voltage swing for microwave?

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Old 03-06-20, 02:30 PM
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voltage swing for microwave?

I have small cheapo microwave only about 6 months old. This only happened once but it suddenly started to surge or fluctuate when heating... similar to how it sounds in defrost mode. The food did not heat correctly. I switched it off and on again and it worked like normal with no surging or lag like before. Im sure the power settings etc were all correct.

What voltage swing does a microwave typically need to see to work correctly? Could a microwave see too low of voltage so much that it wouldnt heat but yet the other functions would work (lights motor etc?)

I want to rule out an issue with my house wiring, I took voltage measurements after this occurred and everything appears normal but I am worried still because the microwave is more or less new.

 
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Old 03-06-20, 03:40 PM
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Plug in your meter and watch the voltage at the receptacle while the microwave is running. (Just heat a cup of water) Sometimes you need to have a load on the circuit in order for a loose connection to create an issue.
 
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Old 03-06-20, 04:20 PM
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I did test while heating a mug of water and i was only seeing it drop from 120 to 118. I even tested with a heatgun and the voltage drop was about the same.
 
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Old 03-06-20, 04:27 PM
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A microwave only heats at one level. It's basically full on. The way it's controlled in defrost mode or a lower heat setting is by turning the high voltage off and on. You should be able to hear it cycle as well as watch the interior light dim slightly when it goes into heating. This function is normally controlled by a relay. That relay may be going bad.
 
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Old 03-06-20, 04:56 PM
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That sounds like what i was hearing. Trying to heat a full power it was sagging back and forth and i could hear slight hum when the HV was likely turning on.

So i guess what im wondering is if low or fluctuating line voltage could cause the HV to cycle like that.

 
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Old 03-06-20, 05:08 PM
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If the voltage was low enough to cause a problem I'd expect the interior light to dim quite a bit.
 
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Old 03-06-20, 05:17 PM
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I say a drop of 120 volts to 118 volts when the microwave oven cycles on is no problem. From 120 to 110 volts (or a greater swing), I would try to do some investigating, look for a loose connection, or worse, might find a problem with the neutral that could damage the microwave or other stuff.
 
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Old 03-06-20, 05:34 PM
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The mw is on the same circuit as the kitchen lights and there was no light flickering at the same time this occured.... so maybe that suggests my problem is with the appliance.

Is testing voltage at the outlet alone sufficient to rule out a loose connection in the house wiring?
 
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Old 03-06-20, 06:52 PM
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It sounds like an appliance problem to me. You could move it to another circuit.
Carry it in and sit it on top of the washer and try that circuit,
 
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Old 03-07-20, 04:59 AM
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Im leaning that way too, it only did it that one time and didnt do it again while i was testing.

As far as hunting a loose connection in general... with a volt meter.... should you work from the outlet back to the panel? Like if a loose connection was somewhere between an outlet and the panel i am assuming you would not see voltage drop at the panel. In other words, you would only see abnormal voltages AFTER the bad joint.
 
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Old 03-07-20, 06:19 AM
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Correct, if you measure a big voltage swing when you draw a large current (a hair dryer is a good device to try) then you need to work back to the panel to look for a bad connection. Pay attention to the last good outlet or at the first bad outlet.

You
leave the hair dryer in the same place as you go test up and down the line.
 
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Old 03-07-20, 06:36 AM
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Thanks I hope it doesnt come to that kind of troubleshooting.

fwiw, I tested at the panel too. With a 1500W heatgun plugged into the microwave outlet I get about 3VAC drop at that outlet. At the panel there is .5 to 1 VAC drop on one leg with the heatgun plugged in. The other leg goes up maybe a half volt tops. I only tested for a few moments with the circuit loaded up like that... approx a minute. Hopefully that provides enough of a sample. Im guessing the additional vac at the outlet is because of the resistance in the wire (length).

To complicate matters, the circuit with the microwave is one leg of a MWBC. When I have the microwave circuit loaded up with the heatgun showing 3VAC of voltage drop the opposite circuit goes up about .5 VAC.

All of these voltage readings are the same as when I have tested in the past over the years. Im guessing a loose neutral would immediately show in the over +/-10% range as AllenJ suggested.


When doing these kinds of tests, how long should I monitor the voltages?... like i said, I only sampled the circuits under load for approx a minute.... does it take awhile for a high resistance connection to heat up and start showing improper voltages?


 
 

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