I read the link and I could only see 65VA.
I also think that is a rare case from a rather unusual design.
It's unusual to get a power factor less than 0.5, so I would still say a smart meter will give a fair estimate. If there's any doubt then you will have to use a true power meter
In some cases, this is considered a transformer failure.
I have a 200 watt iron lump transformer, which has very good idle power wastage. Just because a transformer is big, it does not have to be an energy pig.
When I built my little amp, the plan was to save money by reusing a transformer that had been sitting idle in the junk room. When I fired it up, I noticed it was getting hotter and hotter. After an hour, it was hot enough to be considered to be overheated. And this was with no load on it. I had to buy another transformer, to finish my project, and it is luke-warm to the touch at no-load. So much for my plan to save money by using the old one.
If the oven uses a lump, I would stick a hand on it (carefully), and see what temp it runs at.
"Impedance protected" transformers, they may have different efficiency characteristics. I'm referring to regular take-no-prisoner transformers, which shouldn't really waste power if the eddy current is under control (eddy through breaks in lamination coating). The reason for using a stack of laminations, is to control eddy current.
Paul
You must live in a different universe from me, one where internal electronics in ovens are easily accessible. ;-)
Without pulling my oven out and doing some serious disassembling I don’t think I could get anywhere near putting a hand on my oven clock’s power supply!
Tim
Next line, when talking about the induction hob...
Well we have two kitchen devices Hob & Oven which exhibit this type of behaviour. I know its not a huge sample but pretty sure its not uncommon ...
You say that but from what I have read many devices present almost entirely capacitive loads...
Dave
In general, the smaller the transformer, the less efficient it is. The smallest mains transformer listed by Farnell is a 350mVA one, with a quoted efficiency of 30%. OK, not much power wasted at that size, but even the biggest PCB mount transformers are rarely over 90% efficient.
Same here!
In fact the bigger it is, and the less power you put through it, the better it is likely to be...
That sounds like a very POS cheap transformer. Unless it had a shorted turn. Ive seen transformers that skimped on iron size get hot at idle - if the core saturates a bit current goes up fast,.
Yes. That would be another issue. shorted laminations end up looking like a shorted turn.
We;l to an order of magnitude, yes, but switched mode power supplies draw a very spiky sort of current and unless you have a decent meter that measures true watts you could be out by a large margin.
OTOH they end to draw peak current at pretty much peak voltage. So power factor wont be a huge issue, but the fact that the current is not being drawn over the whole cycle will make the RMS power higher than the 'average current times voltage' .
Bear in mind that with non linear devices - and and SMPS is highly non linear - the concept of power factor becomes as useless as the 'rolling radius' of a flat tyre does.
Ideally you want a meter with the same characteristics as your 'smart' meter so that you measure what you are paying for.
And an idling transformer is almost purely inductive... And switch mode supplies don't have anything resembling a 'power factor'
You misunderstand what "power factor" defines.
For capacitive and inductive loads it is easy to visualise an out of phase voltage and current.
Where current is discontinuous, as in some switched mode power supplies, the definition still holds.
PF = actual power / apparent power.
where: apparent power is I(rms) x V(rms)
actual power is the integral of V x I over a cycle and represent the true power being drawn.
Agreed, except it's rms current and rms voltage and not average. There is quite a big difference, especially with a peaky current waveform.
For those who want to delve deeper:
Transformers with a permanently high load are often designed like this as they provide the best compromise in iron and copper losses. Microwave transformers are a good example.
In my much younger days, I investigated why an upgraded computer device was blowing 25A fuses in the 5V line. With two AVOs paralleled I was reading 18A, but the fuse filament sagged when the power was turned on, and would blow within an hour or so.
Eventually it occurred to me to look at the waveform of the current, and it was pulsed with less than 1 in 3 duty cycle. I measured mark and space as well as I could, and calculated that the 18A average represented about 30A in RMS terms, which was all that the fuse cared about.
The standard engineering answer, of course, was to increase the fuse rating.
I guess a risk analysis was done, but I am sure that was the right decision.
Moving iron ammeters have existed for a long time and generally measure true rms current independent of polarity and ac.
I presume meters which have a stator and rotor and wired in series could also give an rms reading. A similar technique used in true power moving coil meters, where the stator is a current carrying coil and the the moving coil driven by a voltage derived current.
We checked with the power supply and frame designers, there was no problem with the increased load.
What had happened was that the equipment was based on a Data General Nova minicomputer, and DG had the effrontery to discontinue production. The product was still selling well, so the decision was made to clone the Nova using AMD bit-slice processors, and it turned out rather more power-hungry than the real thing.
I believe there were purely electro-mechanical RMS meters, but the AVO was the standard average-measuring device for both voltage and current.
That is not what I was taught power factor meant
In your days of being taught I very much doubt there was much in the way of switch mode power supplies or consideration of a discontinuous current. Phase angles of sinusoidal waveforms ruled the day?
I confess my attention was drawn to the use of power factor following legislation for power supplies for PCs and other modest power consuming equipment.
A peaky current waveform with have a high rms value compared with the same sinusoidal current in phase with voltage delivering the same power. The idea this should be avoided as it would otherwise place a strain on the infrastructure leading to an increased transmission loss.
Equipment such as PCs must now have an 'in phase' near sinusoidal current draw with strict limits on harmonics.
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