Reactive power loading

Hi Jason

Are you saying if i put a meter between my laptop 0 volt and earth it will read 200 volt?

In the past I have had the tingles from equipment because it wasn't earthed, faulty extention. I presumed it had floated up to half mains potential.

Cheers Michael.

With a high enough input impedence meter ..Yes in many cases..

Given that despite terminology we accept US outlets use two wires plus earth.

call one A and the other B..these are often derived "locally" from the secondary of a transformer...with earth connected to the center tap..55-0-55..so earth to "hot" is 55 V rms 60 Hz.

Measuring with your voltt meter will show earth to A 55 rms earth to B 55 rms and A to B 110 rms..

with a 'scope you see 110 V rms sine A to B.

.so how apart from voltage ( and frequncy) does this differ from uk/europe?

in uk A to B 240 (ish) B to earth little or none (in well balanced installation)

consider a filter capcitor A to earth and another from B to earth..for now lets call this mid point M

in us because A moves away from earth in exact opposite to B..===earth stays at midpoint.( more correctly midpoint stays at earth ) thus M to earth= 0V.

In Uk..A moves away from B and M follows at half A to B ..now if B = 0V to earth then M is 120 V rms...

Ok in it self But if the 0V of power supply is connected to M not earth..( or to M then earth via an imperfect current path) then this voltage appears on the power supplies output..above earth.

In double insulated equipment earth is deliberately not connected to the output or even to the input filters..creating this situation.

BTW

REACTANCE IS ......IMAGINARY

Only in the sense that it includes the factor 'j'. It may lead to complex impedance, but still results in real currents flowing in the real world.

Andrew

RESITANCE IS FUTILE!!!

... and all of the foregoing explains why, when teaching aircraft electrics to flight crew, if anyone mentioned reactive loads, the response was short, sharp and (as long as no ladies present) quite rude!! Life is too short!

Rgds

Bill

But IMPEDANCE IS COMPLEX..

( yes Andrew)

BTW as an engineer I want a jphone

Edited By jason udall on 12/03/2013 13:11:33

takes me back to the 1970's and complex numbers !

**LINK**

the forced adoption of a green world wide switch mode power supplies ,very often without a real on /off switch , has compromised safety

from exempting ATX computer power supplies from PAT testing to the use of leaky non earthed switch mode supplies used for digital TV's and satellite receivers laptops , external hard drives , modems etc

very often the insulation barrier between the live part of the psu circuit and what you expect to be the isolated DC output , is bridged by a capacitor and in some designs a 1M resistor

( see C27 on page 2 of the laptop psu design )

European Union has decided that the suppression of RF interference is more important than safety

with a single 25W power supply the leakage current should be "safe" , even though your voltmeter may show 80V AC or more , between the "isolated" DC output and earth

the inexpensive supplies made to a price are likely to be of questionable quality and more of a safety risk

when you have several non earthed pieces of equipment connected together

don't be surprised if the total leakage currents are enough to give you a shock (no prize for guessing how I know )

the now banned non green double insulated adaptors are likely to measure about 6V AC

laptop PSU design        http://www.onsemi.com/pub_link/Collateral/AND8397-D.PDF

ATX PSU design           http://www.onsemi.com/pub_link/Collateral/TND359-D.PDF

without power factor correction , the high pulsed current distorts the mains supply , flattening the top of what should be an AC sinusoidal waveform

this adds to the cost of ensuring supply is within specification

which adds to our utility bills !

     John

Edited By john swift 1 on 12/03/2013 16:44:42

Ah! A real engineer. Everyone else calls it "i".

Russell

Swarf,

The choke input filter is very widely used at high frequencies inside many SMPSU's and several times over on motherboards, it can be just half a dozen turns of wire on a ferrite core. At 240 V 50Hz however it is a bulky expensive part which has a larger than normal core because of the very strong direct current requirement. The peak current without a choke can be very high, if you want to draw 10A DC then the peak current goes towards hundreds of amps for a mS or so. Active PF correction uses an inductor at a much higher frequency so it is small and cheap, as Andrew has indicated you can solve the "chunk out of the mains" issue that happens when you have full wave capacitor input supplies.

