We have an off grid home, solar powered 120v. While running the home on the backup generator (also charging batteries), the drum motor on the propane dryer has an increasing noise, then gets to a point where it trips the breaker on the generator, but the breaker on the manual transfer box (30 amps) and the 20amp breaker in the main panel are unaffected. I have check voltage and frequency at the dryer outlet with the generator running (dryer not plugged in the outlet), 119-121 volts and 59-61 hz which should be very acceptable. Also in a possible related sense, we have 4 ceiling fans 2 of which also get the hum situation but no issue with breakers.

3 Answers 3


Is your generator producing true sine wave, modified sine wave, or square wave output?

The motor's designed to work on sine-wave AC, which is what large generators are happiest producing, and which is generally the most efficient form since all the energy goes into the single frequency and can be easily drawn back out.

Square wave is easier for small alternators to produce, but has a lot of high harmonics which can cause resonance in motors, sometimes to the point of being damaging. Square-wave alternators may also be damaging to some electronics, for similar reasons.

"Modified sine wave" is a compromise between the two -- it adds another few steps between the most-positive and most-negative outputs. This reduces the harmonics to a level that most motors and electronics can handle safely. There are still going to be some harmonics, which may be made audible by the motor, but they shouldn't be strong enough to be damaging.

If you've got a square-wave unit, consider replacing it. If you've got a modified-sine unit and are still having trouble... well, switching to true sine wave will probably solve it, but you might want to get someone with expertise in this system to take a look at your set-up before you commit to that purchase.

  • I'm assuming any sort of doohickey that smooths out the wave costs more than a new gen-set (or don't exist)? Also, what about feedback?
    – Mazura
    Commented Nov 25, 2014 at 2:18
  • (1) Yes, a filter would be difficult. Simplest would be a motor-generator, at which point you might as well just drive the generator directly from the engine... (2) I don't know, what about feedback? I don't see where the concept is applicable to this question.
    – keshlam
    Commented Nov 25, 2014 at 2:25
  • I guess I don't understand where CEMF goes. I thought it would be bad for the gen-set and since you'd have to pay to upkeep it; a concern.
    – Mazura
    Commented Nov 25, 2014 at 2:50
  • 1
    CEMF goes into making the motor work harder to turn the generator. Really. Run a generator with no load and it's just a spinning mass of metal; put a load on it and the back-EMF produces magnetic forces which resist that spin. (Remember that a generator and an electric motor are almost the same concept, just used backward.) Some of it is also lost as heat, of course, just as some of the power the load uses is lost as heat. In other words: It exists, it's normal operation, it's the nature of the beast, and it's designed for.
    – keshlam
    Commented Nov 25, 2014 at 4:16
  • I can run tests again while the genset is running loads-inverter battery charger and the lights, etc that we use while charging which is mostly at night so we're charging AND running the house off the other leg of the generator. I haven't checked the purity of the sine wave (I believe it is from my research on B&S larger generators), I think I'll need an oscilloscope for that. I could see where the engine harmonics could affect the waveform too, if that is what some of the talk here is addressing. I as well thought that the dryer motor may not accept the distortions.
    – user29838
    Commented Dec 15, 2014 at 23:09

It sounds like the inverter is producing a cleaner sine wave than the generator. Your TRMS meter might show 120 volts and 60Hz but isn't telling you the whole story. In other words, the sine wave is distorted with harmonics. The hum is telling you the story.


I am the one that posted the original issue. After research and consideration of what is happening, I believe that what is going on is that there is an imbalance in the way the generator/inverter is wired. I looked at the inverter/charger wire diagram and I am sure that the leg that feeds the charger is the same that is powering a larger portion of the home requirements, that being those kitchen lights, washer and dryer. The other leg is feeding a tv, a couple living room lights and a well pump that has a max of 1200 watts when running which should be a non-issue. We run the generator at night as most usage is lower at that time. I am going to swap the two feeds at the main breaker. Thinking is that what is being heard is the generator going into surge (the hum that starts low and increases) and then trips the breaker. A key thing happened is that I discovered the inverter generator was operating along with the circuits that run the dryer etc, and the twist lock had backed out some and there was no power to a portion of the home, that being the other leg from the generator...I thought the gen had an issue but it was the connection at the twist lock. I'll be back with the answer.

  • 1
    Hello, and welcome (back) to Stack Exchange. You should try to merge your accounts, so that you can get your old reputation. Commented Oct 21, 2016 at 19:51

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