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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Johnny Diaz on January 20, 2014, 07:04:47 pm

Title: AC outlet
Post by: Johnny Diaz on January 20, 2014, 07:04:47 pm
I tested my 120V 15amp outlet with an oscilloscope and this is what I found.  Why are the peaks a little disfigured?  I tested other outlets on different breakers and same result.  What does this indicate?
Title: Re: AC outlet
Post by: John Roberts {JR} on January 20, 2014, 08:30:48 pm
Most electronic equipment power supplies, only draw current at the very top and very bottom or the waveform. When the real resistance of distribution wiring, interacts with the current draw at the peak, you see the waveform flatten out like that.

The whole purpose of PFC (power factor correction) is to spread out the current draw to reduce the flat top. The legislation in Europe talks about harmonics as in distortion and deviation from a pure sine wave.   

You need to review what load is connected to that mains drop to understand what is causing the waveform to flat top.

JR
Title: Re: AC outlet
Post by: Johnny Diaz on January 20, 2014, 09:43:38 pm
Most electronic equipment power supplies, only draw current at the very top and very bottom or the waveform. When the real resistance of distribution wiring, interacts with the current draw at the peak, you see the waveform flatten out like that.

The whole purpose of PFC (power factor correction) is to spread out the current draw to reduce the flat top. The legislation in Europe talks about harmonics as in distortion and deviation from a pure sine wave.   

You need to review what load is connected to that mains drop to understand what is causing the waveform to flat top.

JR

JR,

I didn't have any load on the circuit.  When you say mains drop are you talking about the main hots coming into the service panel from the utility company?  I have a 100amp service panel.  Total load cant be more than 20-30%.  Biggest load would be the refrigerator at the time.

So when there is no load anywhere I would see a nice round sine wave at the peaks?  When there is a big load the peaks will start to disfigure like what I have? 
Title: Re: AC outlet
Post by: Lyle Williams on January 20, 2014, 09:46:22 pm
Everyone else in the street is also applying load to the circuit...
Title: Re: AC outlet
Post by: Ivan Beaver on January 21, 2014, 07:49:18 am
Everyone else in the street is also applying load to the circuit...
Agreed.

The "sine wave" starts at the power generation facility (whatever type it is).

Then everybody "gets a piece of it".  Yes the voltage will go up and down through various transformers-but is not "regenerated" at your house.  The resultant waveform will maintain its shape through th etransformers-only the voltage will change.
Title: Re: AC outlet
Post by: Mike Sokol on January 21, 2014, 07:53:15 am
Everyone else in the street is also applying load to the circuit...

Power isn't always pretty. Many (most) loads are non-linear. Of course 3-phase motors and resistance heaters are linear, drawing constant power from the line. But CFL bulbs, computers, old-school power amps, and all kinds of industrial processes draw power in "peaks".

It would be interesting to run your waveform through an FFT analyzer and look for harmonics. I'll bet these distortions are being cause by both by non-linear loads in the neighborhood, as well as POCO power transformers running near their limits. Power transformer overload will also cause flattening of the AC waveforms.

I've often heard Honda inverter generator owners brag that their power is cleaner than what comes from a wall outlet. There's probably some truth to that...
Title: Re: AC outlet
Post by: John Roberts {JR} on January 21, 2014, 08:42:21 am
It's a little interesting that your distorted waveform is still delivering 123V so perhaps the distortion is not at your drop. If you look closely at the waveform you can almost see where the diodes start conducting current and the sine wave stops rising so fast. You can see the affect if the PS reservoir caps charging up, in the slight tilt up, until the diodes stop conducting until the next peak. Since the current is only being drawn during that small fraction of the entire time, the peak current must be several times higher than the average current. 

For typical unregulated, non-PFC, audio amplifiers how high the waveform sine wave goes on peaks will directly affect the power output.  Imagine a puny extension cord in series with your power amp. Since it's mainly drawing current only during those short peaks, the voltage drop (E=IxR) only occurs then, distorting the waveform even more, and delivering less peak voltage. An average reading voltmeter will understate how much power amp capability you lose while the distortion and peak voltage drop is related to peak current not average current.   

In the euro zone where the power infrastructure in most places is older and smaller** wires than ours, they have legislated the power factor of loads to smooth out the current draw over the entire waveform for less voltage drop. This increases the power delivery capability several fold over rectified power extraction. 

JR

PS For a while it looked like all power amps would have to be PFC but AFAIK they decided the current draw from PA amps was insignificant to the entire power distribution network so postponed the regulations for them. Consumer products (over there) AFAIK do have to be PFC. 

PPS: The ugly secret about CFL lamps is that they use basic diode-cap power supplies, so present a nastier load to the mains than old school incandescent lamps, but this was tolerated because the CFLs use so much less current for equivalent light output, The longer term solutions like LEDs are more mains waveform friendly.   

