I heard some ad on the radio for a power factor device for residences. While I am very familiar with industrial and commercial devices I have never considered one for the home for one big reason:
I had always understood that the residential KWh meter didn’t measure apparent power just straight watts.
So…
A: What do residential meters measure? KW or KVA and…
B: If a residential meter does measure apparent power, will a house-wide pF correction device i.e. one that connects at the panel, realise any savings in a regular 2500 sq ft ouse with a 200A 120/240V service.
I ask because my fridge and outside AC unit already have pF correction. My fluorescent lights have high pF ballasts.
Does the bill from your electric utility have separate charges for actual power consumed and a ‘demand’ charge for apparent power? Isn’t that the point of power factor correction, to reduce the demand charge? If you are not being billed for it, I would say it doesn’t matter.
There are two components of power, the “real” power, which is the stuff that gets dissipated into heat in your heaters and lights, and there is “reactive” power which is the power that goes into charging up energy storing devices like capacitors and inductors. Real power is measured in watts, and reactive power is measured in vars (var = volt-amp reactive). The power that goes into a capacitor or an inductor just ends up getting released later in the AC cycle, so in a way it doesn’t really do anything, but you still have current flowing to charge up and discharge the devices.
Capacitors and inductors work sorta opposite of each other. During the AC cycle, while capacitors are charging inductors are discharging, and vice-versa. Power systems are at their most efficient when the power factor is 1 (also called a unity power factor). When this happens, the inductances and capacitances balance each other out so the generators don’t have to supply extra current to charge and discharge the reactive devices (inductors and capacitors).
Residential loads tend to be slightly inductive, due mostly to motors. The power company balances this out by switching capacitors on and off of the power lines as needed to correct the power factor.
Depends on the meter. Some of the really old ones just integrated the current and assumed that the voltage was constant. The fancy new electronic ones measure both current and voltage and calculate the resulting KVA then integrate that over time.
Industrial customers are often charged extra if their power factor isn’t within a certain specified range, so for them keeping the power factor closer to unity is often very important. For a residence, you probably aren’t going to see any dramatic savings.
I am also an EE so I fully understand the mathematics and application of real power, reactive power, apparent power etc.
What I am trying to acertain is to whether or not a residential meter measures VA and takes into account power factor and whether or not these ‘powersaver’ devices are snakeoil.
There is no mention of pF in any residential bill that I have ever seen.
If the new electronic meters measure current and voltage and then calculate KVA, they must by definition be measuring apparent power. This is the crux of my question because if they are not measuring apparent power and are instead measuring real power, then these devices will do little or nothing.
I haven’t been able to find anything on how a residentia meter works. Only three phase commercial devices which do measure apparent, reactive and real power.
Don’t know how it is in location, but all residential kWh counters that I have seen here in Germany work on the Ferraris principle and by this principle measure real power, no electronics needed. Picture of an old model (marked “Property of the People” because it happens to be from East Germany).
Residential meters of the older type are integrating watt-hour meters and they measure watts and not VA.
They consist of a motor with the stator current being the current into the building and the rotor curreent being proportional to the voltage at the building input. The motor turns a disc (the disc you see rotating in the window) that passes through a magnetic field generating eddy currents in the disc and this regulates the speed at which the disc turns so that the meter can be adjusted to read correctly.
It’s obvious that if the current and voltage are in phase the motor speed is the product of voltage and current. If they are not in phase the motor will run fast for part of a cycle and slower for the other part with the average speed being the product of the in-phase voltage and current.
I typed a description of how the “spinning disk” type of power meter worked, but on preview I see that David described it more clearly using fewer words, so I’ll just delete mine and tell you to read his.
The newer electronic meters read in the voltage and current through A/D converters and do the mulitplication digitally. They can be programmed to read kW-h or KVA-h and which is used depends on your local power company. I think most commonly they charge you for kW-h but then they whack you with an extra charge if the power factor drops below a certain value. Since most residences have a power factor well above 0.9 they never see the extra charge.