This is also a reason why old style PF measurements can be very misleading because they often assume sinusoidial current and voltage instead of a massive current spike. The impulse does not help the Supply Companies or the Consumer, the current demand can cause the rectified voltage to be much lower than expected- certainly so on a long extension lead.

Jason's comments about IT testing are off track. In the US they have 110V line and neutral just as we have 240V line and neutral. Because many US outlets are unpolarised and do not have a third earth connection you cannot say which conductor is line and neutral so the inlet cap is between the two wires. Plastic insulated SMPSU's can have two wire connections with one inlet filter or three pin inlets with a delta filter using a third earth return ( sometimes called FG). There is no path between the input filter and 0V on the two wire types that I have seen.

All ATX supplies that I have inspected have delta filters and require an earth connection otherwise the metalwork floats up to half line due to the capacitor tap. The PAT inspection requirements for IT equipment are different because -in particular- the earth continuity high current test would cause extensive damage to the equipment. This is a legit outcome because only the PSU is a risk item, if you applied the test from power cord to any exposed metalwork you could burn out tracks on the motherboard and any other boards. It is certainly the case that many -if not most- PC's are badly made, leave out EMC filtering parts and put people at risk. That is how you can get an ATX PSU for £7.00 trade in the UK.

Billy.

Edited By Billy Mills on 12/03/2013 17:48:37

We need some consistency in nomeclature here. Consider a 3 phase power supply because most of us are familiar with such things. You can get three voltages from it, A-B, B-C and C-A which are 120 degrees apart and/or for a 4 wire 3 phase system you can get A-N, B-N and C-N, again 120 degrees apart but the voltage amplitudes are different. You can connect a 3 phase motor to A,B,C wires and get a rotating field in the motor which will make it spin. You can take any 2 wires, which some here are calling phases but you will only get single phase power from them and they will only work on a single phase motor which only works because of mechanical and mathematical trickery. so keep in your minds the diffence between "wire" and "phase'.

Merry--- I, in turn am not sure what you mean in 2. I think we are saying much the same thing here.

When I mentioned 2 phase in point 3, I was referring to 2 sinusoidal voltages 90 degrees apart which was used many many years ago. This system can generate a rotating field inside the motor and works perfectly well. It fell out of favour because it takes a bit more copper per HP in the powerlines. In fact with a bit of mathematics you can prove that 3 phase gives the minimum copper per HP of any of them. Actually the difference is not much. To come back to your point, two phases 180 degrees apart is single phase cannot generate a rotating field.

You are quite right, the distortion is high and this brings its own consequences in terms of what voltage a rectifier will deliver for a given AC voltage. The IEEE definition of pf is W/V*I where W is watts, V is volts and I is amps, all measured by electrodynamomer instruments; sort of old technology but there it is.

My understanding of a "rotary phase converter" is a system in which you feed a 3 phase motor with single phase on any 2 of the 3 power supply wires then get 3 phase from the 3 motor terminals. Sort of a buck-she arrangement but it's better than nothing if thats all you have. "M-G set" is a more general term in which you use a separate motor to drive a generator. This term is used for any such arrangement be it 1 ph AC to 3 ph AC, 1 or 3 phase AC to DC or DC to AC, any number of phases.

As I said, it is all in the nomenclature.

Ken

Hi there, all,

I still haven't located my copy of the TI application note - watch this space.

Billy, thank you for your post. The fact that, in a full-wave choke input filter system, the current flows for the whole half-cycle in each diode (rather than the extremely high but narrow current spike with a capacitor input filter ) is the unsung merit of the choke input filter. I did mention this in my initial post but perhaps I didn't talk it up as much as it deserves.

The choke inductance has to be greater than some critical value for the particular value of the load resistance. This varies as the load current varies, leading to the use of a 'swinging choke' in which a gapped core is used to control the saturation of the core and hence the inductance. I guess designing these is a dying art.

Sort of following on from Ken's post, I seem to remember from my college electro-technology lectures (some fifty-eight years ago) that in a three phase alternator, the lap and lead of the stator windings can be chosen so that harmonics are cancelled, improving the purity of the alternator output waveform. The harmonics arise because the rotor flux spacial distribution only approximates to a sinusoidal shape. Any more than that is lost in the mists of time and my aged neurons!

