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Thread: Transformer voltage drop

  1. #1
    Join Date
    Feb 2005
    Wilmington, NC

    Transformer voltage drop

    Without going into too many details, does a transformer maintain voltage as it hits 100% loading? How about when it gets to 130% or 150% loading?

    This is a 1-phase 167-KVA transformer, two 800-amp services are planned to be connected to it (that IMO are undersized according to the NEC, but that's another discussion).
    Lou (wannabe economist)

    If you relentlessly pursue perfection, you will eventually catch excellence.

  2. #2
    Join Date
    May 2003
    Leesburg, VA
    I am assuming this is a 120/240 VAC single phase service.
    Is this transformer a utility transformer?
    Are the 800 amp services for a residentrial property?

    Transformers can operate at 100%, assuming the criteria for cooling is met, in my experience this is seldom the case in electric closets. Transformers with supplemental cooling can (per some manufactures) exceed their nameplate rating, for short durations. But excessive heating (for any reason) will shorten the projected life expectancy.

    Transformers have a difference in output voltage from no load to full load, (voltage regulation). IE. the no load output voltage may be 120 VAC (phase to ungrounded conductor) full load 116 VAC. This voltage level will continue to fall as you load the transformer, but (as explained to me no technical reference to back this up) the transformer will fail due to thermal issues, prior to the magnetic field collapsing completely (though I find this hard to comprehend).

    One consideration with the voltage regulation is if the priimary voltage dips and you are operating close to 100% rating what will the full load output voltage be?

    I was trying to locate the information on how excessive loading as percentage effects transformers voltage regulation but have not been able to locate a source as there are several factors that effect this voltage regulation, winding material, core type, transformer impedance.

    If you really want to know I have a few 30 kva transformers and resistive load banks I could sacrifice for the cause.
    Last edited by brian john; 02-16-07 at 08:51 AM.
    Brian John
    Leesburg, VA

  3. #3
    Join Date
    Apr 2006
    Portland, OR
    No, the transformer will _not_ maintain constant output voltage.

    As the output load current increases, the output voltage decreases. This decrease in output voltage is what limits the short circuit current of a given transformer; under short circuit conditions enough current flows to reduce the output voltage to near zero.

    Transformers have an 'impedance rating', you probably see this number used to calculate the available short circuit current. As I understand the impedance rating, it is measured by short circuiting the secondary, and then adjusting the primary voltage until normal full load current flows. The percentage the test primary voltage to the normal primary voltage is the impedance rating.

    For example: a 2.4KV to 480/277Y 75KVA transformer with 3% impedance. This transformer has a nominal full load secondary current of 90A. If the secondary is short circuited and 72V is applied to the primary, then 90A will flow in the secondary.

    Under the assumption that the transformer is a linear device, the above information is used to approximate the following: with the full 2.4KV on the primary, in the event of a short circuit, the secondary current will be 90/0.03 = 3000A

    Under the same assumption, this means that we expect the transformer output voltage to drop by about 3% as the output load goes from 0% to 100%.

    Now, transformers are not really linear devices; and there are transformers which will _regulate_ their output voltage. Large power distribution transformers include 'tap changers' which adjust the turns ratio of the transformer as the load changes, so that the output voltage remains constant. Finally, if the load is relatively constant, you can probably use fixed taps to adjust the output voltage to the desired value.

    (Note: working from motor theory here, not installation experience!)


  4. #4
    Join Date
    Oct 2004
    Antipolo City
    voltage will certainly lower as loads are added to it. how much load the transformer can take depends on several things, mostly it is how cool/hot the transformer gets.

    have seen transformers loaded beyond 100% but no oil leaks since the transformers are inside a basement maintained at 20 degrees celsius. however, a building AC motor on cycling service tends to cause a voltage sag everytime it is on the 'start' cycle.

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