Power Factor What Does It Mean

BroomJM

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These are some amusing illustrations, but I have a question about "RMS". In some limited reading I did, this seems to be a measure of how many watts a given device consumes. The device I'm looking to power claims "Power consumption at full brightness •9W RMS".

Does this mean it runs at just 9 watts, like a 9 watt lightbulb? If I had a 12v battery with 90wh of capacity, this would run for 10 hours?
 
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svetz

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Does this mean it runs at just 9 watts, like a 9 watt lightbulb?
An incandescent light bulb is purely resistive and has a power factor of 1. Anything that shifts the phase of the frequency (inductive load) alters the power factor of an AC circuit. LEDs are DC, so any related power factor is from their AC-to-DC driver.

If I had a 12v battery with 90wh of capacity, this would run for 10 hours?
If you were running AC it depends on the power factor which they didn't tell you. So, if a device consumes 9 watts of "true power" and has a power factor of .5, from your supply it'll consume 9 / .5 = 18 watts.

But, since you're running from a battery then they should draw 9 watts so ~10 hours sounds right.

These are some amusing illustrations, but I have a question about "RWS" [RMS?]. In some limited reading I did, this seems to be a measure of how many watts a given device consumes. The device I'm looking to power claims "Power consumption at full brightness •9W RMS".
RMS is about how the AC power is measured, the RMS voltage in an AC circuit gives about the same amount of power as an equivalent DC voltage.

RMS = root mean square

What folks in the U.S. think of as 120VAC it's really 120 VRMS, which is about 340V peak to peak (see image right).

VRMS = Vpk / √2. So, 170Vpk / 1.41 = 120 VRMS.
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tigerwillow1

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There are two different types of power factor: displacement and distortion. Displacement power factor is associated with motors, when the current draw is in the form of a sine wave, but not synchronized, or in phase, with the voltage. Distortion power factor is associated with power supplies, when current is pulled only at the peak of the voltage sine wave, but in phase with it. Battery chargers and non-incandescent lights cause distortion power factor. AC power generation and distribution is most efficient when the voltage and current are both sine waves in phase with each other. Any deviation from this causes the power factor to be less than 1. "Apparent power" is the RMS voltage multiplied by the RMS current, measured in volt-amps. "Real power" is the useful power transferred to the load, measured in watts, which is less than the apparent power if the power factor is less than 1. If you're the power company, your cost of generation and distribution is based on the apparent power. If you're the customer, your benefit is based on the watts. Say you're using 50 watts with a power factor of 0.5, To support this the generators, power lines, transformers, etc. have to be sized to supply 100 watts, even though only 50 watts are being consumed. In general, residences are billed for watts and industrial customers are billed for volt-amps. I consider the reactive power more as wasted infrastructure than wasted electricity. Some of it goes up in heat, but a lot of it is never generated in the first place.
 

curiouscarbon

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There are two different types of power factor: displacement and distortion. Displacement power factor is associated with motors, when the current draw is in the form of a sine wave, but not synchronized, or in phase, with the voltage. Distortion power factor is associated with power supplies, when current is pulled only at the peak of the voltage sine wave, but in phase with it. Battery chargers and non-incandescent lights cause distortion power factor. AC power generation and distribution is most efficient when the voltage and current are both sine waves in phase with each other. Any deviation from this causes the power factor to be less than 1. "Apparent power" is the RMS voltage multiplied by the RMS current, measured in volt-amps. "Real power" is the useful power transferred to the load, measured in watts, which is less than the apparent power if the power factor is less than 1. If you're the power company, your cost of generation and distribution is based on the apparent power. If you're the customer, your benefit is based on the watts. Say you're using 50 watts with a power factor of 0.5, To support this the generators, power lines, transformers, etc. have to be sized to supply 100 watts, even though only 50 watts are being consumed. In general, residences are billed for watts and industrial customers are billed for volt-amps. I consider the reactive power more as wasted infrastructure than wasted electricity. Some of it goes up in heat, but a lot of it is never generated in the first place.
thank you for education!
 

curiouscarbon

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As a "professional nitpick" ;), I should like to add that I like the analogy with the pack of crisps (chips) a little bit better because while binging on beer head may eventually get you drunk, no amount of compression will turn the extra air in the bag into crunchy bits of potato solids.
very good point

sad chip flavored air has no power, whereas beer bubbles still have some power ;)
 

HaldorEE

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These are some amusing illustrations, but I have a question about "RMS". In some limited reading I did, this seems to be a measure of how many watts a given device consumes. The device I'm looking to power claims "Power consumption at full brightness •9W RMS".

Does this mean it runs at just 9 watts, like a 9 watt lightbulb? If I had a 12v battery with 90wh of capacity, this would run for 10 hours?
RMS is a way of equating AC power to the DC heating equivalent assuming the power factor = 1.0. This is based on heat rise measured in a non-inductive resistor. What this really means is that RMS only means anything for purely resistive loads which is virtually never the case in real life.

When specifying industrial AC electrical equipment the power capacity is rated in always in VA (Volts x Amps). This absolutely tells you what the equipment is capable of dealing with without the marketing games flaky companies engage in.

AC power that is not specified as RMS is pure smoke and mirrors (they are not even pretending to tell you the truth). Like those 200W powered PC speakers that come with a 12V @ 1A power supply (6W per channel). Even when they use the term RMS they are still probably lying to you. A $400, 2000W inverter will typically ends up delivering 1000VA at best.

