Reactive power loading

The better versions of the little plug in power meters you can get for a domestic outlet actually give a phase anglereadout, for those interested (sad?) enough to want to compare reactive and resistive loads.

Don't make too much of it though, of the power companies will be asking us all to install capacitor farms behind our worshops...

Neil

Surely there's only one type of inverter, one with active components. The methods using capacitors and slave motors are not inverters.

The electricity supply companies like capacitative VARs because it tends to counteract the inductive VARs caused by three phase motors. That's why they promote the energy saving light bulbs because, according to my power meter, they have an awful power factor, it tends to be capacitative.

Regards,

Andrew

Most inverter drives I 've seen rectify inoming phase or phases .strore on great big cap..then chop it about as required....looks like a "capcitve" load to input...

Btw..'.llecy bill records power factor PER PHASE and applies correction..but it appears no penalty anymore

Edited By jason udall on 08/03/2013 00:20:00

Smaller inverters use a rectifier and capacitor on the input. The problem with this is not so much power factor as non-linear loads leading to significant current harmonics, which are also a problem for the electricity companies. Cheaper inverters will normally incorporate a passive input filter to reduce the harmonics. Higher power or more expensive inverters may include an active power factor corrector on the input. For an inverter this would normally be a boost converter where the control loop is designed to draw input current proportional to input voltage. This reduces harmonic content as well as improving power factor.

Ultimately I suspect that the odd model engineer using a 'dodgy' inverter isn't likely to register with the electricity comnpanies, unless they start monitoring power factor and/or harmonics on individual premises. And who's going to pay for the army of people who are going to oversee all the data collected?

Regards,

Andrew

Andrew to answer you question ..

"And who's going to pay for the army of people who are going to oversee all the data collected?"....the user

as to inverter drives,,, I describe a 5 to 35 hp FANUC unit. piddling I know but there it is.

Add to this mix a whole lot of solar panels dotted along the lines.

As this is a 3phase topic can I bend it slightly to ask if anyone has domestic (not small industrial) 3 phase and what the charges are?

My neighbour has a three phase supply as when his house was built it was fitted with electric storage heating. We live in rural Lincolnshire and the supply is overhead cable.....he has four cables going to his house which I presume are the neutral and 3 phases but someone allegedly "in the know" says that this is not true 3 phase. I have no idea but if it is 3 phase, to answer Bazyle's question, he doesn't pay any more for it than standard single phase users. I wouldn't like a quote to have it installed now though.

Mick

Michael: Thanks for your concern, but it does seem slightly gloomy. Your idea seems to be that once a problem, real or imagined, is identified we'll all be banned, or similar. If we take that idea to it's logical conclusion, given that life has so far proved to be 100% fatal for all concerned, everybody should be killed at birth, until life is extinct, and there is no danger of injury or death for anybody.

Bazyle: I have installed a 3 phase supply at home, running into a 3 phase distribution board in the garage and thence to the machine tools and a single phase garage ring main, plus a 32A single phase outlet. One phase is tapped off from the electricity board fuses via a normal consumer unit and supplies the rest of the house. As far as I'm aware I pay a normal domestic rate per kWh. I'm not at home to check, but I don't think my electricity bills even mention 3 phase, they just list the kWh at the two rates I pay per unit according to usage.

Regards,

Andrew

My house also had 3 phase electric heating when built before gas came into the street. So I have the feed (underground) right into my garage. That's why I'd like to know if it is viable compared to inverters. If I call the power company I will get "3 phase oh yes sign up with us for the 3 phases gas electric phone".

Andrew, that;s encouraging. Probably would be worth going further. I assume you have a separate 3 phase meter.

Edited By Bazyle on 08/03/2013 18:17:18

I'm curious, what is the voltage-per-phase in the UK? Is it actually 230 (give or take) volts or is it done in a similar fashion to North America?

Here, the phase voltage is nominally 115V. For domestic supplies, 2 phases are brought into the home and approx half the oulets are run off each phase. For such appliances as the stove (cooker) and clothes-dryer which are impractical on 115V, oulets are wired across both phases deriving 230V.

(And building a 115V device using more than one line-cord is strictly illegal!)

If, in the UK, a single phase is brought into each house then presumably different houses will use a different phase (of the 3) to balance the load. In that case, the supply authority would see all domestic consumption as a 3 phase load and if an odd (some might say very odd) homeowner used an actual three phase supply there would be no reason to charge him differently to anyone else.

(And yes, Andrew, CF's are net-capacitive and part of the admitted - here at least - reason for pushing their adoption is to offset the industrial inductive load).

Technically an inverter is a DC-AC converter. The digital two to thee phase converters are so called as they go via DC.

I suspect adding phase monitoring to modern electronic meters would be a doddle, and in the future I'm sure they will all wirelessly report our usage in near real time over the net - no humans needed for the monitoring

Spotting 'quirky' domestic users might become a hobby for companies increasingly forced to pit 'straight' users on cheaper., simple tariffs.

Neil

Hi Sid,
230 volt mains in the UK is the neutral to live (One of the three phases.) voltage. The voltage between phases is 230 x root 3 (1.732) 230 x 1.732 = 398 volts. I do not know how the distribution system is designed in the US but the voltage between phases of a three phase system with 115 neutral to live voltage woud be 199 volts. The way you describe it it looks like the two phases are 180 Deg out of phase. So it would look like being fed from a transformer with a 230 V secondry with the centre tap as the neutral. (Neutral being connected to ground at the substation here the transformer is located.)

Les.

