Electronics > Beginners

DMM Hz wrong? Or equipment under test problem

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horse_ebookmarklet:
Hey,
I am getting a reading I don't understand on my ExTech EX330, 10 years old, but fresh batteries.
I have this old junk generator (not gen inverter!) 130232-1839-01 I've been fixing. Its loud and heavy, but it was free.
I'm trying to set the unloaded frequency to ~62Hz, but my meter is giving me inconsistent readings. Sometimes they're believable (off by a few Hz), sometimes its off in the weeds (double the frequency)

In the below video you see:

* Me turn the meter on
* Meter reads ~55Hz for a bit
* Meter jumps to ~160Hz
* Me turn the meter off, then back on
* Meter reads ~55Hz for a bit
* Meter jumps to ~160Hz
Since it gives a plausible answer when first powered on (55Hz) and the motor speed is not varying, I think my measurement is suspect.
Whats my source of error here? Generator kicking off such noisy power the meter is getting confused?

BeBuLamar:
I think your generator generates a lot of harmonic and the meter is picking up on the harmonic.

RoGeorge:
My guess would be that's either a bad DMM, or the generator is putting out a waveform with lots of 3rd harmonics (probably without any load, the generated fvoltage goes too high, then some other circuit is limiting to keep the AC at 110V, resulting a square waveform instead of sinusuoidal AC - square waveforms have a lot's of odd harmonics, the 3rd one is almost as big as the main frequency).

I would try adding a few incandescent lightbulbs as a purely resistive load (so not LEDs or CFL lights), then try to measure again, to see if adding some resistive load to the generator will make the frequency measure a constant 60Hz.

If not interested in extra testing, don't bother connecting light bulbs, just let it be and divide the number on the DMM by 3, so adjust the motor for 62x3 = 186Hz on the DMM without load.

BrokenYugo:
Number leads to a Briggs engine, assuming typical 3-5kW chinese import style genset and 60Hz power.

Hz is heavily tied to RPM, a 2 pole genset should have a unloaded high idle around 3700-3750 for about 62hz (max) no load, right on 60.0 around 50% load (space heaters work well, know how the outlets are wired so you can balance the loads between the two 120 windings) the engine would experience connecting rod failure or valve float before it spun fast enough to make 160Hz, so this has to be the typically dirty waveform they put out confusing the meter.

I usually use a killawatt or similar for generator governor setup, no annoying auto off issues. My AN870 (only DMM I have that does frequency) also has never failed to give me a sane reading as far as I can remember, that said this isn't something I do often.

Also, remember, these things are far from perfect and absolutely not high precision machines. Don't chase a perfect reading, just get it close enough, verify the no load speed isn't too high, and confirm the governor behavior is stable when you put a big load on it. Run a good tough oil in it for reliability, I prefer 5w40 diesel truck oil. Always flip the fuel valve off and run it dry before storing.

Zero999:
Use an oscilloscope and a small mains transformer to look at the output waveform.