Hello,

Today I tested 3 different Inverters for the quality of their sine waves and got what I consider unusual results.
If you are proficient with Oscilloscope and could shed a bit of light on two of the odd ones it would be appreciated.

1). Test of House Grid AC (just to verify oscilloscope was functioning properly)



2). Brand New Bestek 500Watt Pure Sine Wave Inverter. I have never seen a double trace like this and don't know what it means. Do you?



3). HarborFreight 400Watt Chicago Electric inverter - I have no idea what this is or how it could make usable power. What could this mean?



Thank you for any assistance

 

#2 likely a scope trigger artifact.
#3 is what you would see if the scope probe is not connected properly (high pass signal only) across the inverter output lines. The spikes are likely the edges on a modified-sine multi-step inverter.

 

Would a Scope Artifact be unique to that one and only device?

The artifact does not show up when checking the house AC current nor anywhere else nor have I ever seen it.

Is it also more likely to be a defect in the brand new inverter I received?

Very curious as to how the problems are not present when testing the house AC current and no matter how many times I go back and forth between the three there is consistency in the results?

In other words, if it was a problem with the probe, how would it only affect one device and none of the others consistently no matter how many times I test each of them?

 

Hello there from a safety perspective with common probes connected to the mains is a bad idea!
#3
Harmonic distortion from a non-pure sine wave inverter.

My mistake I just made this more confusing for you @nsaspook answer to your question is eloquently precise.

 

I have no doubt that any person with advanced degrees in electronics or a lifetime of experience in the field would wholeheartedly agree.

#2 likely a scope trigger artifact.
#3 is what you would see if the scope probe is not connected properly (high pass signal only) across the inverter output lines. The spikes are likely the edges on a modified-sine multi-step inverter.

"Likely" a trigger artifact. A Google search did not necessarily support that assumption.

For the fun of it.......

Let's say #2 is not a trigger artifact........

What else "could" it be ?

Thanks

 

I have no doubt that any person with advanced degrees in electronics or a lifetime of experience in the field would wholeheartedly agree.


"Likely" a trigger artifact. A Google search did not necessarily support that assumption.

For the fun of it.......

Let's say #2 is not a trigger artifact........

What else "could" it be ?

Thanks

Your inverter shouldn't produce, just by itself, a real-time waveform that looks like that trace on #2. It's possible some utility 60Hz signal is mixing with the 60Hz inverter signal for a beat (the difference in phase and frequency between the two) trigger that results in the strange double sine wave scope display.

 

If you are proficient with Oscilloscope and could shed a bit of light on two of the odd ones it would be appreciated.

The intrinsic faintness of the information provided requires illumination please.

Make,model ,oscilloscope ,calibration date, scope probes specifications active,passive,differential,
LF compensation. where in the circuit are you taking your measurement. Oscilloscope settings.

 

2). Brand New Bestek 500Watt Pure Sine Wave Inverter. I have never seen a double trace like this and don't know what it means. Do you?

Try this again using slower timebase speeds. You may see some some lower frequency amplitude modulation of the waveform.
Or possibly using a faster timebase to look at a small section of the doubled part of the waveform where you may see some higher frequency signal added to the sine wave. I think this is the more likely of these two.

 

Just for clarification, Bestek sells many Inverters both MSW and PSW. This one is advertised as is a PSW Inverter.
It also oscillates the amplitude of the signal.
I have never seen this before and I have many other other inverters and NONE do this.
I do not understand how it can provide usable AC if the sine wave drops flat periodically as the video shows.
Maybe an oscilloscope expert can identify this pattern?

By the way.....I bought another, it arrived....and produces EXACTLY the same oscilloscope results.
This leads me to belive it is some odditity in the way they designed the circuit and therefore the way it appears on an oscilloscope, but is ok otherwise?

Interestingly, when the unit is switched OFF, the sine wave becomes a normal, expected AC Pure Sine Wave.

 

Just for clarification, Bestek sells many Inverters both MSW and PSW. This one is advertised as is a PSW Inverter.
It also oscillates the amplitude of the signal.
I have never seen this before and I have many other other inverters and NONE do this.
I do not understand how it can provide usable AC if the sine wave drops flat periodically as the video shows.
Maybe an oscilloscope expert can identify this pattern?

By the way.....I bought another, it arrived....and produces EXACTLY the same oscilloscope results.
This leads me to belive it is some odditity in the way they designed the circuit and therefore the way it appears on an oscilloscope, but is ok otherwise?

Did you measure the AC voltage with a good meter under load when you were having sine wave drops?

It seems to me your o-scope is seeing something funky on the inverter AC output but the visual presentation on the screen of that distortion is strange. This guys scope pictures are a clear and stable display of the inverters AC output changing under load.

 

Interestingly, when the unit is switched OFF, the sine wave becomes a normal, expected AC Pure Sine Wave.

That, IMO tells the tale.

 

Interestingly, when the unit is switched OFF, the sine wave becomes a normal, expected AC Pure Sine Wave.

That, IMO tells the tale.

Sure. But you still cannot understand the amplitude changes?

 

Did you measure the AC voltage with a good meter under load when you were having sine wave drops?

