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Microwave not working properly with generator


ZFM

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Any help will be appreciated. Recently the microwave has not been working properly when using the generator. Its a 7.5kva so the load cant be too much. The microwave does switch on and work, only the clock runs down very fast. 30 seconds on the timer takes about 10 seconds.

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31 minutes ago, PsyWulf said:

Interesting,is this an older microwave that could be using direct power frequency/steps to generate a timer? Similar to how Fans act weird on a square-wave

As the timer is motor driven it would be strange to see it turning 3 times the normal speed. Also the genny cannot provide a frequency at 3 times normal 50Hz. The genny will then take off. :)

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1 hour ago, ZFM said:

@WannabeSolarSparky thanks, will get someone to check it out. Is this common though? I only have the generator for about 1.5 years and probably used less than 100hrs.

I am assuming it is a portable home type generator.
Most of these are mass produced in china so bad electronics creep through the cracks sometimes.
These machines endure lots of vibration, so if the regulator is not properly sealed the electronics can come undone and end up shorting or losing connections on the circuits.

My generator did the same sort of thing, turned out to just be a blown regulator. was about R250 to replace, though that was a few years back.

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54 minutes ago, WannabeSolarSparky said:

I am assuming it is a portable home type generator.
Most of these are mass produced in china so bad electronics creep through the cracks sometimes.
These machines endure lots of vibration, so if the regulator is not properly sealed the electronics can come undone and end up shorting or losing connections on the circuits.

My generator did the same sort of thing, turned out to just be a blown regulator. was about R250 to replace, though that was a few years back.

A petrol generator of tubular frame type does not produce pure sine wave but modified sine wave and you will also have fluctuating frequency when loading and unloading the generator. The AVR will not rectify this problem. Portable petrol generators should not be used in domestic enviroment they are build for power tool use on construction sites with extension cords. An inverter generator or diesel generators should be used if true sine output is required for domestic appliances.

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Petrol and diesel generators do produce "pure" sine wave power, just as Eskom does. The AVR circuit is used to excite the field coils in the alternator, and varies the coils voltage to maintain a "constant" output voltage. The frequency is a mechanical function of the RPM of the petrol or diesel motor, and the faster it runs or slows down, will change the frequency output. The mechanical governor on the motor tries to maintain a constant RPM. Petrol generators have poor torque compared to diesel, so the frequency can vary more under changing loads, but this should not cause your microwave clock to go so far off.

As others have suggested maybe something is loose or faulty on the generator, and causing all kinds of spikes and dirty power. Get someone with an oscilloscope to check the output under load.

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2 hours ago, TaliaB said:

These machines endure lots of vibration, so if the regulator is not properly sealed the electronics can come undone and end up shorting or losing connections on the circuits.

Yip, normally the AVR is sealed in potting compound to help with vibrations, but they do fail with use. When mine failed years ago, the voltage shot up to crazy highs, but  lucky my  high / low voltage protection saved my house.

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23 minutes ago, TimCam said:

Petrol and diesel generators do produce "pure" sine wave power, just as Eskom doe

Only the more expensive diesel generator will have pure sine wave like escom has the tubular petrol generators will not have a good sine wave THD>5% some close to 40%. Most tubular type generators are not allowed to be connected to your db board as most are V0V configurations.

 

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Just because someone posts something on Youtube doesn't make it the gospel  truth.  A generator is a rotating machine and by definition will produce a sine wave.  Yes, it has a finite output impedance and can be yanked around by a difficult load.  This could indeed produce a non-uniform wave with more THD distortion.  However there will be no switching noise from a genie and thus no high frequency harmonics.  The waveform will still be rounded.

The guy in the video above made one mistake.  No load.  An oscilloscope has a typical input impedance of 10 Megaohm.  There is something called the wetting current.  This is applicable to any form of electro-mechanical contact such as a switch or slip-ring.  A slipring might look smooth and clean to you, but under a microscope it looks like a mountain range covered with dust, oil films, oxides, moisture and all sorts of other gung.  The wetting current is the minimum current that needs to flow through such a contact to make a reliable connection.  I am not surprised to see the noise on that scope.  Different generators will have different slip-ring alloys, different finishing techniques applied, different brush loads, oxidation, etc.  When I was still in the defence industry, I spend many, many hours in technical review meetings discussing the non-compliance of slip-rings in missile systems and our supplier spend 100s of hours experimenting with all imaginable variables to try and improve matters.

@TimCam  is right about the RPM -> frequency variations and also the RPM induced voltage variations that an AVR tries to regulate but often fails to do adequately.   Diesel engines normally have much bigger flywheels due to the high compression and this would give a higher mechanical damping, improving the load regulation.  There is no reason why they can't mount a bigger flywheel on a petrol genie, except that it adds cost and weight... so they don't bother.  Run a genie at low loads or at constant loads.  Failing to do so will give voltage fluctuations that can be problematic and damaging.  The waveform distortion and even the frequency variations should not be a problem for most equipment.

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Re that youtube video: Was about to post the same thing, no load! but @Modina beat me to the punch.

These smaller gennys are terrible when they have no load. They pretty much use the same amount of petrol regardless of load, so just get a big fat 500W halogen lightbulb, and plug it into one of the generator's outlets.

Regarding the OP, I don't think it's an overspeeding engine, 3x the RPMs will no doubt cause other problems, like taking flight. Something is off with the AVR or other associated electronics. 

