NM
“Wattage” can be useful for specifying the units you want. If I ask “what’s the power of that motor?”, the answer might be “One horsepower”. But if I ask “What’s the wattage of that motor?”, then the appropriate answer would be “750”.
It’s just the signage of the times.
Generators are almost always rated in KW.
Power plants are almost always in Net MW, just in case folks want more random info.
Also, last time I looked, 1 MΩ is one megohm (no a) and a meter that measures current is an ammeter (no p).
I’ve always just heard H-V-A-C.
Used by professionals I have only heard HVAC or HV&AC.
Sequoia school grads, property managers, and personnel employees. Will use H-VAC. If the interview was giving me the feeling that I did not want the job, when asked to explain my experience with H-VAC systems. I would. The look of confusion on their faces were great. Finally they would ask something like don’t you have any air conditioning experience. " you mean HVAC systems, yes I do, along with high vacuum systems.
Considering that P=IE and E=IR, I try to be consistent with those formulas.
P = power and the unit of measurement is the watt (abbr “w”)
I = current and the unit of measurement is the amphere or amp for short (abbr “a”)
E = energy and the unit of measurement is the volt (abbr “v”)
R = resistance and the unit of measurement is the ohm (abbr “Ω”)
I’ll confess when I do the Ohm’s Law triangles, a V might slip in where an E should go. And it’s always V2 measured across R2, not E2.
I never spell out the unit. Why do it? Keep it simple by using the proper symbols. Examples:
5 ADC
3.6 VDC
45 MΩ
12.2 W
13.28 mV
Um, “volt-amp-reactive” as an explanation isn’t making it past the reptile portion of my brain. Could you restate for people like me who are afraid of wall outlets?
I could look it up, of course, but I’ve looked up and posted a zillion cites this morning already, and my thumbs hurt. So, thanks.
Many loads draw current somewhat out of phase from the supplied voltage. If the current is drawn 90deg out of phase then there is no net power consumed.
The load stores power during part of the cycle, and returns it all later.
This is reactive power. It is generally a problem because it increases distribution losses without doing anything useful.
So in AC systems, multiplying volts x amps gives you VA, not Watts.
To get watts, you multiply VA by the cosine of the phase angle between voltage and current waveforms. AC wattmeters take care of this by design.
Multiplying VA by the sine of the phase angle gives reactive power or VARs.
Note that VAR would be the mathematically “imaginary” part of a complex number. Watts are the “real” part. Electrical engineers use j instead of i to denote sqrt(-1) to avoid confusion with the symbol traditionally used to denote current.
This is why I hate the term “imaginary”. I can go out and measure “imaginary” power any day of the week. Go ahead and bring me e grams flour. Or for that matter, exactly half of a stick of butter. “Real” numbers are way more imaginary in a lot of ways.
To be nit-picky, E in this case stands for electromotive force (emf), not energy. Energy is emf multiplied by charge.
If you take a coil of wire and run electricity through it, it causes a magnetic field to form (basically, you’ve created a simple electromagnet). When you remove the electricity, the magnetic field collapses and the energy that was stored in that magnetic field gets converted back into electricity.
If you take two metal plates and put them close together (but not touching) and apply a voltage, this will also store energy, except in an electric field instead of a magnetic field.
The coil of wire is called an inductor, and the two plates is called a capacitor (there are other ways to construct inductors and capacitors but this is just to give you the basic idea). Inductors and capacitors don’t consume energy like a light bulb or an oven heating coil does. Inductors and capacitors just temporarily store energy and release it later. Inductors and capacitors are called reactors.
Even though reactors don’t consume energy, it still takes energy to charge them up, so when you apply a voltage there is still current that flows, resulting in power Power is therefore made up of two parts, the watts, which is the real energy that gets consumed by light bulbs and ovens and such, and vars, which is the energy that goes into all of those reactors.
Most homes are slightly inductive, due to motors in things like vacuum cleaners, refrigerator compressors, clothes dryers, hair dryers, etc .
Home electricity is AC, which means that it is a sine wave that swings positive and negative. Inductors and capacitors with a sine wave applied to them charge up and discharge according to where the sine wave is in its cycle. The way it works, inductors are charging during the part of the AC sine wave where capacitors are discharging, and capacitors are charging while inductors are discharging. So you can use one to balance out the other.
