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Author Topic: AC outlet  (Read 8674 times)

Johnny Diaz

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AC outlet
« 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?
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John Roberts {JR}

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Re: AC outlet
« Reply #1 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
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Johnny Diaz

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Re: AC outlet
« Reply #2 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? 
« Last Edit: January 20, 2014, 10:23:32 pm by Johnny Diaz »
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Lyle Williams

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Re: AC outlet
« Reply #3 on: January 20, 2014, 09:46:22 pm »

Everyone else in the street is also applying load to the circuit...
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Ivan Beaver

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Re: AC outlet
« Reply #4 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.
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Mike Sokol

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Re: AC outlet
« Reply #5 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...
« Last Edit: January 21, 2014, 07:55:53 am by Mike Sokol »
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John Roberts {JR}

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Re: AC outlet
« Reply #6 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.
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Johnny Diaz

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Re: AC outlet
« Reply #7 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?
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John Roberts {JR}

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Re: AC outlet
« Reply #8 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


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Johnny Diaz

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Re: AC outlet
« Reply #9 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 )? 
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