Multimeter question - reading Hz

[wayne50]

Kind of related to welding. I just bought a fairly cheap multimeter (Electronic Specialties 480A) that also has a Hertz scale. On my house current it reads 60.0 as expected. On my Devilbiss 5000 Watt generator the meter will show 60.4 for an instant and then show 181.2 Hz. It should read about 60 Hz. I know this is 3 times what it should read but why? Cheap meter??? Lower price generator??? The generator will run fans, grinders, radios just fine. Just curious if anyone else had seen this.

[MAC702]

Hmmm, 181.2 is exactly 60.4 x 3. Something going on there. Any other generators to try? Have you reconfirmed the tester on household AC again?

[84ZMike]

It could be low batteries or a bad connection

[Zrexxer]

I don't possibly see how a bad connection or low battery could result in frequency readings three times what they should be. However, I'm watching this thread with interest, since I'm planning on investing in a meter capable of giving me frequency readings from a generator, also.

In some of the research I've done online, I have read that inexpensive meters that don't read true RMS voltage can be off by several multiples when trying to read AC frequency. I can't reconcile that with your results of exactly 60 Hz from the mains, but it's maybe something to consider if your meter isn't RMS.

[bandsawguy]

I had a Greenlee meter when I was working for a different company. It was reading voltages too high. instead of 600 or 604 it was giving me readings of like 620. I took it in to a local supplier and before we sent it out for repair he tried a fresh set of batteries. Just like magic it fixed it.

[84ZMike]

I had a Greenlee meter when I was working for a different company. It was reading voltages too high. instead of 600 or 604 it was giving me readings of like 620. I took it in to a local supplier and before we sent it out for repair he tried a fresh set of batteries. Just like magic it fixed it.

WOW go figure that muliti meters with low batteries will cause crazy readings..... I have one that will read 145 volts form a 12 volt battery

[Sandy]

Odd that it's just happens to be the third harmonic. Wonder if it goes away with a little load?

[Vector]

The output of the generator probably isn't a pure sine wave by any means, and as Sandy suggested, teh harmonics could be strong enough to be counted by your meter.

[enlpck]

lower end generators often have less than great waveforms. Many frequency meters have, in addition to a low pass filter with cutoff near the max frequency it will read, also have a high pass filter with a half power frequency in the ballpark of a bit over 60Hz to help reject line frequency interference on low signals, the idea being that a signal you are looking for is likely to be higher in frequency than 60Hz and may be lower in amplitude than the 60Hz pickup in some situations. The waveform doesn't need to be that bad to have a strong enough third harmonic at 180Hz to read that rather than the 60Hz fundamental.

[Jake_SS]

The frequency is relative to the speed, are you measuring with the generator under load? If not try that, or reduce the speed until you get 60Hz and then measure the output at load and unloaded

[Blacksmith]

I suggest he leave it alone and load it. The odd numbered harmonics are stronger and I think he is seeing the third harmonic; load may change the relationship such that the meter starts picking up the first i.e. 60 Hz.

[moe1942]

On a generator you are better off just just measuring voltage, amps and RPM.

Hz will be RPM divided by 60. No load RPM should be about 3700-3750. AC induction motor equipment doesn't like Hz under 58 or over 62 for any length of time. If you want to check gen Hz plug in an elec clock and time it.

If all you power is universal motors don't worry what you feed them.

[wayne50]

Thanks for all the replies. As mentioned it must have something to do with the harmonics in the output of the generator. I have only one generator to check with and I did check it when it was pretty well loaded with an electric heater. I go back and check my house current with the same meter and it reads 60.0. Maybe a more expensive meter such as a Fluke would read as supposed to or a much more expensive generator might give a cleaner output which would read correctly.

[enlpck]

On smaller generators, you want to check voltage and frequancy under a moderate load, as you did. They will vary some with load, though the inverter types (like the little Honda's) do a much better job than the straight alternator types.

If you really want to see, hook up an oscilloscope. (disclaimer: If you don't have some experience, this can be dangerous, as you are dealing with lethal voltages) You will likely see the waveform is flattened a bit at the peaks relative to a pure sine wave. If you know anyone that is deeply into electronics, see if they have a low frequency spectrum analyzer (or a "harmonic analyzer" intended for analyzing power sources)

In the grand scheme, I doubt very much that it matters. If you are between 59 and 61Hz, you are fine. If you are seeing anything between 177 and 183Hz, then you are seeing the thid harmonic, and the frequency is fine.

[usmcpop]

A little blurb about portable generator harmonics: http://bremacair.com/Gillette.htm