3 ms·

Same. I was kind of hoping for an in-depth explanation on the difference between Watts and VA, which should be none, W = VA, but apparently for AC they're used

by jolmg 4y ago

Same. I was kind of hoping for an in-depth explanation on the difference between Watts and VA, which should be none, W = VA, but apparently for AC they're used for 2 different ways of measuring average power, one being the product of peak current and voltage divided by 2, and the other being the product of the root mean square voltage and the root mean square current. I still wonder why they both matter, rather than just sticking with one or the other. According to Wikipedia, > Some devices, including uninterruptible power supplies (UPSs), have ratings both for maximum volt-amperes and maximum watts. The VA rating is limited by the maximum permissible current, and the watt rating by the power-handling capacity of the device. Is there something besides the "maximum permissible current" that influences the "power-handling capacity" in a circuit? These sound the same.

InitialLastName 4y ago
The key concept you're missing is Power Factor[0]. With reactive (inductive or capacitive) loads, the current and voltage will be out of phase with one another, leading the peak current to occur at a time other than the peak voltage. This reduces the power delivered to the load to less than |V|*|A|; in a situation where you hold power demand and voltage steady, you would need to deliver more peak current. For the two power measurements, one is instantaneous power, the other is average power. Different situations call for different measurements. [0]https://en.wikipedia.org/wiki/Power_factor https://en.wikipedia.org/wiki/Power_factor
marcosdumay 4y ago
The power consumption is the integral of the current your device takes for each voltage of the AC cycle. Different devices do all kinds of different things during that cycle, so it's not simple to communicate it in a number. Your grid power consumption is probably measured in terms of current alone. That means that the number on your power bill is different from the amount of energy your devices dissipate as heat. That measurement is converted into power using the RMS trick, but there is no guarantee that it will exactly match the actual power consumption of your house. Your UPSs has different limitations for real power, current, and peak current. And your house wiring (or better, any wiring) has limitations for (windowed) average current (with many different limits for different window sizes). Wiring also has a limitation for voltage, but it's usually high enough to not matter on normal usage. As a rule, with AC, there are so many degrees of freedom that any question becomes hard to answer.
- otter-rock 4y ago
  You cover a lot of interesting topics. Some exposition: Reporting power is easy with the power triangle. It's a mathematically exact concept. Measuring power is a different story. Distortions of the assumed perfect sine wave create measurement errors. Meter bandwidth matters, but most have low bandwidth. Grid meters at least use voltage phase, if not magnitude. Residential customers are billed only for real power, so the phase information is necessary to exclude the reactive power from the measurement. One rating is for maximum output. The other describes a short-circuit that you can create with an overly reactive load. The degrees of freedom complicate the implementation, but the bottom-line discussion abstracts away those details.
otter-rock 4y ago
There are actually three parts of the power triangle: real (W), reactive (VAR), and the hypotenuse apparent (VA) power. If voltage and current are exactly in-phase, VA=W and VAR=0. All of the power is doing work. Being out-of-phase is like turning a crank with one hand while resisting with the other - wasted effort. Some of the VA becomes VAR instead of W. The math thing is a simplification so you can take measurements with a multimeter, and do just a bit of arithmetic to get all three power values. It comes from the average power integral of trig functions with complex numbers. Watt rating is what you'd expect. Voltage and current are working together to do work. The limit is probably thermal. VA rating at max current is a short-circuit. If your load draws enough reactive power, the voltage and current get so far out-of-phase that a huge current sneaks through without much voltage. It could be many times as much current as the max current at max Watts.

by jolmg 4y ago

InitialLastName 4y ago

The key concept you're missing is Power Factor[0]. With reactive (inductive or capacitive) loads, the current and voltage will be out of phase with one another, leading the peak current to occur at a time other than the peak voltage. This reduces the power delivered to the load to less than |V|*|A|; in a situation where you hold power demand and voltage steady, you would need to deliver more peak current. For the two power measurements, one is instantaneous power, the other is average power. Different situations call for different measurements. [0]https://en.wikipedia.org/wiki/Power_factor https://en.wikipedia.org/wiki/Power_factor

marcosdumay 4y ago

The power consumption is the integral of the current your device takes for each voltage of the AC cycle. Different devices do all kinds of different things during that cycle, so it's not simple to communicate it in a number. Your grid power consumption is probably measured in terms of current alone. That means that the number on your power bill is different from the amount of energy your devices dissipate as heat. That measurement is converted into power using the RMS trick, but there is no guarantee that it will exactly match the actual power consumption of your house. Your UPSs has different limitations for real power, current, and peak current. And your house wiring (or better, any wiring) has limitations for (windowed) average current (with many different limits for different window sizes). Wiring also has a limitation for voltage, but it's usually high enough to not matter on normal usage. As a rule, with AC, there are so many degrees of freedom that any question becomes hard to answer.

otter-rock 4y ago
You cover a lot of interesting topics. Some exposition: Reporting power is easy with the power triangle. It's a mathematically exact concept. Measuring power is a different story. Distortions of the assumed perfect sine wave create measurement errors. Meter bandwidth matters, but most have low bandwidth. Grid meters at least use voltage phase, if not magnitude. Residential customers are billed only for real power, so the phase information is necessary to exclude the reactive power from the measurement. One rating is for maximum output. The other describes a short-circuit that you can create with an overly reactive load. The degrees of freedom complicate the implementation, but the bottom-line discussion abstracts away those details.

otter-rock 4y ago

There are actually three parts of the power triangle: real (W), reactive (VAR), and the hypotenuse apparent (VA) power. If voltage and current are exactly in-phase, VA=W and VAR=0. All of the power is doing work. Being out-of-phase is like turning a crank with one hand while resisting with the other - wasted effort. Some of the VA becomes VAR instead of W. The math thing is a simplification so you can take measurements with a multimeter, and do just a bit of arithmetic to get all three power values. It comes from the average power integral of trig functions with complex numbers. Watt rating is what you'd expect. Voltage and current are working together to do work. The limit is probably thermal. VA rating at max current is a short-circuit. If your load draws enough reactive power, the voltage and current get so far out-of-phase that a huge current sneaks through without much voltage. It could be many times as much current as the max current at max Watts.