David
I don't think that's what Angel measured: I understood it was measured using a
regular current meter, therefore the results are VA reactive not Watts.
David
G3UNA
>
> From: David Ackrill <dave.g0dja@tiscali.co.uk>
> Date: 2008/04/24 Thu AM 10:31:14 BST
> To: amps@contesting.com
> Subject: Re: [Amps] current in primary of microwave oven transformer
>
> 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
>
-----------------------------------------
Email sent from www.virginmedia.com/email
Virus-checked using McAfee(R) Software and scanned for spam
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|