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What is an isolation transformer

  1. Aug 17, 2011 #1
    Does anyone know what is meant by electrical isolation?

    From this schematic:

    Swer.gif

    url=http://upload.wikimedia.org/wikipedia/en/1/12/Swer.gif

    I don't see the significance of calling the transformer an "isolation transformer". What is the difference between a transformer and an isolation transformer? What is the isolation transformer doing in the circuit? Why is the step-down a measly 3 kV?

    Also, just to make sure I understand the definition of electrical isolation, is this correct:

    If you have two separate circuits A and B, and take one (and only one) wire and connect them together, then that has no effect on the voltages and currents in each individual circuit. It establishes a common ground, but does not effect the circuits' behavior. So do two circuits that have a common ground but are not connected to each other at any other point, isolated circuits? Or does electrical isolation mean floating, in which case A and B are not floating with respect to each other when you connect a wire between them?
     
  2. jcsd
  3. Aug 17, 2011 #2

    vk6kro

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    A transformer is an isolation transformer if the windings have only magnetic coupling between them.

    However using this expression also implies that isolation is a function of the transformer.

    You could convert from 220 volt mains to 110 volts by having a center-tapped winding or you could have two distinct windings, one for 220 volts and one for 110 volts.

    The second one would be more expensive, but you could connect the secondary any way you wished without having to worry about the connections to the primary.

    In the center-tapped version, you could get the 220 volt mains active as part of the output if you connected it incorrectly.

    Isolation also implies that if there was a DC voltage on one winding, then another "isolated" winding would not have this DC voltage on it.
     
  4. Aug 17, 2011 #3
    What other coupling does a transformer have other than magnetic? I thought that was the definition of transformer, two windings that don't touch but share flux.

    That makes sense. So can the definition of an isolation transformer be a transformer that is not tapped, i.e., the transformer has only two leads coming out of it in the secondary? So for example the electricity coming out of the split-phase transformers that deliver power to residential areas are not isolation transformers (since it's center tapped at 120, 0, -120), but if you take one of the phases and the neutral, and connect them to the transformer that is at the end of the power cord for your computer, then this transformer is an isolation transformer, even though it also serves the purpose of stepping down the voltage?

    But surely the secondary is always affected by the primary, whether the secondary is center tapped or not?
     
  5. Aug 18, 2011 #4
    Isolation transformer is used only when magnetic coupling is needed between 2 circuits. It allows ac power to be transferred from one circuit to another without electrically connecting the 2 circuits.

    Also isolation transformers are used for impedance matching to get maximum power transfer between 2 circuits.

    Yes the secondary is always affected by the primary winding with magnetic flux from the primary winding which is the common effect in both configuration(center tapped or not).
     
  6. Aug 18, 2011 #5
    Not sure but i think that if a short circuit happens on the primary winding, the secondary will not be affected by the short circuit current, since in case of short circuit the primary voltage drops to zero thus it gives zero voltage on the secondary but since they are electrically decoupled short circuit current from the primary will not pass to the secondary and thus it ensures protection for the secondary circuit.
     
  7. Aug 18, 2011 #6
    Isolation transformer is to isolate the secondary from the primary. For example if you have a 110 to 110 isolation transformer with the primary reference to ground, if you stand on the ground and touch one side of the secondary, you would not get electricuted because there is no return path to ground and no current will flow.

    Secondary can also be floated to some bias voltage since there is no reference to ground. I designed various Mass spectrometer in the pass which involve different high voltage supply and we want to have different signal control that is floating on top of 10KV. My design was to bus hundreds of watts of power from ground level to 10KV lever so I can power up micro computer, filament power and more HV supply floating on top of the 10KV. The main way to transport the power up the deck is using a high isolation voltage DC to DC converter that convert 24V and ground level to 24V floating at 10KV. This involve a switching converter with a high voltage isolation transformer and I use opto coupler to feedback the voltage from the 10KV back to the ground to control the PWM in order to regulate the 24V. the isolation transformer is everything in this design. We even design and wind our own transformer to get the power and high speed switching capability.

    Isolation transformer is use for safety to break the path from ground.
     
  8. Aug 18, 2011 #7
    I think vk6kro has put it succinctly, but there is more in this than meets the eye so some amplification is in order.

    The isolation is not only a function of the construction of the transformer it is also a function of the associated circuitry as well. It is not just a question of whether the windings are isolated from each other.