** The euro zone also uses 230V for outlet distribution and appliance use, so current draw is roughly 1/2 for the same power from US 120V outlets.
Title: Re: AC outlet
Post by: Johnny Diaz on January 21, 2014, 09:09:05 am
Mike,

"Power transformer overload will also cause flattening of the AC waveforms" 
Is this similar to when you drive a line level signal to clip and it turns from a sine wave to square wave? Would I be correct In saying that my picture shows that my signal is partially clipped?  When I call the utility company what exactly should I tell them? 

John,

"Most electronic equipment power supplies, only draw current at the very top and very bottom or the waveform. When the real resistance of distribution wiring, interacts with the current draw at the peak, you see the waveform flatten out like that. "

Is this always the case when slipping occurs?  So only the peaks start turning into straight lines.  I know the source is bad but can this be resolved with a voltage regulator?
Title: Re: AC outlet
Post by: John Roberts {JR} on January 21, 2014, 09:26:47 am
Mike,

"Power transformer overload will also cause flattening of the AC waveforms" 
Is this similar to when you drive a line level signal to clip and it turns from a sine wave to square wave? Would I be correct In saying that my picture shows that my signal is partially clipped?  When I call the utility company what exactly should I tell them? 

John,

"Most electronic equipment power supplies, only draw current at the very top and very bottom or the waveform. When the real resistance of distribution wiring, interacts with the current draw at the peak, you see the waveform flatten out like that. "

Is this always the case when slipping occurs?  So only the peaks start turning into straight lines.  I know the source is bad but can this be resolved with a voltage regulator?

Power transformers have simple resistance too, just like wires. The magnetic circuit inside transformers can saturate if there are not enough magnetic domains to hold all the energy being passed. A 50Hz transformer need to be 16% bigger than 60Hz transformer, since the same average current must be transferred in 50/60 th the number of waveform peaks per second.

What exactly is the problem you are trying to fix? FWIW I haven't metered that many outlets so I don't know if yours is all that horrible, or even unusual..

JR


Title: Re: AC outlet
Post by: Johnny Diaz on January 21, 2014, 09:41:52 am
Power transformers have simple resistance too, just like wires. The magnetic circuit inside transformers can saturate if there are not enough magnetic domains to hold all the energy being passed. A 50Hz transformer need to be 16% bigger than 60Hz transformer, since the same average current must be transferred in 50/60 th the number of waveform peaks per second.

What exactly is the problem you are trying to fix? FWIW I haven't metered that many outlets so I don't know if yours is all that horrible, or even unusual..

JR

I just tested the outlet out of curiosity and this was the result.  No real problem.  Is this something I should look into further with the utility company?  Is it unrealistic to think that outlets can deliver a clean power ( clean sine wave )? 
Title: Re: AC outlet
Post by: Keith Broughton on January 21, 2014, 10:17:44 am
I just tested the outlet out of curiosity and this was the result.  No real problem.  Is this something I should look into further with the utility company?  Is it unrealistic to think that outlets can deliver a clean power ( clean sine wave )?
I would say it's unrealistic to get a pure sine wave...IMHO
Title: Re: AC outlet
Post by: John Roberts {JR} on January 21, 2014, 10:49:14 am
The only time I ever called my utility was when my home outlet voltage hit 130V one night and was still climbing. I noticed that my incandescent lamps were unusually bight.  The guy on the phone didn't believe me, but it was slow night so they sent out a truck. They metered my drop and discovered I was not imagining things, and then fixed "their" problem at the sub station.

Waveform purity is generally not a major concern unless it gets significant. I am not aware of throwing distortion analyzers on mains power being common practice.

JR
Title: Re: AC outlet
Post by: TJ (Tom) Cornish on January 21, 2014, 11:21:55 am
I just tested the outlet out of curiosity and this was the result.  No real problem.  Is this something I should look into further with the utility company?  Is it unrealistic to think that outlets can deliver a clean power ( clean sine wave )?
Nope, and Yep.

This doesn't look unusual to me, and as JR said, since you're still getting 123 volts, it's perfectly within spec.

We spend a lot of time fretting about "power quality", but unless things are really bad - much worse than your scope trace shows, things are totally fine.  The biggest issue we face is voltage drop from high-current loads at the end of too long/too small cords.  Our industry should host a mass bonfire to rid the world of 16-ga orange extension cords, followed by mass psychotherapy to rid the idea that that 20,000 watts of amp power and 100,000 watts of PAR lights are necessary for 50-cap bars.

On a related note - some folks say that generator power is cleaner than the mains.  This is true in many cases - scoping the output of a Honda EU generator looks textbook great - until you actually start using the generator - then you're back in the same game as with any other power - it's the load that messes up the power, not the source.  The power grid has a lot of electrical inertia to deal with this - small generators have considerably less.
Title: Re: AC outlet
Post by: Kevin Graf on January 21, 2014, 12:52:19 pm
To worry about a clean sine wave from the power company is a total waste of time.  If you examine the current wave form (not voltage wave form) to a power amplifier, you will see a wave form with between 75% and 150% THD (Total Harmonic Distortion).
Title: Re: AC outlet
Post by: Tim McCulloch on January 21, 2014, 01:23:14 pm
To worry about a clean sine wave from the power company is a total waste of time.  If you examine the current wave form (not voltage wave form) to a power amplifier, you will see a wave form with between 75% and 150% THD (Total Harmonic Distortion).

