Volt Amps?

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  #1  
Old 12-11-05, 08:32 PM
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Volt Amps?

The door bell trans I just put in says 20va. Does this mean volt-amps? If so, and volts x amps = watts anyway, why not just say 20 watts instead of 20va? Or am I missing somrthing else?
 
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Old 12-11-05, 09:11 PM
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Not a big deal on something like this; but technically when you are speaking of inductive loads like transformers and motors, voltsXamps is not true watts because of phase issues.
 
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Old 12-12-05, 10:30 AM
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As 594 says, va and watts are mostly the same thing. But they are not exactly the same thing, and in this case, "va" is more technically accurate. A doorbell chime is an inductive load.
 
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Old 12-12-05, 02:13 PM
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A transformer is rated in VA because the heat build up in a transformer is a function of how many amps are goin through it, not the amps relative phase angle to the voltage (power factor).
Watts will always be equal to or less than the VA by the factor of the cosine of the angle between the current and the voltage. What this means is that you cannot interchange the two terms except in a case where the load is purely resistive (say a light bulb or heatstrip).

Your transformer is rated 20 va . If you put a 20 watt load on it at unity power factor, it would still be 20 va and everything is fine. But if the 20 watt load has a 80% power factor (a motor for example), then the va would be 20 watts/.8 = 25 va. Your transformer would be overloaded.

As 594tough says, it's not really an issue in this case, but would be if you were ever trying to size a power supply or a UPS system.
 
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Old 12-12-05, 08:32 PM
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Good

Thanks for all the replies. If I understand correct, VA is power measurement for inductive loads and watts is power measurement for resistive loads-- Right?
 
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Old 12-13-05, 04:34 AM
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Kinda.
Think of va as the available amount of usable energy from a source. Think of the watts as how much of that usable energy is actually used (as work) by the load, inductive or otherwise.
 
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