Peak RMS power is basically calculated as the maximum power an inverter is cable of delivering into a dead short right before the inverter emits the magic smoke and stops working. That is why that same $400, 2000W inverter claims 200% surge capability. Yah, for less than half a cycle. Then the overload protection engages (you hope) and turns it off. Good luck starting a motor with that.

I worked in product marketing while I was going to school for my EE degree. We used to joke that the difference between salesmen and marketing people is that at least the marketing guys knew when they are lying to you. For the good of my soul, the company I worked for at the time was a top quality manufacturer.
 
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BroomJM

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RMS is a way of equating AC power to the DC heating equivalent assuming the power factor = 1.0. This is based on heat rise measured in a non-inductive resistor. What this really means is that RMS only means anything for purely resistive loads which is virtually never the case in real life.

When specifying industrial AC electrical equipment the power capacity is rated in always in VA (Volts x Amps). This absolutely tells you what the equipment is capable of dealing with without the marketing games flaky companies engage in.

AC power that is not specified as RMS is pure smoke and mirrors (they are not even pretending to tell you the truth). Like those 200W powered PC speakers that come with a 12V @ 1A power supply (6W per channel). Even when they use the term RMS they are still probably lying to you. A $400, 2000W inverter will typically ends up delivering 1000VA at best.

Peak RMS power is basically calculated as the maximum power an inverter is cable of delivering into a dead short right before the inverter emits the magic smoke and stops working. That is why that same $400, 2000W inverter claims 200% surge capability. Yah, for less than half a cycle. Then the overload protection engages (you hope) and turns it off. Good luck starting a motor with that.

I worked in product marketing while I was going to school for my EE degree. We used to joke that the difference between salesmen and marketing people is that at least the marketing guys knew when they are lying to you. For the good of my soul, the company I worked for at the time was a top quality manufacturer.
That is a very detailed response that I can only presume is accurate.

The device I'm powering is a sonar unit on a boat. It is a 12v device and rated at 9W RMS. I'm presuming that means it draws about .75 amps, is all? What I'm really trying to understand is how long a battery of a certain size will run the sonar unit. I recently tested the LiFePO4 battery I'm intending to use and it's about 13.0 volts and 18 amp-hours. My limited understanding is that would be 234 watt-hours. Divide that by 9W (RMS?) and it should run the sonar unit for about 26 hours?
 

Hedges

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What folks in the U.S. think of as 120VAC it's really 120 VRMS, which is about 340V peak to peak (see image right).

Although to clarify, 340V never appears between the wires. They have zero to +170V between them, then down to -170V.
340V only exists as peak at one moment in time compare to negative peak at a different moment of time.

Similar for 240V of 120/240V split phase. The pair of wires get 340V between them of one polarity at one time, later 340V of opposite polarity at another time. 680Vpp, we can call it.

Makes a difference when evaluating insulation rating. Also matches what peak current flows through a resistive load.

(I got that wrong, or misinterpreted "170V zero to peak" specification, for RF in an instrument I helped develop. When I went looking for N2+ ions, the tiny peak I found was N2++ because I was looking at the wrong voltage.)
 

bdserv

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There is no such thing as "RMS power" - it is likely a term invented by some sales or marketing department.
 

curiouscarbon

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would "RMS voltage" be less of a shibboleth?

that is, the root mean square of the voltage value sampled at more than double the frequency of the AC waveform. squaring each value, summing them all together, and then taking the square root of that sum? ensuring that the entire sample occurs over an integer number of cycles? preferably beginning at zero at ending at zero volts?
 

curiouscarbon

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it makes sense to me to do root mean squared algorithm on the voltage, but i would assume amps would be linearly time averaged, that is mean average over entire time sample
 

curiouscarbon

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This is an interesting subject. Both RMS Power and RMS Watts are a fiction created by the Federal Trade Commission.
*grabs popcorn and reading glasses*
i love learning!
 

curiouscarbon

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The FTC also incorrectly assumed that the measurement of the power in Watts would be RMS Watts. It's not. It's Watts.
There's no such thing as RMS Watts.
In summary, RMS Voltage is correct, but there's no such thing as RMS Power or RMS Watts.
Or stated differently, the Voltage that's measured is RMS Voltage, but the resulting power is Average Power and it's measured in Watts.
📝
 

Hedges

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it makes sense to me to do root mean squared algorithm on the voltage, but i would assume amps would be linearly time averaged, that is mean average over entire time sample

At first I was going to argue that "RMS Watts" was valid, just not common terminology. But I've decided it doesn't have useful meaning.

RMS Voltage is an AC measure that represents average power delivered to a resistive load (instantaneous power = V^2/R)
RMS Current similarly represents average power dissipated in a resistance (e.g. heating element, transmission line, fuse or breaker) (instantaneous power = I^2 x R)

If you multiple Vrms x Irms that gives VA, apparent power. But not necessarily delivered power. If the load is only a combination of L and C (ideal, lossless), then zero watts are delivered. Each cycle, current is out of phase with voltage part of the time and delivers power back to the source.

If you multiply instantaneous voltage times instantaneous amperage, you get instantaneous watts. Mean average (rather than RMS average) is the way to calculate power. I don't find a use for RMS average of watts.

RMS calculation in affected by DC offset. When using an oscilloscope and documenting ripple that is present on a DC signal, I realized that "standard deviation" equals RMS with DC component removed.

For battery current (when feeding an inverter) I use RMS amps rather than mean amps to calculate fuse size. I've determined that capacitors in inverter are no where near large enough to smooth out current draw, and at full load it will be close to a rectified sine wave going all the way down to zero. RMS current is about 1.11 times larger than mean.
 
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