I have a three phase supply direct into my house. It was put in for free by the old Norweb electricity company before the sell off under Maggie. Yes I do have three separate meters and have installed a three phase distribution board along side my single phase household system. The pro's are the ability to run three phase machines directly without having to fit new motors or inverters etc. this is great for grinders and most other machines where speed control is not really an issue. On ex-industrial machines there is always a speed change system either belts or gears or similar which give a useful range, however I still fit inverters (three phase models) to some of my lathes for the better control they give together with soft start and easy reverse etc. also suds pumps and feed motors work as intended.

The main con's with three phase is the problem of finding a supplier who can handle it !!! most companies who supply the home market simply have no provision to handle three meter readings they usually assume one meter to be an off peak one and the other to be a separate granny flat supply or something. This results in some pretty odd situations for e.g. when I got a bill for £73000 for one months bill because they thought my meter had gone completely round the counter and started again; they could not understand it was two separate meter readings.

There is only one supplier I have found who is willing to supply to me without classing me as an industrial user and all the problems that brings such as VAT, accounting, carbon levy or whatever. The other main con is undoubtedly the cost of installation now. I would imagine it is prohibitive.

I would be very reluctant to give up my three phase set up because it is undoubtably better for the type of machinery I have where everything is industrial in origin. Indeed some of my machines would be imposible to run in any other way because it is not possible to convert the specialised motors without huge expence and loss of power. The other great advantage of course is the amount of power I can draw without worrying about tripping the circuit breakers or blowing the mains fuses.

In N America, the simplest 3-phase is "208V" phase-to-phase voltage, which is formed by 3 phases of 120V phase-to-neutral, in the same way that the 240V in Europe translates to "415V" 3-phase. Just as in Europe, higher power (proper?) loads use a variety of higher voltages like 480V or 600V, as 208V isn't much of a voltage.

Domestic wiring in N America provides 2 "opposite" phases of 120V nominal, giving 240V for high power loads (cookers, driers, kitchen receptacles, workshops etc). If you move over from the UK, you can use your 240V gear with a bit of rewiring. All my stuff was fine, it just spins a bit faster!

In Europe, if you have a poor power factor, you are paying for power you aren't using, so the utility companies aren't too bothered. If you don' t like paying for that, you can fit your own remedies.

There are regulations governing the manufacture of high power equipment that limits the distortion (=harmonics) and power factor (angle), so unless some incredibly high power homebrew system gets connected up the the network there really isn't a problem. Some guy in a shed isn't going to even register on the radar.

I believe more than 50% of motors produced nowadays are inverter-driven unless I completely misremember. There's no real disadvantage to them apart from the initial cost, yet the benefits are several. For home users with no 3-phase, they are the obvious solution. The main requirement is that you need to connect your 3-phase motor in delta (240V) instead of star (415V). Some machines can't be reconnected this way. However, 3-phase motors are quite a bit more compact than single phase motors, so replacing one with a modern motor and inverter may still make a lot of sense.

Merry

Reactive power? - no such thing. Reactive current, ie., that drawn by an ideal capacitor or inductor is 90 deg. out of phase with the voltage so there is no power. Power is he vector product of current and voltage.

So to answer your question, Michael, as to what happens to this reactive power, the answer is nothing as it doesn't exist!

Russell.

Bazyle: I only have one meter, which is 3 phase. Before I had 3 phase installed I asked two questions. One, would I pay the normal domestic rate per kWh and two, would the 3 phase meter cope with the highly unbalanced load of the house on one phase and nothing on the other two, if I wasn't using the workshop. The answer to both questions was yes.

Murray: Thanks for the info on the North American supplies. While the UK is nominally 230V for single phase my voltage at home is normally above 240V, and it's not unusual to see it above 250V.

Regards,

Andrew

Official spec. for single phase in UK is now 230 -6% +10% so 216V to 253V, The UK suppliers like to keep it at the high end as it increses consumpltion and thus revenue! If they keep it normally at 230V instead of 240 their revenue would go down by about 8%

Russell.

Russell,

I'm confused, if something comes up on this forum I dont understand I research it. The reason i'm confused is that I have found several pieces that comment on Reactive Power, **LINK** but you say it doesn't exist, is it just the terminology that is confusing me.

Regards,

Martin Perman

Martin: It's a case of terminology. In an AC system there may be current flowing in the load that is 90 degrees out of phase with the applied voltage. This is generally known as the reactive current as it usually arises from reactance in the load from inductors, capacitors, or a combination thereof. Since the reactive current is at 90 degrees to the voltage the actual power dissipated by the reactive current is zero; which is the point I think Russell is making.

However, from the point of view of the generating company the reactive current at the load has to be generated in phase with the voltage at the power station, and hence it takes real mechanical power to generate it, which obviously costs money. I think the older style UK electricity meters, with the dials that went whizzing round, only measured the real power used and did not account for any reactive current. Hence the electricity board was paying to generate a certain amount of power, but only charging the customer for a lower amount; which could be regarded as a bad business model.

Regards,

Andrew

Yes it is the terminology. Power is the rate of flow of energy.

In an ac system power is given by P = I x V x cos( θ) wher I and V are the rms current and voltage respectively and θ is the phase difference between the voltage and the current. Thus, if θ is 90 degrees as it is with a reactive load then there is no net power flow. There is, of course a flow of current into a reactive load but it doesn't transmit power. If you like it "borrows" it. What they are talking about in that link is what we used to call VA or volt-amps.

If you are sure that you can work safely you could connect a large capacitor such as a motor capacitor across the mains output of a plug-in mains power and energy monitor. Tou will see that you can measure a significant current but the power will be zero.

Russell.