    A voltage transformer transforms voltage from the primary to the secondary with the current depending upon the secondary load. For an ideal transformer the output voltage does not vary for any change in output load current (resistance), instantaneous or otherwise.

    A current transformer transforms a current from the primary to the secondary with the output voltage depending upon the turns ratio and the secondary load resistance. For an ideal transformer, with no output resistance (infinity) the output voltage theoretically goes to a very high value equal to the voltage source on the primary times the current transformer turns ratio (which is typically something like 1:100). This high voltage can be enough to break down the transformer winding insulation. This is not a function of any transformer inductance.
     
  10. jubaitca

    Thread Starter New Member

    Jan 30, 2013
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    In other words, the voltage spike seen on the secondary of voltage transformers is not due to the same reason as that of current transformer.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Yes. Any voltage spikes from a voltage transformer are due to leakage inductance. High voltage from an open circuit current transformer secondary is inherent in its basic design and will occur even with an ideal transformer.
     
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