power factor

[Max Demian]

They could have made half of them with the rectifier the other way round so the chassis was connected to HT +ve.

[NY]

Nowadays if that happened, you'd achieve the 50% one way and 50% the other if the appliance had a removable figure-of-8 power lead because that can be inserted equally well either way round.

But back in the days of live chassis, you'd have wanted to make damn sure that the plug could only be inserted one way round into the wall socket and that its live and neutral were wired the correct way round in the plug.

My dad once nearly electrocuted himself on a toaster that had been manufactured with the switch in the neutral rather than live wire. He used a knife to lever out a bit of toast that had got stuck, without unplugging it but believing that it was safe because it was switched off at the switch that is operated by inserting a piece of bread. I rewired the plug with the live and neutral wires the wrong way round (and put an explanatory note on a scrap of paper inside the plug) to get round that design fault. I doubt whether the toaster would have passed CE-mark or kite-mark safety checks.

[Jack Harry Teesdale]

A current of 20A at 240 volts will always be 4.8kw on a watt/hour meter but at a pf of 0.5 lagging it could as much as be 9.6kVA on a kVA meter.

[Chris Green]

No:-

20A at 240 volts, power factor 1 - 4.8kw on kw meter and 4.8 kva on kva meter 20A at 240 volts, power factor 0.5 - 2.4kw on kw meter, 4.8kva on kva meter.

[The Natural Philosopher]

Yes. Meters measure real power, not imaginary power. It's not hard with electronics to measure both voltage and current samples at high speed and perform the average of V*I at a given instant, or set thereof.

[The Natural Philosopher]

Wrong way round

It will always be 4.8kVAm but could be as little as OkW..

[Andrew]

Or since they would exhibit the same problem in customers homes, presumably just spread the sets on soak test across all three phases ?. Or is is too simplistic ?.

[Jack Harry Teesdale]

Only in theory, a lagging power factor will always show a higher kVA reading than the corresponding kW figure.

That's why power factor correction works and saves on max demand charges.

[Jim GM4DHJ ...]

that is the worrying bit

[Jack Harry Teesdale]

Exactly, when compact fluorescent lamps were introduced for 'energy saving' they did save money for customers with kWh meters but because they had an inherently poor power factor when fitted in commercial premises with kVA metering the savings were much less hence the viabilty of changing over was much reduced.

[The Natural Philosopher]

That is what I said, but exactly not what you said

It doesnt save on charges because it uses less power. It saves on charges (if it does at all) because it reduces losses in the grid, so large users of un power factor corrected electricity are charged, not for just power, but for out of phase current as well

actual real watts is ALWAYS less than or equal to KVA

You said "A current of 20A at 240 volts will always be 4.8kw"

No, it will always be *4.8KVA*. What power it represents is down to the phase angle between the V and I components. In quadrature (pure induction or pure capacitance or a mixture of both), power will be *zero*.

[Chris Green]

Not just lagging, leading will have exactly the same effect.

[PeterC]

IIRC there was a reqirement for CFLs over 20W to have a high PF. Certainly the 30W ones that I bought were close to unity. LEDs are lower power, so the problem is less. I don't know if there's the same requirement for higher power LEDs to be corrected as I don't have anything over 15W.

[John Walliker]

I remember hearing about that problem - I assume it was the Decca factory. The version I heard was that the dc component caused core saturation in the substation transformer resulting in overheating.

John

[Jim GM4DHJ ...]

DECCA DECCA DECCA

[John Rumm]

If by "worrying" you mean "wrong" then yes :-)

(I will concede that the poster probably knew what he was trying to say, but worded it in an arse about fashion)

If you know the voltage and current feeding a reactive load, but don't know the PF then you can't make a statement of "will always be 4.8kW". Since for any load with non unity PF, the only certainty is that the power shown on the watt/hour meter *won't* be 4.8kW

If you replaced "will always be" with "reading" and replaced the later "could be as much as" with "will" then it makes a bit more sense.

[rick]

PF is irrelevant in domestic situation. Its industrial installs where it is key ... its about efficiency and is the ratio of working power to apparent power. If PF is lagging, it means the supply authority would have to install much more capacity to meet the need, so they want you to keep PGF low.

I worked for a cable manufacturer with loads of induction motors in the mill. There was a Power Factor alarm ...... if it rang the works electrician would be frantically running around the factory shutting things down .... if you exceeded agreed PF figure for more than a few minutes you had a penalty applied for the whole Quarter.

As induction motors cause a lagging PF, for every so many induction motors you would install a motor which run with leading PF (usually a Large Synchronous motor) more expensive hence few were used.

A very good guide here;

and this explains teh motoros

[JNugent]

Yes. Well aware of that, even after fifty years!

Exactly.

Wasn't it more usual to "install" a large capacitive bank?

But I don't think I'll be looking for another job in the electrical field. ;-) The last years of my working life were spent in more amenable conditions!