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I need to measure the AC frequency of a 230 V electric generator (gasoline powered). I have a cheap but ok multimeter that does Hz but the reading is erratic (some 1 kHz...) The reading on the mains is a perfect 50 Hz. I also have a TDS-220 Tektronix oscilloscope but I'm scared to blow it. Any idea why I would get that strange reading, or how to safely use my scope?

Edit: I got myself a cheap Wall wart meter and got 60Hz. I suspect that its got only 2 readings, 50 and 60Hz and because of the noise, it switches to the highest. It should read 50Hz. I will try some more tomorrow. One question though: is it normal that there is so much noise on the line?

Edit 2: I managed to get correct readings. I was getting like 53Hz+ but managed to find how to regulate the engine RPM so I can get around 50Hz, but I'm still getting fluctuations of about +-2.5Hz without load, and about -+1Hz under load. Voltage also fluctuates around +-1V at 228V. Do you think this is normal for an entry price generator from a known brand? The wave looks noisy with no load and distorted under load.

enter image description here

And this under a 2.4K load from a crappy electric heater. enter image description here

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    \$\begingroup\$ Erratic reading is probably due to "noise" on the voltage. a simple RC low pass filter MAY work. Try eg a series 10 K resistor and a mains voltage rated and mains use intended capcitor. 0.1 uF are reasonably available - giving about 1000 Hz filter frequency - which may be too high. Note that Winny's excellent idea requires an iron core AC in AC out plug pack. ie NOT an electronics based plug pack. \$\endgroup\$
    – Russell McMahon ♦
    Sep 19 at 12:28
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    \$\begingroup\$ @RussellMcMahon Good point, I’ll clarify. \$\endgroup\$
    – winny
    Sep 19 at 15:48
  • \$\begingroup\$ thank you @RussellMcMahon. Just one question: is it normal that there is so much noise on the line? This is a fuel-powered electric generator. \$\endgroup\$
    – brunobhr
    Sep 20 at 21:38
  • \$\begingroup\$ @brunobhr Murphy says that there will lways be noise :-). How much will vary with implementation. They may be regulating the output in some manner. The waveforms may be trapezoidal and not sinusoidal and upsetting the meter. || Long ago I had an el cheapo petrol powered 230 VAC output alternator. Some equipment was not overly happy with the output. I put it through a 1:1 isolating transfomer which usefully impoved the results. \$\endgroup\$
    – Russell McMahon ♦
    Sep 21 at 1:43
  • \$\begingroup\$ hello @RussellMcMahon, I had to give the answer winny, but I finally achieved the measure through an AC-AC transformer..But as you helped me also a lot figuring that thing out, would you mind giving me your insight about my Edit 2? thanks! \$\endgroup\$
    – brunobhr
    Sep 25 at 13:33

2 Answers 2

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A small AC-AC wall wart (low output voltage) or otherwise transformer will interface the high voltage towards your oscilloscope nicely and will preserve the frequency unless you go too low and saturate it. Please note that it needs to be a real line frequency transformer (heavy), not a switch mode AC/AC intended for low voltage halogen lamps (lightweight). Your load is practically zero so a very low VA one is fine.

If you have to measure frequencies below 50 Hz, feel free to simply pick one with higher primary voltage rating. Urms*f is constant in this regard so 25 Hz at 230 V would require a 460 V rated transformer at 50 Hz.

Your other options are a high voltage probe or your own voltage divider.

If you want to stay with your multimeter, you could consider a low pass filter for it.

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A simple option that might work is to download a mobile phone frequency meter application and see if you can reliably measure the engine speed by sound. You might have good success with a guitar tuner application as these tend to try to pick out the fundemental frequency of the guitar string which will always have harmonics (multiples of the fundemental frequency). You can probably assume that the generator is close to the rated frequency and figure out how to scale the meter readout. e.g. If the meter reads 26 Hz then the alternator is probably outputting 52 Hz.

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