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Old 29th October 2011, 07:36 AM   #1
Mooly is offline Mooly  United Kingdom
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Default Do domestic Kwh meters (ordinary electric meters) record reactive power.

Haven't been able to find a definite answer to this.

When a capacitor is placed across an AC supply although it draws current it doesn't dissipate power.

So a specific example.

If a 0.22uf capacitor has a reactance of approx 15k at 50hz then over a year (8760 hours) how many Kwh would a domestic meter clock up ? If it were a resistor it would average out at around 34Kw over the year at 240 Vac.

What would the cap clock up ?

Does the meter type make a difference as the old mechanical ones with a rotating disc have been replaced with small electronic types ?
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Old 29th October 2011, 07:50 AM   #2
Elvee is offline Elvee  Belgium
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No, the domestic meters implement a true vectorial calculation, P=U*I*cosφ

Of course, they are relatively crude electromagnetic devices, and the quadrature may deviate slightly, causing a "leak" of the reactive power into the active one.
But the leak can work in either direction.

If you connect a very large polypropylène capacitor to your mains, you will see some consumption though, because the wiring of your home will introduce a very slight resistive component, but for "normal" values ,like suppression capacitors, this is negligible.
Except if they are paper type, as they have ~2% losses.
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Old 29th October 2011, 08:14 AM   #3
Mooly is offline Mooly  United Kingdom
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Thanks Elvee...

So you are saying that apart from wiring resistive losses the meter would not record the current (power) that has flowed (because no "real" power has been generated).

What about the newer all electronic meters which have the capability (but is it used ?) to record reactive power/power factor etc.

So the 15k resistor would consume a real 34Kw over a year and cost say £5 a year.

So would the cap "cost" essentially £0.00 over a year ? (apart from miniscule power due to resistive losses).
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Old 29th October 2011, 08:32 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Mooly View Post
Thanks Elvee...

So you are saying that apart from wiring resistive losses the meter would not record the current (power) that has flowed (because no "real" power has been generated).
No, only active power is recorded
Quote:
What about the newer all electronic meters which have the capability (but is it used ?) to record reactive power/power factor etc.
In the industry, reactive power is billed under the form of penalties when it exceeds a certain level.
For domestic installation, there is at present no similar scheme.

Quote:
So the 15k resistor would consume a real 34Kw over a year and cost say £5 a year.

So would the cap "cost" essentially £0.00 over a year ? (apart from miniscule power due to resistive losses
Yes, and also the dielectric losses: negligible for PP, medium for mylar, and high for paper.
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Old 29th October 2011, 11:01 AM   #5
Mooly is offline Mooly  United Kingdom
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Thanks Elvee
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Old 29th October 2011, 11:19 AM   #6
agdr is offline agdr  United States
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Here is an interesting read on it all

http://www.nlcpr.com/Deceptions1.php
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Old 29th October 2011, 11:57 AM   #7
Mooly is offline Mooly  United Kingdom
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Thanks...

Reason for the question was actually to do with "watless droppers" vs small transformer for a particular circuit. A small transformer that gets hot obviously consumes real power and you are charged for it, the cap supplying the same circuit doesn't and from what I gather you don't get charged for it (although obviously the circuit it's powering consumes real power that is chargeable).

Not trying to make the meter run backwards... honest
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Old 29th October 2011, 12:04 PM   #8
AndrewT is offline AndrewT  Scotland
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Look inside a mains voltage LED lamp bulb.
You can see the series cap that "drops" the voltage down to a LED suitable voltage.
They tell us on the label it is a 240Vac, 1W or 3W or whatever bulb.
It registers/consumes ~1W of Power.
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Old 29th October 2011, 12:06 PM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Look inside a mains voltage LED lamp bulb.
Never looked in one of those... but yes I see your point.
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Old 29th October 2011, 12:11 PM   #10
Elvee is offline Elvee  Belgium
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Small transformers are a disaster from an energetic point of view: a typical 1 VA transformer can use 3 watt or more open circuit. (of course, you don't use them without a load, and you have to add copper losses).
From that perspective, capacitive droppers are much more effective, even when most of the quiescent power is dissipated in a zener.
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