And that’s exactly what the power company does. They install capacitor banks in substations to balance out the inductance of all of those motors in homes. This way, the vars required for the inductance in your house is supplied by the capacitors, then when those inductors discharge they end up charging those capacitors. In a perfectly balanced system, all of the reactive energy ends up just bounding back and forth between the inductors and the capacitors. Then the power company’s generators only have to supply the current for the watts, and not the vars, which reduces the load on their generators and makes their power generation as efficient as possible.
We use the term Power Factor to describe how well balanced a system is.
You may have seen things that advertise that they will save you money on your electric bill by correcting your power factor. First of all, these things are just capacitors, and since unlike the power company’s capacitors they aren’t switched on and off of the line, they can’t possibly compensate for a varying power factor (in other words, it doesn’t know when you’ve turned your dryer on). The second thing is that for home users, the power company doesn’t charge you for vars. They only charge for the watts. So balancing out your vars eliminates something that you aren’t charged for anyway. So these devices don’t work, and even if they did, they wouldn’t save you money.
Another way of describing it: In AC, the voltage and current are both constantly changing. When you refer to the voltage of an AC circuit with a single number, you either mean the peak voltage, or (more likely) the root-mean-square voltage. Sometimes the voltage is at the peak voltage, sometimes it’s zero, sometimes it’s at the negative of the peak voltage, and most of the time it’s somewhere in between.
Similarly for the current. Sometimes the current is high, sometimes it’s low, and so on. But what about the power? Well, at any given instant, the power is equal to the voltage at that instant times the current at that instant. So the power changes with time, too, and it’s zero any time either the voltage or the current is zero. But usually we’re just interested in the average power, averaged out over many cycles.
If the only thing that’s on the AC circuit is a resistive load, like an incandescent light bulb, then the peak current happens at the same time as the peak voltage, and hence the peak power happens at that time, too (or when they’re both at the negative of the peak-- Two negatives multiply to a positive). In this case, we can just multiply the RMS current times the RMS voltage, and get the average power (this is one of the reasons why RMS is the typical measurement used).
But what if the load isn’t purely resistive? If the load is partly inductive, or partly capacitative, then the peak current and the peak voltage will be out of phase with each other. The instantaneous power will still be equal to the product of the instantaneous current and the instantaneous voltage, but the average power will not be the product of the RMS current and RMS voltage any more. It’s part of it, but there’s also the vars which are also part of it.
Gracias, amigos.
So things that run on electricity are (while running) always drawing power and have to play catch-up, like when your blood supply stream gets wonky when the pumping isn’t maximized by matching what should be a designed-in system of pressure drops and surges, when disregulated by wonky valves.
(Having written that, voltage as water upstream in a hose, I know remember, was taught to me in high school…)
Can they make motors, whatever, that are happiest with the on-off hits of phase cycles?
A weird way of “efficiency,” I admit…
*ETA: I just thought of how you push someone only on high points on a swing, or the times a ballet partner (male) spins the ballerina in a pas de deux. And then I realized I just invented the flywheel.
Should have said: healthy body deals with pressure drop just fine out of heart pump phase; bad valves illustrates by pathological breakdown.
The voltage at your 120 VAC receptacle is a simple sine wave. There is no DC-component or bias; the average voltage is 0 V, and there is a zero crossing every 8.3333 ms. When the voltage is exactly 0 V, the power at the receptacle is 0 W, so energy cannot be transferred from the receptacle to the device that’s plugged into it during this time. And when the voltage is very close to 0 V, there is very little power available at the receptacle, and thus very little energy is transferred from the receptacle to the device during this time.
This is a problem, since you obviously want the device that’s plug into the receptacle to be powered 100% of the time. How can you ensure the device is powered 100% of the time when the receptacle can’t deliver power 100% of the time (and can only deliver very little power during a significant portion of the cycle)?
There are a three solutions to this problem:
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The device must contain components that store energy (mechanically, chemically, electrically, thermally, etc.). When the voltage at the receptacle is at or near 0 V, the energy storage component(s) in the device release energy, thereby powering the device during this brief “down” time.
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Polyphase power. Instead of powering the device with just one voltage sine wave, power it with more than one voltage sine wave, and make sure the sine waves are not “in phase.” (Most polyphase systems use three voltage sine waves that are not in phase.) That way, when one sine wave is at or near 0 V, the other sine waves are not at or near 0 V, and thus positive power is always supplied to the device.
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DC. Scrap the sine wave and just use a constant, steady, DC voltage.
There are pros and cons to each of the above, obviously.