The death of the Watt (He was an old friend)


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For the better part of the last century, we measured electrical work in Watts. It was a simple and good life! Volts times Amps equaled Watts! It is sad to report that our friend has passed on as an accurate measurement. Mr Watt is survived by his brother VA.

"What?", you say, "I didn't even know he was ill!"

It is a long and sad story that started with the introduction of electronic equipment, and has spread to larger devices, such as electronic ballast systems. Most all of these devices change power from ac to dc and store them in a capacitor which then feeds switch-mode power supplies and other circuitry. In the process, something strange happens: No power flows in the primary circuit until the waveform voltage of the ac line exceeds the current voltage in the supply capacitor. The Diode blocks it. In other words, the only part of the AC waveform that is doing any work is the portion near the peak voltage. In calculating wiring loads, the Watt no longer works as it is based on a liner draw of power across the full waveform. Now, part of the waveform is left dead, and the peak of the waveform draws an unexpectedly high value of current. To try to rate this draw, a new term came out called VA. Anybody who has dealt with battery back-up supplies or computer power supplies knows the plates rate them in VA and the VA number is higher than the wattage draw of the unit.

"What is VA then? Isn't it Volts times Amps?", you may ask.

Nope! VA is the calculated voltage multiplied by the maximum waveform draw. Since all of the work is being done during only part of the waveform, the VA value must be used to calculate wiring and power source capacity requirements. Wire works like a big resistor. If your 575 watt mover is drawing 7 amps during part of the waveform, it may still be considered a 575 watt load, but power distribution must be handled based on the 7 amp draw, or wiring voltage drop may become a problem. This is not to be confused with strike and startup draw, which requires even more current. 7 amps would be the running draw. (Also, this should not be confused with "Power Factor" which is a whole other subject!)

"Huh? 120 times 7 equals 840? Why is it drawing so many watts?", you may ask.

Its not! Confused? The ballast is only drawing 575 watts, plus a few for itself. (Maybe 625 total) But.... the VA draw is 840VA. The reason there is a difference is that there is a portion of the waveform where the fixture is drawing nothing! So, for wattage, you have to calculate the VA draw, then prorate it by the percentage of the waveform the draw is occurring over. (Kind of a pain) For power distribution, we can only be concerned with the VA rating, making the watt obsolete!

I loved Mr Watt. He was married to Mrs Incandescent, who I fear is not doing well either. Both lived in the town of Resistance-ville, but I hear that is being demolished to make way for the new community of Inductor City. :(


CB Mods
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CB Mods
And I thought technology was supposed to make life easier...


Well-Known Member
It's the conversion between real power and apparent power.

"Power Factoring" is a little bit different. It is associated with uncorrected magnetic ballasts and motors. In a power factor error, these devices will draw their peak current at a later time then the peak voltage. In other words, the voltage waveform crests and is headed downwards as the current waveform crests. To correct this, a capacitor can be put across the load. Basically, the capacitor takes current charging as the voltage waveform rises, and releases it as the waveform heads down, offsetting the additional current draw of the uncorrected device. This helps nullify the phase lag on the current draw, and the device is considered "corrected." As you can guess, choice of the value is critical.

"Apparent Draw," can actually be considered the parent category for both Power Factoring and VA non-liner draw error, as both require us to design distribution based on a draw above the rated wattage. The error introduced by electronic equipment differs from the PF error in that peak current draw is still in phase with peak line voltage. Both could be identified by the following lab bench test: (It is assumed that all equipment would be run on an isolation transformer and that all lab safety protocol would be followed.)

The Test Rig: A one ohm resistor of sufficient wattage connected between the isolation transformer and the equipment to be tested. A dual trace oscilloscope with the "A" input monitoring the output of the isolation transformer, and the "B" input monitoring the drop across the resistor. This rig would monitor voltage on trace A and the represented current by voltage drop on trace B. In both cases, trace A would be used for the scope trigger, and would display a sign waveform.

Test 1: Bad power factor-
Motors: The waveform on trace B would be sinusoidal but would lag behind trace A timing.
HID Ballast: The waveform on trace B would be somewhat squared and would lag behind trace A timing.

Test 2: VA / Wattage variation-
The waveform on trace B would be somewhat squared on the leading and trailing edge, with a visible gap (0 volts) between the later section of the leading wave and the start of the downward waveform. Trace B would however maintain phase alignment with trace A.

End Test

Conclusion: As more and more lighting equipment becomes "ballasted", the "Apparent Draw" becomes the primary distribution design factor as compared to the actual wattage. One other note- As electric meters are basically motors that work off of the current draw of a circuit, they tend to ignore PF errors and bill at the correct wattage. This is why power companies get so upset over PF errors! Their equipment must handle an unbalanced waveform but they don't get to bill for it. VA errors get billed to the customer as the peak current is indeed higher! Thus, no complaints from the power industry!

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