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1 hour ago, Superfly said:

Well.. IMHO (as always..) ..lots of my colleagues use generators during loadshedding  (for work PC, router etc) and seem to be OK electronics-wise.

 

Most stuff doing a AC->DC Conversion would somewhat clean up whatever junk they receive,that's why I alluded that perhaps the microwave is doing something analogue with the AC Input causing the clock to act funny

See this as an example of a Mains-connected Clock crystal

https://hackaday.com/2018/03/29/ask-hackaday-is-your-clock-tied-to-mains-frequency/

Edited by PsyWulf
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@JustinSchoemanNo, the load on the secondary of a microwave transformer is full-wave.  A microwave has one diode and one capacitor, so it can easily be assumed that it is using a half-wave rectifier.  However, the diode and capacitor are exchanged.  The transformer feeds the capacitor, and the diode is connected thereafter.  This acts as a voltage doubler and produces about 4000 to 4500VDC.  Voltage doublers rely on both half-cycles to be able to work.

I have no idea whatsoever what could cause a 3x clock speed.  This microwave uses an electro-mechanical timer or an electronic clock?  It isn't explicitly stated.  I am not even 100% sure if the generator is to blame.  I am asking myself if it might not be some peculiar earthing issue, but I can't put my finger onto anything specific.

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22 minutes ago, Modina said:

@JustinSchoemanNo, the load on the secondary of a microwave transformer is full-wave.  A microwave has one diode and one capacitor, so it can easily be assumed that it is using a half-wave rectifier.  However, the diode and capacitor are exchanged.  The transformer feeds the capacitor, and the diode is connected thereafter.  This acts as a voltage doubler and produces about 4000 to 4500VDC.  Voltage doublers rely on both half-cycles to be able to work.

I have no idea whatsoever what could cause a 3x clock speed.  This microwave uses an electro-mechanical timer or an electronic clock?  It isn't explicitly stated.  I am not even 100% sure if the generator is to blame.  I am asking myself if it might not be some peculiar earthing issue, but I can't put my finger onto anything specific.

Good point - never bothered looking how it was connected.  But even as a doubler, the current draw will be drastically asymmetric. Current will flow for the 1st quarter cycle (charging the capacitor), and then nothing until near the next peak in the opposite direction (when the magnetron threshold is reached, and it almost instantly goes to max possible current draw). So it will actually have far worse harmonics than I originally thought.

But the test stays the same, turn on a big resistive load and see if the problem goes away.

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@JustinSchoeman  Yes correct.  The overall load will have a poor power factor.  It will look very much like any other device where DC rectification is performed without additional PF-correcting circuitry.  The difference here being that a microwave is an extraordinary heavy load, while normally DC power supplies are seldom over 300W or so.

I recently noticed that one can now buy "inverter" microwaves - at a hefty price.  Perhaps I was asleep and have not noticed them before, or they are now new on the market.  So obviously they chucked out the big and heavy 1.5KVA+ transformer and replaced it with a PF-corrected high-voltage SMPS.   I wonder if an inverter can somehow drive a magnetron at less than full power.  Currently magnetrons are driven bang-bang on-off, in the same way that cheap solar chargers, euphemistically referred to as PWM, work.

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The clock problem on the microwave is frequency related as microwave clocks work on input frequency. If the frequency from the generator is out by 5hz it will influence the timer speed. If the formula is used f= 1/t then 50hz = 20ms. Check your generator frequency with a kill a watt meter especially when load on generator changes.

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9 minutes ago, TaliaB said:

The clock problem on the microwave is frequency related as microwave clocks work on input frequency. If the frequency from the generator is out by 5hz it will influence the timer speed. If the formula is used f= 1/t then 50hz = 20ms. Check your generator frequency with a kill a watt meter especially when load on generator changes.

Not easily to take only 10sec for a 30sec time? 

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I still don't know if this is an electro-mechanical or an electronic timer/clock.

All electro-mechanical timers use the 50Hz mains.  In the old days, all mains powered clocks where also using the 50Hz mains.  These days many clocks are using 32Khz crystals with battery back up.  So the rule is this, if the clock looses it's time during a power outage, it is not battery backed up and relies on 50Hz.

@TaliaB nobody disputes your claim but what is in dispute is a 200% error.  A genie can be out by 20% but if it runs 3x faster than designed for, you will likely pick up pieces of the conrod that slammed a hole through your engine block and spewed hot oil all over the place.  You would also have run for cover before that engine disintegrated...  What does a genie run at?  I think 3000rpm ?   Now imagine it doing 9000rpm.🙉🙈

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1 minute ago, Modina said:

I still don't know if this is an electro-mechanical or an electronic timer/clock.

All electro-mechanical timers use the 50Hz mains.  In the old days, all mains powered clocks where also using the 50Hz mains.  These days many clocks are using 32Khz crystals with battery back up.  So the rule is this, if the clock looses it's time during a power outage, it is not battery backed up and relies on 50Hz.

@TaliaB nobody disputes your claim but what is in dispute is a 200% error.  A genie can be out by 20% but if it runs 3x faster than designed for, you will likely pick up pieces of the conrod that slammed a hole through your engine block and spewed hot oil all over the place.  You would also have run for cover before that engine disintegrated...  What does a genie run at?  I think 3000rpm ?   Now imagine it doing 9000rpm.🙉🙈

We don't need to find out what type of clock mech as it appears the microwave has gone dead-LOL

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