    Another definition might read two circuits are electrically isolated if there is no electrical connection (wiring, metalwork etc) between them.

    An auto transformer fails the above since the primary circuit is electrically connected to the secondary circuit via the common winding.

    (@redx this is what was meant about 'tappings' when the primary and secondary are tappings on one and the same coil)

    In an old fashioned amplifier with transformer coupling the speaker circuit is isolated from the amplifier circuit by the output transformer.

    In an old fashioned superheterodyne radio the IF transformers do not isolate the sections of the superhet as both are fed from the same power supply, although the windings in the IF transformers are isolated from each other.

    The internal power transformer stepping the mains down (or up?) within electrical apparatus is usually an isolation transformer, as others have described here.
    However lowest cost switching supplies sometimes use autotransformer techniques and then do not provide isolation.

    Wrapping a few turns of pickup wire round the outside of the HT or spark leads in an auto engine to provide an isolated signal for an oscilloscope constitutes an isolation transformer.

    Isolation transformers are also sometimes used to avoid hum or other pickup in audio work with microphones.

    Finally there are 'degrees' of isolation. It is possible to put numbers to the issue.
    National codes specify the voltage difference necessary to break down the isolation between two circuits.
    This is often noted in opto-isolators.

    go well
     
  9. Aug 18, 2011 #8
    There are lots of electrical noises on power lines. Corona, lightning, motors starting and turning off........
    Most transformers have capacitive coupling between the primary and secondary. This capacitive coupling puts all this noise on the transformer secondary.
    A "isolation transformer" has a shields between primary and secondary that reduces this capacitive coupling.

    The meaning of isolation transformer could also be interpreted to mean any two winding transformer where the windings are not connected together.

    You pick the definition.

    Dictionary of Electronics:
    ISOLATION TRANSFORMER-A transformer designed to provide magnetic coupling (flux coupling) between one or more pairs of isolated circuits, without introducing significant conductive (ohmic) or significant electrostatic (capacitive) coupling.
     
    Last edited by a moderator: Aug 19, 2011
  10. Aug 18, 2011 #9
    Studiot's post points out how isolation is such an important consideration in all aspects of electricity and electronics. Seems I can never learn enough about it. There are other methods of isolating currents other than transformers, but that's probably most common. It becomes a whole science for electronics on the PCB level.
     
  11. Aug 19, 2011 #10
    In this case, I would say the isolation transformer enables to use a floating voltage on the primary side, such as one produced between 2 phases of a delta transformer.
     
  12. Aug 20, 2011 #11
    When lightning strikes near power lines, it breaks down the air which shorts the power lines, but why would this have an effect on the load? The short and the load are in parallel, so the load should not be affected. When you have two things in parallel what happens to one branch should not really affect what happens to the other, assuming that the resistance elsewhere is negligible. Or does the lightning strike induce an electric field through the magnetic field created by the lightning flow, and that emf propagates between transformers and to the load?

    Also, wouldn't motors turning on and off be on the secondary side and not the primary side?
     
  13. Aug 20, 2011 #12
    I always thought it'd be dangerous to ground the primary side since it's at such high voltage. So maybe isolation transformers allow you to ground the secondary side, rather than, as you say, allowing the primary to float when you have a grounded secondary?
     
  14. Aug 20, 2011 #13
    Lightning can be viewed as a source of current with a very sharp front (in the order of kA and microseconds), so it does not necessarily short the line. There are different possible scenarios. The lightning can hit the line or it can hit the ground (or shield wire). Both scenarios could affect the load (depending on the location of the lightning), but the surge arrester will conduct to avoid the overvoltage.

    22kV is not really a high-voltage. Plus the voltage is also that "high" on the secondary side and it is grounded... I would doubt that the primary side is connected as a floating Y, since that would mean that if there's an unbalance in the load (which is always the case in distribution), the floating neutral potential will move (resulting in smaller voltage on phase A, for instance, and higher voltages on phases B and C). This means the primary is delta connected and the secondary is Yn. Grounding or not (through impedance or solidly grounded) of different parts of the system depends on where you live. There are arguments for both. If the short-circuit currents are too high, you might ground through an impedance. On the other hand, that would mean that a low short-circuit might pass undetected by the protection, set for higher currents.

    M.
     
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