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 12/03/2013 21:45:06

There's a comrehensive explanation of electronic PFC here for anyone interested.

Russell.

Russell

Thanks for that reference. There is a lot of good stuff in it and it goes into my favorites

Ken

Swarf Mostly

My education in electrical engineering is the same vintage as yours.

In a rotating AC generator the major contributers to non-sinusoidal waveform, as I remember it are slot ripple which is high frequency and non sinusoidal flux distribution in the air gap which can give either a flat topped or peaked wave shape. Various games were/are played to reduce these effects such as slot skewing, shaping of the pole face and, I presume your lap and lead of stator windings although I'm not familiar with these names.

At one time I owned a small cheap and dirty AC generator which had a very large slot ripple and also an almost flat topped wave shape. About all I can say for it is that it would run a light bulb or small hand tool. I have a better one now which uses inverter technology and I'm very impressed by its wave shape. You cannot see any non sinusoidal component in it though it's probably not perfect.

In a large utility with many generators in parallel things like slot ripple tend to cancel out as the various machines have different ripples.

Ken

... and mine. This is where I learned about electrical machines. HSE would have a field day!

Russell.

Edited By Russell Eberhardt on 13/03/2013 16:06:10

Michael: I'm unable to find any specfic rules applying to domestic electricity consumers regarding the use of converters; can you elaborate?

Andrew

Billy - "The peak current without a choke can be very high, if you want to draw 10A DC then the peak current goes towards hundreds of amps for a mS or so." That's an old chestnut (myth). The theory is actually very simple and the worst case current (even with a hard supply and ideal components) is actually part of a (modest) cosine waveform, not a peak of indeterminate amplitude. I know, having spent many years in this field.

"It is certainly the case that many -if not most- PC's are badly made, leave out EMC filtering parts and put people at risk. That is how you can get an ATX PSU for £7.00 trade in the UK." It's illegal to sell PSUs (or any other potentially hazardous electrical equipment) in the EU that doesn't meet a whole raft of EMC and safety standards. If it's got a CE (or UL or CSA) mark, then you can be fairly certain it's significantly compliant. If you find that someone's selling such a component with the marking on but that you think isn't compliant, you should contact trading stds etc.

Nothing personal but we need to get a few things straight!

Interestingly, "passive PFCs" are simply large inductors on the input of passive diode rectified capacitive filter PSUs that smooth out the input current so that it's much closer to a sine wave. Of course, it doesn't actually lead to a very good power factor but it's cheaper than an active PFC for small (<300W) loads. The filter sections at the front of PSUs are actually aimed at preventing the switching voltage (noise) being conducted back into the mains and generally have very little effect on the current waveform. The legislated limits for conducted noise are of the order of millivolts and cover all frequencies up to 30MHz.

Merry

(1) Re : Electrical equipment .

**LINK**

All supply companies have a similar guidance document .

This is for industrial users requesting the utility company make a new connection for their facility. I think generally speaking, there will be very few plonkers in sheds that find their existing connection inadequate.

Meanwhile, the regulations governing the development and sale of electrical equipment address these concerns, so said plonkers will struggle to find equipment that cause those kinds of issues in the first place. Unless said sheds contain some very impressive experimental electrical equipment!

Those regulations cover the issues in the link (flicker, harmonics, EMC etc), as well as the concern in (2). You can start with EN61000 but there are quite a few others to be met as well. And that's before you even start on safety certification..... **LINK**

Merry

Edited By Murray Edington on 18/03/2013 18:36:53

Michael: Thanks for the link to the electricity supply guidance document. I'm pleased to say that in general I meet the requirements, as the biggest three phase motor I run DOL is 5hp. My TIG welder also comes in under the single phase limits. However, the less said about the air compressor the better.

Reading between the lines I assume that the document is aimed primarily at commercial users? I doubt many domestic users would understand the terms used, know how to measure the parameters mentioned or have the equipment for the measurements.

Regards,

Andrew