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Volt Amps and MilAmps

Discussion in 'Lighting and Electrics' started by ship, Feb 26, 2004.

  1. ship

    ship Senior Team Emeritus Premium Member

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    Ok, since I'm not electronic, it's time for me to verify some stuff.

    First, if I have a 120v input and 12VAC output that is rated for 500 mA, than it's good for 0.5 Amps at 12v or 6Watts/12v?

    Thus a 850mA transformer is going to be good for 850mA than it's good for 10.2 Watts/12v Correct???

    A 1000mA transformer at 12v thus is 12Watts. Or is something wrong with my calculations? Am I supposted to be taking the mA reading off the 120v input side which would allow for a much higher wattage?

    Next is Volt Amps. I realize it's Volts x Amps or at least I think I do. If I have a single phase International step down transformer rated for 200VA, than the amperage of the fixture able to be run would at 110VAC is1.8 Amps or 198 Watts Correct?

    On the other hand, if I have a Power Boosting transformer starting with 110V for the inlet, and it's stepping up to 120V, the kVA rating is how many thousands of Volt Amps the transformer is rated for right??? In other words, if given the above, I have a 0.5kVA transformer, than it's worth 500 Watts at 120v. Right?

    Such things I just don't deal with enough to remember.

    And now for the fun part.
    A three phase transformer transfering 208v to 240v at 9.5kVA is good for how many watts per leg of power?
  2. DMXtools

    DMXtools Active Member

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    Elgin, IL, USA
    Pretty much correct. For the majority of transformers, the current rating is specified for the output. However, power isn't always I x E when you're talking about AC. For a purely resistive load, for example a lamp filament, you can pretty much treat AC and DC the same, in which case, yes, it would be 6, 10.2 and 12 wattts.

    The problem is that many loads are not purely resistive. A reactive load, for example an electric motor, causes something strange to happen - the current gets a little out of phase with the voltage. When that happens, the Volt-Amps stays the same, but the actual power delivered is less. That's why transformers are often rated for Volt-Amps instead of watts. In the simplest sense, Volt-Amps is the power the transformer sees, Watts is the power the load sees. If the load is purely resistive, the Volt-Amps and the Watts are the same.

    Power Factor is the ratio between true power (Watts) and apparent power (Volt-Amps) = W/VA. A resistive load has a power factor of 1. Any reactive load will have a power factor less than one. A typical electric motor might have a power factor of 0.85 - in other words, if it's an 85-watt motor, it needs a 100 volt-amp transformer to drive it without burning up.

    There's also the little matter of the direction of the phase shift - an inductive load causes the current (amps) to lag behind the voltage. A capacitive load causes the current to get ahead of the voltage. The power companies use this to make their transmission systems more efficient.

    The idea is that electric motors make up a large part of electricity usage, shifting the overall current to lag behind the voltage. They intentionally put large capacitors across the power lines at most of their substations to shift the current the other way. The right amount of capacitance can shift it right back to where it's in-phase with the voltage again, reducing the stress on their transformers, because when the total phase shift is zero, true power = apparent power.

    3.166 KVA. The actual watts, again, depend on the power factor - if it's strictly lamp filaments, 3.166 KW per leg. If there are cooling fans in some of the fixtures, or scrollers or movers, I'd limit the total to about 2.5 KW per leg, just to play it safe.

    xander and (deleted member) like this.

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