Plus 1, bingo, etc. Not sure why the OP thinks this is even worth pursuing.
Title: Re: AC outlet
Post by: TJ (Tom) Cornish on January 21, 2014, 01:39:23 pm
Plus 1, bingo, etc. Not sure why the OP thinks this is even worth pursuing.
OP was trying to learn.  I have no issue with that.  Much of one's success in life is being able to tell the difference between things that matter, and things that don't.  Now the OP has one more thing to add to the "doesn't matter" list.  I'd like a few more of those in my life, too.  :)
Title: Re: AC outlet
Post by: Jonathan Johnson on January 21, 2014, 01:49:32 pm
To worry about a clean sine wave from the power company is a total waste of time.  If you examine the current wave form (not voltage wave form) to a power amplifier, you will see a wave form with between 75% and 150% THD (Total Harmonic Distortion).

Yep. A lot of people (not accusing the OP of this) are conditioned to expect perfection in everything and are unwilling to accept reasonable variations from the ideal**. They simply don't understand what constitutes acceptable deviation, or why. Pretty much everything (except anally-retentive consumers) can tolerate some irregularities in the delivered product, whether it is the water going to your dishwasher, the gas pressure to your furnace, or the electricity to your whatever.

I applaud the OP for asking here before complaining. Seeking information -- "is this normal?" or "do I need to look into this further?" are the right questions to ask. Going to the power company with a zero-tolerance attitude saying "MY POWER ISN'T PERFECT!!!1!!" annoys the power company, irritates those whose equipment generates harmonics if the power company makes them do something about it, and ends up raising everyone's prices in a cumulative, roundabout way. And the customer receives no benefit from perfect power, except for a pretty picture on his O-scope.


**Except when it comes to the spelling and grammar of their own complaints in public forums and social media.

P.S. -- To paraphrase a famous theologian, "Understandest thou what thou meterest?"
Title: Re: AC outlet
Post by: Johnny Diaz on January 21, 2014, 01:59:58 pm
The reason why I posted this question is for informational purposes only and would like to thank everyone for their contributions.  I actually never metered an outlet before with an oscilloscope.  When I saw the sine wave I noticed it appeared to be clipping at the peaks and posted it here knowing that the experts would chime in. 
Title: Re: AC outlet
Post by: Stephen Swaffer on January 21, 2014, 04:27:32 pm
I worked in an industrial facility and checked the waveform out of curiousity from time to time it typically looked much worse than yours.  That was understandable since we were running a bunch of inverter based welders, VFDs -a bunch of ugly stuff from a noise standpoint.  My understanding is that a waveform like that can cause motors to run less effeciently and thus warmer-but from a practical standpoint it is not worth pursuing unless you are running really big stuff.  My employer at the time only got concerned when the poco decided to charge more because we were causing so much distortion that it affected their ability to deliver clean power to other customers-then we installed PFC correction at the incoming switchgear.
Title: Re: AC outlet
Post by: Josh Millward on January 21, 2014, 04:28:30 pm
The reason why I posted this question is for informational purposes only and would like to thank everyone for their contributions.  I actually never metered an outlet before with an oscilloscope.  When I saw the sine wave I noticed it appeared to be clipping at the peaks and posted it here knowing that the experts would chime in.
Which was exactly the right way to approach this kind of thing.

Hopefully this discussion will encourage others to understand the situation and realize why, when they see the same kind of thing that you saw, it looks that way.

Great discussion! This is why I frequent this forum and not others.

Many thanks to everyone who contributes.
Title: Re: AC outlet
Post by: Mike Sokol on January 21, 2014, 04:34:24 pm
Great discussion! This is why I frequent this forum and not others. Many thanks to everyone who contributes.

This really is a great brain trust of contributors. I really enjoy moderating it... Keep those questions (and answers) coming.
Title: Re: AC outlet
Post by: Mike Sokol on January 21, 2014, 07:57:14 pm
I just split this topic to cover overheated extension cords at

http://forums.prosoundweb.com/index.php/topic,147961.0.html (http://forums.prosoundweb.com/index.php/topic,147961.0.html)
Title: Re: AC outlet
Post by: Tom Bourke on January 22, 2014, 12:38:23 am
Which was exactly the right way to approach this kind of thing.

Hopefully this discussion will encourage others to understand the situation and realize why, when they see the same kind of thing that you saw, it looks that way.

Great discussion! This is why I frequent this forum and not others.

Many thanks to everyone who contributes.

Something else to consider is if the scope is safe to hook to mains on the measurement side.  For example my RIGOL is only rated 300V CATI.