It seems to me your o-scope is seeing something funky on the inverter AC output but the visual presentation on the screen of that distortion is strange. This guys scope pictures are a clear and stable display of the inverters AC output changing under load.

Yes, he has a newer oscilloscope. I have an old Hitachi V1050F.
Sry, can't invest in a new one at this time.
Also, it's probably my poor use of the camera more than anything.
I wasn't concerened about making a good visual presentation, just one that showed the issue.

A few problems (or differences) with his test.......

1). His initial sine wave was with the unit switched ON. He got a steady sine wave.
Neither of the two I got exhibit that behavior.
Both only exhibit a steady sine wave when switched OFF.
Once switched ON with no load, the sine wave amplitude changes. So off the bat his is different.

2). He used the cables that came with it to connect it to the battery. Big No No.
The cables that come with these have been shown to be only capable of carry 300watts (+/-)
I made 2 custom cables using 10Ga OFC copper with solid tinned copper ring terminals soldered to both ends.
Far better. in addition, I also simultaneously connected the OEM cables that he used so that my connection was capable of carrying far more load.

3). His sine wave picture at 4:20 was completely different than mine. No idea why.
I am wondering if his was provided to him by the company for testing. Both of mine were purchased directly and randomly from Amazon with my own money.

4). Neither of my units presented the "noise" he displayed. Under load mine remained much cleaner as far as the sine wave.

I also did another load test today. it passed with flying colors. 499Watts for 10 minutes with no issues.

At this point, I'm inclined to chalk it up to some fancy electronic wizardry that no one here can fully understand. Me included.
It seems to work fine and holds the load. I did send an inquiry to Bestek Tech support about this so let's see if they reply what they say.
I've decided to keep this one and hope for the best. I'll be back for advice on repairing it when it fails haha.

 

Good visual presentation is very important when the only factual information we have as clues is that visual presentation.

Sure. But you still cannot understand the amplitude changes?

Sure I can. If there is a 60Hz background on the scope display (inverter switch off) mixed with a very close to 60Hz signal (inverter switch on) they will add and subtract as the phase of the two signals change in relationship to each other.

https://forum.allaboutcircuits.com/...s-please-help-me-identify.178113/post-1617555
It's possible some utility 60Hz signal is mixing with the 60Hz inverter signal for a beat (the difference in phase and frequency between the two) trigger that results in the strange double sine wave scope display.

 

Yes, he has a newer oscilloscope. I have an old Hitachi V1050F.
Sry, can't invest in a new one at this time.
Also, it's probably my poor use of the camera more than anything.
I wasn't concerened about making a good visual presentation, just one that showed the issue.

A few problems (or differences) with his test.......

1). His initial sine wave was with the unit switched ON. He got a steady sine wave.
Neither of the two I got exhibit that behavior.
Both only exhibit a steady sine wave when switched OFF.
Once switched ON with no load, the sine wave amplitude changes. So off the bat his is different.

2). He used the cables that came with it to connect it to the battery. Big No No.
The cables that come with these have been shown to be only capable of carry 300watts (+/-)
I made 2 custom cables using 10Ga OFC copper with solid tinned copper ring terminals soldered to both ends.
Far better. in addition, I also simultaneously connected the OEM cables that he used so that my connection was capable of carrying far more load.

3). His sine wave picture at 4:20 was completely different than mine. No idea why.
I am wondering if his was provided to him by the company for testing. Both of mine were purchased directly and randomly from Amazon with my own money.

4). Neither of my units presented the "noise" he displayed. Under load mine remained much cleaner as far as the sine wave.

I also did another load test today. it passed with flying colors. 499Watts for 10 minutes with no issues.

At this point, I'm inclined to chalk it up to some fancy electronic wizardry that no one here can fully understand. Me included.
It seems to work find and holds the load. I did send an inquiry to Bestek Tech support about this so let's see if they reply what they say.
I've decided to keep this one and hope for the best. I'll be back for advice on repairing it when it fails haha.

View attachment 235263

We can understand it, your measurement and presentation system is defective. His works correctly.

 

We can understand it, your measurement and presentation system is defective. His works correctly.

True. I'm not going to make YouTube channel quality videos to ask a simple question.
I hear those high quality YT presentations are quite labor intensive.

Thank you so much for the assist.

I have to respectfully disagree with your statement above, however because the evidence points to me having superior test cables,
That alone could easily account for the noise and other differences. Also, the measurements seem to be fine.....it's the interpretation that I lack. Hitachi knew what they were doing. Me? Meh, it's just an occasional interest.

The reference to phasing possibly causing the shifting sine wave seems helpful.
I'll look into that.

 

Hi,
Did you tried it with a consumer (a light bulb or something)?
If the reading becomes sinusoidal, then I think you are looking to a bad capacitor.

 

Brand New Bestek 500Watt Pure Sine Wave Inverter.

With this inverter, when it is showing the strange output on the 'scope, what does the LED show?

 

...consumer (a light bulb or something)?

In other words did you try placing a load on your inverter output. An incandescent light bulb makes a good test load as it should have minuscule reactive impedance. Your sine wave will not be distorted by such a load.

 

There is a ground clip on oscilloscope probes.
Where did you connect the ground clip?
Is your oscilloscope grounded through the power cable?