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Re: Exact meaning of KVA??

Discussion in 'Electrical Engineering' started by daestrom, Feb 1, 2004.

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  1. daestrom

    daestrom Guest

    As others have pointed out....

    Transformers ratings are typically their current capabilities and the design
    voltage. Of course, these vary from the primary side to the secondary side
    by the turns ratio. But a convenient thing to notice is that while the
    voltage may step down by the turns ratio, the current capability steps up.
    So the VA rating is the same for the high-voltage side and the low-voltage
    side. So rather than list each winding's voltage and ampacity, often just
    supply voltage ratio and VA. To keep numbers easy, for larger units list in
    kVA or MVA.

    Since a Watt is a Joule/sec, if you integrate the power over time you get
    the energy that has 'flowed' over time period. If the power of the load is
    constant, the integration is easy, just multiply the power times the time.
    Traditionally, the amount of electric energy used has been stated in
    Watt-hours, although Watt-seconds is also used. For larger amounts,
    kiloWatt-hours is often used. It is somewhat 'redundant' since you are
    taking a unit that is "unit of energy per unit time" and multiplying by
    "time" to get back to "unit of energy", but it has been with us a long time
    now, and not likely to be replaced by kiloJoule anytime soon.

    Ratings of generators are actually quite complicated. The sales literature
    of portable units leaves a lot to be desired ;-)

    Commercial generators actually have a whole set of curves (called
    appropriately, "capability curves"). The main windings are limited in the
    total current they can carry continuously. The nominal voltage is fixed by
    insulation, terminal spacing, electronic regulators etc... So, combining
    the nominal voltage and current of the armature winding, one rating is kVA
    or MVA.

    Another rating of generators is the amount of current the field winding they
    can handle. This limits how poor a power factor (lagging) that a unit can
    operate with rated kVA and still maintain nominal voltage. If the unit must
    supply a load with a lower pf, the total armature current capability must be
    reduced or terminal voltage suffers. If the unit is to supply a leading pf
    load, the capability is reduced because of severe iron-end heating of the
    rotor.

    If it is a three phase unit, the total imbalance of the three line currents
    must be limited to avoid iron heating of the stator caused by
    negative-phase-sequence currents.

    Notice, that I haven't mentioned kW (what you found some portable generators
    rated in). This is because the kVA is the true *generator* limit. Of
    course, if a *generator* has a rating of 10kVA, then it cannot safely handle
    more than 10kW, assuming the load has a unity power factor. But the maximum
    kW of a genset is more a function of the prime mover (engine, turbine,
    windmill, whatever makes it go 'roundy 'roundy) than the generator.

    You can hook a 20kVA generator up to a 5kW engine. Although the generator
    *could* handle a 20kW load (if pf is unity), the engine will stall out long
    before you reach 20kW.

    Conversely, you can hook a 20kVA generator up to a 4000 hp engine. Load it
    up with a few 100kW and the engine won't even blink, just keep right on
    spinning. But the windings of the generator will start to smoke in no time
    at all because you have exceeded the generator's kVA limit.

    Small portable units for retail sale are rated based on a few assumptions.
    That the load you will connect is 'typical' with some lights and light-duty
    motors (pf of between 1.0 and .8 lagging). So manufacturers build/buy an
    engine for 5kW, and a generator for 5kW / 0.8 = 6.25kVA and call it a '5kW'
    unit. Keeps the average consumer's life simple.

    But some manufacturers would like to cut corners and boost the sales 'hype'.
    So some might build a 5kVA unit (slightly cheaper) and *assume* the pf is
    1.0 and call it a 5kW unit (sells better than a 4kW unit). And the engine
    might only be able to supply 5kW for a short time, not continuously. So,
    BUYER BEWARE...

    daestrom
     
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