Generator "Dirty Power" 4

[VicP]

My son bought a cheap, 5kw no name brand Chinese made generator to power his RV when not connected to utility power. The generator is a true AC generator, not an inverter. If he runs only the microwave from the generator, the clock/timer on the microwave runs too fast, about twice normal speed. If he runs the air conditioner with the microwave, the clock runs at normal speed. I have checked the frequency of the generator and it is very close to 60Hz. Researching the problem on the internet, it seems this is a common problem with certain generator/microwave combinations. The general consensus is the generator is producing "dirty power", i.e. a distorted sine wave which is producing extra triggering of the clock circuit. I do not have an scope available to check the waveform. The fact that running the motor in the air conditioner makes the clock run correctly would seem to indicate this is the case. The AC motor smooths out the waveform.

How could a generator produce a bad enough sine wave to make the clock run so fast?

 

[davidbeach]

Harmonics producing additional zero crossings.

 

[waross]

It is at times a challenge to check the frequency of a small set with some frequency meters.
But not only generators. Some frequency meters, although they are rated down to 60 Hz, are intended primarily for RF and have a hard time checking the frequency of the grid due to harmonics and RF noise on the power line.

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

It is not the generator that is at fault. It is the Graetz (two-pulse or full bridge rectifier) of the Micro that, by "sucking" two Heavy current pulses each period adds even harmonics to the generator voltage.

You can usually overcome this if you add a PF capacitor to the load. Properly chosen, the capacitor will handle the 120 Hz current surges and reduce harmonics. A larger generator with less impedance will also work. But that would not be very practical.
An easy way out would be to set time to twice what you want when you run the generator with no other load than the Micro.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[HamburgerHelper]

I think that you would want a generator with more impedance. Adding an additional choke would help block the harmonics that are causing your clock to incorrectly trigger. I think the power electronics in your microwave with its resonant transformer is generating harmonics.

http://www.wseas.org/multimedia/journals/systems/2014/c085702-389.pdf

You could choke the microwave like this.

http://palomar-engineers.com/rfi-kits/acdc-power-line-chokes

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If you can't explain it to a six year old, you don't understand it yourself.

 

[Skogsgurra]

HH, the clock is IN the Micro. So you can't better the situation by adding impedance to the source. It only makes things worse.

And the toroids you refer to are for common-mode noise not normal mode. And not for noise in the 100 Hz region, which makes the clock run twice as fast, but for noise in the MHz+ region.

Less impedance it is.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JG2828]

It would be interesting to see what the output of that generator looks like on an oscilloscope.
I came across the below forum showing a few different examples of generator output waveforms. I could see this one causing some issues with clock timing if it is based on falling edge triggering. Good luck filtering that out.

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28115992/print/true.cfm

 

[HamburgerHelper]

Skog,

What about buying a 60hz power line filter from mouser?

https://www.mouser.com/Power/Power-Condition/Power-Line-Filters/_/N-axgnp?P=1yzu6vb

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

A couple of things to consider:
The clock will reset after a power failure and the time will not be correct.
The generator frequency will vary between 61.8 Hz and 60 Hz with the droop governor that is common on small sets, so even after you set the time and correct the issue, the clock won't keep accurate time.
Your best, cheapest and most dependable option is to buy a cheap, battery powered clock and set it on top of the Micro-wave.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

That's a dreadful generator waveform, probably from a cheap & nasty capacitor-regulated type rather than a machine with an AVR. I agree with Bill about buying a cheap battery-powered clock. Most microwave ovens either don't care about the time or just need it setting to some arbitrary value to enable operation.

HH - those filters are intended for much higher frequency noise. Massive distortion of the generator output due to harmonic currents / high peak currents isn't really 'noise' - more of a mismatch between generator and load.

 

[Skogsgurra]

HH, no matter from where you buy a filter. It will not help because it is INSIDE the Micro the problems are. Not in the generator. The phenomenon is well known, as shown by JG2828 6 Aug 18 19:22. The Schaffner filters and all others increase impedance and makes the problem worse. BTW, they have cut off frequencies way above the "problem frequency".
I think that I will have a look at my own Micro. With and without filters and/or added impedance. Stay tuned.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[HamburgerHelper]

I suppose a simple fix would be to get a mechanical timer switch or just get a microwave that runs with a mechanical timer.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[VicP]

Thanks for the replies. I wasn't looking for a fix for the problem, he doesn't use the generator that often so he just adjusts the run time to compensate for the fast clock. I was more interested in the reason why this was happening. I think it is probably a combination of the distorted waveform from the generator and the harmonics produced by the microwave.
ScottyUK, I believe the generator is a capacitor regulated type, there is a large capacitor mounted behind the panel. I didn't think the output waveform would be so bad.

 

[Skogsgurra]

OK, this is what an old (around 15 yrs) does to a perfectly Clean mains voltage when there is some inductivity (around 50 mH) in the path.

So, the generator isn't necessarily the culprit. High source impedance (expected in a small generator) plus distorted load current are to blame.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Great work Gunnar.
Can we add to that a cheap and dirty triggering circuit for the timer/clock counter?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Thanks. Primitive zero cross detection. Absolutely. I have been designing trigger Circuits for SCR (DC drives) and we had to use the mains voltage with commutation notches in it. Needed a two-pole filter for that. Don't think they have that in Micros.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VicP]

Gunnar
Thanks for doing this. Out of curiosity, was the microwave clock keeping the correct time with this waveform?

 

[Skogsgurra]

There is no clock. Just a set of LED:s that step down until finished. I didn't check the pace of them. Should've done that. But it wasn't that easy to arrange the measurement. So I don't think that I will do that again. Someone else, perhaps? With a more modern Micro...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[krisys]

Such a high harmonic generating equipments shall not be connected to the mains unless the harmonic injection form the load is brought down to the acceptable level.

So the equipment under discussion is non complying to the EMC requirements. Hence should not be used.
So you cannot claim anything from the generator.

 

[ScottyUK]

krisys,

I imagine that the microwave oven complied with whatever regulations prevailed at the time of manufacture. When regulations change they are rarely applied retrospectively. Saying it shouldn't be used isn't reasonable.