The power company’s losses depend upon how much current is demanded by their costomers so the rate for actual real power used were set so as to recover their costs plus a profit, etc. Although you paid only for actual watts used the rate per watt accounted for the power factor. Industrial users of gobs of power were monitored and they got a break in rates if they kept their power factor above a certain value.
The power company, like all comapnies, has to get enough from the customers so that they stay in business and except in unususl circumstances the power factor was academic.
Looking over my description I discover that I should add that the motor is geared to a set of dials that, in effect, counts the total number of revolutions made and reads it out in kwh.
So your meter probably reads the real power you are consuming, if it’s the spinning disk type (I agree with Simmons’ “They consist of a motor with the stator current being the current into the building…” post).
There is still a reason adding capacitors will lower your bill. The purely reactive part of the apparent power that flows into your house (whether or not it’s measured) still requires current to flow in your wiring, and the wiring creates more heat when more current flows through it. That is, although inductance in your house just stores energy and releases it later in the cycle, whatever resistance is in series with it will consume additional real power as a result.
But in a residence this difference should be much too small to measure. Your time and money will be 10 or 100 times better spent fooling around with weatherstripping.
As stated by ECG, modern power transducers measure instantaneous power using the definition of power, i.e. p(t) = v(t) * i(t). In other words, instantaneous power = instantaneous voltage * instantaneous current. Voltage and current are measured using a high speed, dual channel, simultaneous sampling ADC, thus p(nT) = v(nT) * i(nT), where n is the sample number and T is the sampling period.
But the power company doesn’t charge you based on power… they charge you based on the amount of energy you use over a certain amount of time. To arrive at this, the microprocessor integrates the power signal over time, and stores the accumulated sum in memory. One easy way to do this is to calculate the amount of energy used during each sampling period, and then keep a running sum total:
Let T = sampling period (s) and n = 0 = current sample,
p(n) = v(n) * i(n) current instantaneous power (W)
p(n-1) = v(n-1) * i(n-1) previous instantaneous power (W)
Energy used during previous sampling period = (T/2)*(p(n) + p(n-1)) (J)
Then:
Running sum total of energy = (T/2)*(p(n) + p(n-1)) + Running sum total of energy
Note that this technique does not use a fancy reconstruction filter… it integrates the signal using straight line approximation. Another problem with this approach is that T has to be known to *very * high degree of accuracy. Even a slight amount of uncertainly in T will propagate into a huge offset error at the end of the month.
I’m not sure how they actually implement it in a power meter. But you get the idea…
An experimental approach would be to run a device that is highly reactive and relatively high power in your household with and without one of those gadgets that is said to save power by making the power factor close to unity (the gadget MrFloppy is referring to) and measure the rate of energy consumption that the residential electric utility meter is showing.
The meter should show a lower rate of energy consumption with the gadget (and higher without it) if the meter is measuring volt-amps rather than true power. If the meter is measuring true power, there shouldn’t be any difference with or without the gadget.
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The simple answer is that household power meters are true power (its misleading to use units to indicate true vs apparent ,etc) , and the power saver devices are not saving anyone anything. They can only be sold by tricking the owner with a skewed meter, such as showing a 1 % imaginary power reading (eg a vacuum cleaner using 1% more current than true power suggests.) as something nasty and abhorrent.
But what if I had a defective piece of equipment that for some reason only consumed power 1/2 cycle at a time, and only once every 30 cycles…? Does the “old school” induction motor type meter have a fast enough response to catch that very low duty cycle load?
No matter how sporadic the load is, it’s still going to produce a magnetic field that will spin the disk. Theoretically, if the current is low enough and the duty cycle is short enough, you will eventually reach the mechanical limit of friction within the meter, but at that point you are probably talking about very small current loads. For higher current levels, even with a short duty cycle, the current is going to induce enough electromotive force to move the disk.
