4

I have a question about a 3-phase 400V motor used by an hydraulic pump.

I read its currents and their graph is quite strange to me, far from the usual (deformed) wave.

What could be going on here? enter image description here

Alex Poca
  • 169
  • 1
  • 8
  • 2
    Looks like some significant harmonic content. Some kind of mechanical resonance maybe? – Solomon Slow Dec 20 '17 at 14:34
  • 2
    This graph is very satisfying to look at for some odd reason. – Redja Dec 20 '17 at 14:52
  • 1
    Not at that time based/frequency. –  Dec 20 '17 at 14:55
  • 1
    What's your sampling rate? Are you performing any average in your plotting (kind of default in Excel)? – winny Dec 20 '17 at 14:58
  • what type of pump? (i'm guessing a gear pump) ... how did you obtain the measurement? – jsotola Dec 20 '17 at 17:57
  • @winny and jsotola: it is possibly a gear pump (but I am not sure), sampling rate is 32kHZ, obtained with a Yokogawa Power Analyzer. I was graphing the values and was surprised to see such a big distortion. – Alex Poca Dec 21 '17 at 07:49
  • Graphing yes, but are you averaging by any chance? It looks just like a folding phenomonon. That, or some strange drive with peak rectification. – winny Dec 21 '17 at 08:49
  • No, I am not averaging. I need to check if there is some hidden param in the Yokogawa but after a first check there wasn't. – Alex Poca Dec 21 '17 at 08:56

1 Answers1

4

A motor like transformers has some excitation current that causes some <10% phase lead on current with respect to voltage (not given/shown.)

But what is shown is the current of each phase with a dominant 3rd harmonic current from the non-linear core ( partial saturation at peak voltage).

By hand drawing a similar sine wave with a 3rd harmonic and adjusting the phase and amplitude, I see that the 3rd harmonic is approximately 41% of the fundamental and the phase is 1.95 radians phase lead. Since reactance increases 300% with 3rd harmonic, we expect this for an inductor. But the core having partial saturation properties shows that the 3rd harmonic content is between 1/3 and 1/2 of the fundamental. THis is stored energy and not contributing to work by the motor. (VAR)

The pump load will also emphasize the current harmonics and sub-harmonics if a gear ratio is involved.(not observed or analyzed) as the load is not linear throughout the pump cycle.

enter image description here The fundamental above is normalized to 1 @ 0 deg phase near 50Hz with the 3rd harmonic only shown in yellow with composite waveform in red. You can see the website where to analyze FFT and settings that I used.
enter image description here

Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182