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I have a beautiful old frequency meter from a power station, a "System Frahm" type panel based on vibrating reeds. It has two rows with 21 reeds in each, each going from appx. 45 Hz to 55 Hz. It says that it accepts 110 volts.

I want to be able to provoke the instrument to indicate frequencies of my choice, between 45 and 55 Hz. If possible, different frequencies on the two rows, but it's not mandatory.

I have access to a VFD, the Delta VFD004S21A 220V 1-Phase model. I know it can generate 3 phase power at the frequency and voltage that I need, but will it be able to run without driving a motor, or any other load? Would it work if I added a small 3 phase motor? Would I be better off with some kind of audio power amp + step-up transformer solution?

Frequency meter

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4 Answers 4

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You can get stereo sounds synthesized in Audacity (free) and sweep each channel then playback with a PA and stepup XFMR using a stepdown wallwort.

I 've never seen these ABB historic instruments but I see they have medical interest in cochlea demonstration, perhaps in your area of interest. You probably only need 10 watts at rated voltage. (guessing)

They actually are tuned at 2f each using relay armature mechanical pulse being broad spectrum. (I think)

https://peerj.com/articles/1333/#fig-1

enter image description here

Alternatively you could easily measure the armature R to estimate current and feed it from a transistor and clamp diode from a sig. gen with 10Hz +/-10% with 10% duty cycle using lower as this may be enough energy with pulsed DC voltage and 5th harmonic.

  • or many other methods... the Q of 50~100 of resonant reed wil reject any PWM noise or VFD carrier noise.
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    \$\begingroup\$ I like this idea. A VFD does make "sinewaves of any frequency" but this is PWM at some carrier frequency (8kHz is common.) If this is looked at on a 'scope or better yet FFT - Yowzers! VFDs are a noisy mess. Would it affect this instrument? Unknown. But Tony's idea is the proper way to go. \$\endgroup\$
    – rdtsc
    Commented Aug 31, 2017 at 1:29
  • \$\begingroup\$ Sorry, I'm not very experienced with this kind of electronics. What would the wiring with a sound signal going through a power amplifier and a stepup transformer combined with a stepdown (wallwort) transformer look like? What kind of power amplifier and stepup transformer would I need? \$\endgroup\$
    – Magnus
    Commented Aug 31, 2017 at 14:12
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    \$\begingroup\$ 120:24 Vac doorbell transformer with 10VA (or 10W rating) minimum is 5:1 used in reverse from a stereo amp with at least 10W/channel can then boost signals 1:5 at bass frequencies 45-55 Hz to drive meter. Audacity is free software like a Studio Sound Mixer/recorder with generators AM/FM sweep, single/ dual tone with tons of features including Spectrum Analyzer. Just use AUX out to AUX in or speaker out to power amp in whatever works to drive transformer secondary input instead of speakers. Use a DMM to verify levels before connection and after. \$\endgroup\$
    – D.A.S.
    Commented Aug 31, 2017 at 16:16
  • \$\begingroup\$ Your suggestion worked perfectly, Tony. I tried with the VFD as well, but as soon as I connected one of the phases (a single wire), the meter growled loudly. It may have found a low-resistance way to ground through some existing wiring? I didn't dare try again :) \$\endgroup\$
    – Magnus
    Commented Sep 25, 2017 at 12:09
  • \$\begingroup\$ good to know.. The Q of the tuning fork is define by half power f/delta f. so if you can ring 1 bar only that implies Q= 50/1 . where a few hundred is possible, \$\endgroup\$
    – D.A.S.
    Commented Sep 25, 2017 at 13:22
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The VFD should have no problem operating without a load. If that is easily available, I see no reason to use something else for a one-time demonstration. If you want a permanent setup, you might want to construct something. You might want to run a test to see how much power is required.

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Yes, it is not only possible but very doable.You may need a step up transformer based on your voltages.

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    \$\begingroup\$ Please don't use links, they break and then the answer is worthless, put all relavent info in post and\or provide a synopsis or screenshot of the video \$\endgroup\$
    – Voltage Spike ♦
    Commented Aug 31, 2017 at 2:53
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Very cool meter by the way. Given that it says "Kristiania" on the faceplate, that dates the meter to pre-1925 when Kristiana's name was officially changed to what we know today as Oslo, Norway.

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  • \$\begingroup\$ The question was "Can I use a VFD to send power of a specific frequency to a frequency meter?". Your answer doesn't address this at all. It should be a comment. \$\endgroup\$
    – Transistor
    Commented Sep 5, 2017 at 22:39

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