Buddy bought a transformer that states 100 Va output per terminal. Logic says that V+A=watts. He says he was told that something called PFC power factor is .99 or 99% which still is 99 watts? Capacitor input suppy is .55-.75. Can someone explain this?
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Both watts (W) and volt-amperes (VA) are units of measurement for electrical power. Watts refer to “real power,” while volt-amperes refer to “apparent power.” Usually, electronic products show one or both of these values to provide information about how much energy they will consume or how much current they will draw. Each of these values can be used for various purposes.
These ratings are useful if you have to get rid of the heat generated by the device consuming the watts or if you want to know how much you will pay your utility company to use your device since you pay for kilowatt-hours (power used for a period of time). To combine the real power of multiple dc or ac devices, you can just add up the individual power ratings in watts of each device to get the total power (watts add linearly).
The apparent power in VA is used to simplify power ratings, making it easier to calculate current draw. Since VA = RMS volts x RMS amps, you can divide the VA rating by your RMS voltage to get the RMS current the device will draw. Knowing the RMS current helps you properly size wires and circuit breakers or fuses that supply current to your device.
Volt-amperes provide insight into the amount of current drawn by a product or circuit, assuming you know the voltage. For example, the standard residential voltage in the United States is 120 VRMS. If a product is rated for 300 VA (the rating implies this is the maximum VA the product will draw) and is powered from a 120-VRMS ac line voltage, you can calculate the expected maximum current as 300 VA/120 VRMS = 2.5 ARMS maximum (see the figure). Thus, you would want to ensure that the wires and associated circuitry supplying this product accommodate at least 2.5 ARMS.
Now...lest you think I am some kind of electronics expert, I got all this information from THIS PAGE.
Rich has it down pretty well there. For what it's worth when working with DC power, Watts and VA are the same thing. In AC, not quite so much. Over all, Watts are best used when measuring power over time, where as VA measure power at any given instant, ex: maximum peak power.
JGL
"Logic says that V+A=watts." Careful... not "plus" but "times" is the correct formula.
Typo error. Meant V x A = watts. This is one of those MRC AH101 transformers. Claims 270watts of Ac power. Now I understand it completely. LOL Thanks guys
bob
Just for fun went over to buddys house to see how new transformer worked. He has Volt & amp meters set up on it. We ran (3) '51 PW 2343 ABA' s with a total of 30 PW cars on 1 track. Didnt run them wide open cause they wouldnt stay on the 042" curves. Ran the volts up to 12V. The amp meter showed 10.75A. That should equal 129 watts! It did trip the breaker after about 5 minutes of run time. We swapped out (1) F3 for a geep 2349 with the same amount of cars. Same 12V, Amp meter ran up to 9. 8. Never triped breaker. Thats still 117W. cooling fan blowing air. How can that be when its listed on the panel for 100VA? Powerful transformer! My reburbished ZW wont do that. I know Im missing something here. Was that a fair test?
bob
Now check the track voltage during those same two tests & report back.
Your ZW, once warmed up, should be at least 200, up to 220 watts continuous if the breaker is the correct one and it's working right.
VA=W only when resistive AC load, because the current and voltage sine wave peaks and valleys are in phase together, thus true power consumption. Once you throw in motor and transformer winding inductances and/or capacitances, the voltage and current phase angles change relationships (lead/lag one another) and readings need corrections to get the true power consumption as Rich explains above.
My rough rule of thumb is W= 0.75 of VA rating. Crude but first order with a safety factor.
I bought a wattmeter several years ago on a whim at a university lab garage sale. It is a pain to connect properly, but the results were impressive. In question, at the time was the power consumption of a "attic/roof" ventilation fan that I had bought and installed. The motor did not look that substantial and didn't seem to move that much air. I compared the wattage used by the store bought fan against an exterior AC unit exhaust fan I had installed in the other attic. I found that the store bought unit used about 4x the electricity and moved about half or less the air.
But I got the store bought unit on sale and at a good price. I ended up getting another exterior exhaust unit fan and modifying it to fit in the store bought frame. Now this gets me to thinking of measuring the total wattage used on my layout, Hmmm. Now I could use that data against the wife's hair dryer and easily win the argument about which costs more to run, the trains or the hair dryer.
You know unless I can be diplomatic about this hair dryer thing, I may not be here to post in the future.
Whew RRMAN way over my head. My ZW will run (2) F3's on one track but not (3). Ill have to do the other voltage test tomorrow. Blizzard going on rite now.
bob
Second "test" results Rob. Using all 3 f3's. Voltage at transformer, 12V. "Track voltage" in far corner, (30' away) read 10.5. Amps at 10.6. for 111.3W? Didnt trip the breaker. Swapped out 1 F3 for the Geep. Same 12V at transformer. Far corner read 11V, amps at 9.7A for 106.7W? I took my mulitmeter along just to varify against his. They were within .2 of each other as far as Volts go. Is that voltage loss acceptable for 3 rail lionel "made in usa" "O ga" track? My ZW will run 3 F3's on 1 track if I disconnect everything else. Interesting test.
bob
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