Angel Vilaseca wrote:
> Good mews then! My MOTs are OK.
>
> Bad news: although no real power is delivered to my house, the power
> company will bill me for it! :-)
>
> Thanks for your help, guys!
>
> Angel Vilaseca HB9SLV
>
Only if you get charged for kVA, rather than just kW...
The 147 Watts you measured is 'real' power, the Watts part, so you will
spin the meter by 147 Watts per hour, or 0.147 kWh at what ever your
rate is. This was probably due to copper losses in the transformer and
any other electronics that was associated with the microwave in its off
state. Unless you had removed the transformer 1st of course.
The Watt-less element, or reactive power, cannot usually be measured by
a domestic kWh meter. Which is why I was a bit surprised that the 1st
two respondants mentioned it as, without a suitable meter, you can't
measure it...
If you are on a non-domestic tariff, and I assume this is the same in
other parts of the world, but no doubt someone will put me right if not,
you may have an element related to the kVA demand as well as the kWh
consumed. The metering system has to be more complicated to do this and
it's not normally cost effective for domestic supplies. At least, in
the UK it isn't.
However, your statement that 'no real power is delivered' is only partly
the story. The reason the electricity companies worry about kVA is they
do have to supply that out of phase component through the supply cables
and ensure that there are transformers big enough to supply both parts
of the electricity supply, plus generators supplying VAr (Volt Amps
reactive) to the grid to support it. Sometimes they are generating no
'real' power and just supplying reactive power to support the Grid.
However, they often do this when on 'hot standby' ready to take up the
load if demand increases as it takes a long time to run up boilers and
synchronise a set onto the bars at a large powerstation, so whilst they
'do nothing' in waiting they will often excite the windings in such a
way as to generate 'capacitive' VArs. That's where the current leads
the voltage, in an inductor current laggs behind the voltage.
The way that industrial companies, who have alot of big motors which are
often running at poor Power Factors, and hence high demands for kVA, can
avoid the extra charges is to fit power factor correction capacitors. If
you think of how you resonate an antenna, you put enough capacitance and
inductance together to make the radiating element resonate. Inductors
are the opposite of capacitors, and vice versa, so if you have alot of
indictors (motors) you put some capacitors next to the incoming supply
to give the opposite effect and reduce the amount of kVA you demand off
the system and reduce the size of cable/transformers etc., needed to
support your supply. By the same thinking, putting capacitors next to
the motors means that your cables in the factory can be smaller as well,
because they only need to be sized for the kW part and a much smaller
kVA component.
I'm sure all this is available on the internet, or a good electrical
engineering book. ;-)
Dave (G0DJA)
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|