isolation transformers constant power usage?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Mar 11, 2015.

  1. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    So you have a primary and secondary say a 1:1 ratio. It runs at 120vac.
    Imagine it is a 30 amp in and 30 amp out.

    To get power out the secondary, you have to energize the primary.
    When you turn on the primary how many amps are used? Is it using full 30 amps all the time flowing on the primary windings?

    If you want power from the secondary, do you have to turn on some kind of primary switch, then turn on the secondaries switch to a device?

    How do people use these things in a practical way if they constantly use power?
  2. Ike
    Joined: Apr 2006
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    Ike Senior Member

    They don't constantly use power. They only use power (other than some parasitic loads that are in the milliamps). Power is only used if there is a functioning load, in other words if you turn something on, like an appliance, or other electrical device. If you have a battery charger running then it uses power to charge the batteries.

    The amount of power use depends on the power needs of the device. If it only needs 1 amp to operate then you use 1 amp times 110v = 110 watts. If it needs 15 amps then it would be using 1650 watts. Most device have a range of power needs and the amount of watts listed on the label is usually the max used. For example a radio is using very little power in receiving mode, but max power in transmit mode. A refrigerator is drawing very little power between cycles but when cycling uses it;s max power.

    Yes you have to turn on the secondary, but it is not just a switch. It is usually a circuit breaker. Of course by it's very nature a circuit breaker has to be on for anything downline to function.

    Most equipment has an on/off switch/ some don't

    The point is if the circuit breakers (usually there are more than one) are off no power is being used. AC systems are usually divided up in circuit branches that power just a few things, or maybe one appliance that needs a lot of power. Circuit breakers are sized so that if the amperage exceeds a certain amount, say 15 amps, 20 amps, 30 amps depending on the circuit needs, then the breaker trips and shuts off the circuit. On some complex circuits there is a main breaker on the secondary that cuts off all power. Normally the main breaker is on. You only switch it off if you want to kill all power.

    An important thing to remember here is the Circuit breaker is there to protect the wire from the power source to the equipment, not the equipment. That is why some equipment has it's own fuse or circuit breaker built in or installed next to the device.
  3. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    There are losses in transformers... They are hysteresis, magnetic fringing, some heat, and also eddy currents in the cores although minimised by the laminations.

    Hysteresis loss is minimised by the type of material used in the core, expensive transformer core material outperforms the the cheaper options.

    So when you energise the primary, you won't use much current unless there is a load connected the to secondary. The load determines the current, the magnetic effect through the core will draw a similar current through the primary, although it will be slightly higher due to the losses mentioned above.

    I mentioned isolation transformers in the other thread because it highlights the importance of earthing. If you touch an active wire from the secondary isolation transformer winding which has no earth reference, you will not get electrocuted. This is why they are used, for safety, generally bench testing equipment etc. current will not flow unless your body forms part of an electrical circuit which goes back to the source, not between active and the planet. The circuit is completed through the planet, only if the source is referenced to it (earth neutral bonding) and then some current can flow through the ground, back to the nearest earth stake, then through the earth neutral link, and finally back to the transformer winding.

    You can't have everything in isolation however, if faults develop and in reality they do, then parallel paths exist back to the source through the faulty equipment, so a person can still find themselves in an earthed situation in reference to raised potential. This is why the earthing system was put in place as a standard. It serves as a prevention rather than a cure if you like. It's better to have a very low impedance path for fault currents to flow, where they are easily detected by circuit protection devices, the moment a fault occurs. This is far more preferable than having a fault liven up a metallic part and having it sit there at raised potential , just waiting for someone to touch it... Then after the fact, trying to save someone via a Gfci or similar once they have already become part of a circuit and hope the device works as designed...

  4. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I did find out you can make you own cheap isolation transformer from two microwave oven transformers joined back to back, with the high voltage connections joined together. Or you can modify one like this.

    Powerwise wont be a lot, maybe at most 15 amps pushing it!, which would be ok for a mid sized inverter. But I wonder if could be used with an MSW inverter so that you can isolate them and have a bonded neutral on the output. I suppose you could parallel several together to get more power capacity, just for fun of course...
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