vfd and multimeter

[BaronJ]

Hi Guys,

The few VFD's that I've looked at and measured the carrier frequency of, have all been in the 35 to 100 Khz range. Frequencies in the audio range tend to be avoided.

The scope pictures posted show the sort of wave forms coming out.

[PStechPaul]

There are analog true-RMS-DC converter ICs that implement the conversion function:

https://circuitdigest.com/tutorial/t...using-ad736-ic

https://www.maximintegrated.com/en/p...er/MX536A.html

There are also "old school" analog meters which use a small heating element and thermocouple which can measure much higher frequencies.

https://www.electronicspoint.com/for...mmeter.278388/

I also worked on a design that uses an incandescent lamp and photocell, but there was a problem with the lamp aging and changing its luminous output over time. It may have been possible to compensate for this by using two matched lamp/photocell pairs and driving one with a known DC current to match the RMS current in the other. This meter could not be externally powered because it was needed to very accurately measure the anode filament voltage of some sort of X-ray tube operating at 20,000 volts or so. IIRC the original design used two thermocouple bulbs, and they were extremely expensive.

https://www.eevblog.com/forum/beginners/how-to-see-effect-of-true-rms/

[J Tiers]

Hi Guys,

The few VFD's that I've looked at and measured the carrier frequency of, have all been in the 35 to 100 Khz range. Frequencies in the audio range tend to be avoided.

The scope pictures posted show the sort of wave forms coming out.

What units were they?

Generally, the higher the power of the VFD, the lower the frequency. I can clearly hear the traction VFD signal on the local transit trains, sounds like around 3 to 5 kHz, and locomotives I have also heard, seems like they are n the same range, although there is a lot of noise for the inverter noise to be heard through.

Most lower power do not (or did not) go to the frequencies you mention, but with better IGBTs that is possible.

I did one "inverter" with a 450kHz carrier, it was about 3 HP, but it actually drove a speaker, it was an amplifier of 2500W output. No IGBT at the time could do that without a fancy resonant system that we did not have time to develop, so it used large MosFets doing "hard switching". (It passed emissions testing, too!)

[PStechPaul]

I have seen videos of subway and light rail trains where it sounded like the carrier frequency increased in discrete steps as the output drive frequency increased. That makes sense, as more PWM cycles will produce a cleaner waveform at higher frequency. The duty cycle will also be higher because of V/F characteristics, and perhaps the longer ON time might cause saturation of the motor windings.

[BaronJ]

There are analog true-RMS-DC converter ICs that implement the conversion function:

https://circuitdigest.com/tutorial/t...using-ad736-ic

https://www.maximintegrated.com/en/p...er/MX536A.html

There are also "old school" analogue meters which use a small heating element and thermocouple which can measure much higher frequencies.

https://www.electronicspoint.com/for...mmeter.278388/

I also worked on a design that uses an incandescent lamp and photocell, but there was a problem with the lamp aging and changing its luminous output over time. It may have been possible to compensate for this by using two matched lamp/photocell pairs and driving one with a known DC current to match the RMS current in the other. This meter could not be externally powered because it was needed to very accurately measure the anode filament voltage of some sort of X-ray tube operating at 20,000 volts or so. IIRC the original design used two thermocouple bulbs, and they were extremely expensive.



https://www.eevblog.com/forum/beginn...t-of-true-rms/

I've got a couple of those thermocouple sensors kicking about, they look a bit like the old "Acorn" tubes !

[BaronJ]

What units were they?

I didn't take a lot of notice ! I believe that one was a Siemens 2 HP unit on a conveyor drive. You don't get a lot of time to prat around on a production line.

[J Tiers]

Ok, figured since you had the specs, and may have been setting parameters, you might remember.

It is rather a high frequency even for a tiny 2 HP drive.

[BaronJ]

Hi JT, Guys,

There was all sorts of kit in that place, VFD's of various makes. One of them was "Motion Control Systems" device. That one was nasty, driving a guillotine across the width of a moving belt and no synchronisation to the belt speed. So you had to adjust the chop speed to match the conveyor speed to get a fixed product length. I lasted about a year before they put me on permanent nights. I left soon after.