[Amps] current in primary of microwave oven transformer

[d.cutter at ntlworld.com]

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 at tiscali.co.uk>
> Date: 2008/04/24 Thu AM 10:31:14 BST
> To: amps at 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)
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