Speaking of noise and troubles with VFD's. Those who followed my thread about the market truck with the VFD's: we had this week serious roubles with them and it's even not solved yet. Leak current devices are tripping all the time.
But yesterday, we think we found the cause of it and it's really a difficult theoretical case.
Now it comes, trying to explain this in English.
The earth leak current protectors (LCP) of 30 mA trips constantly due to an 'imaginary' leak, a leak it thinks there is but it isn't. It's a theoretical leak.
So I need some feedback from you guys in this forum regarding the fact that a VFD has zenerdiodes in it's electronic circuit to generate the sine wave.
A zenerdiode measures the 0-passage of the sine wave.
There is somewhere a theory that this device has a voltage drop of 0.6 V before it let through the current. This drop results in a DC signal.
This DC signal is present/transposed in the total sine wave signal.
Now, those who understand the function of a LCP, this device has coils in it who measure the flux of the voltage going in and flux of voltage going out.
As long as both fluxes are equal, everything is OK because what goes in must go out your motor (or any connected to the LCP device)
So, if they’re equal, there's no leak.
Now,... what we have and in fact with every VFD is that all what's going toward the VFD is a pure sine wave (with the common garbage on the net) in his pure AC-form but what returns is 99,5% (just a number) and a little bit of DC, but the algebraic sum together remains the same.
But the DC returning can't generate a flux in the coils of the CLP. Remember school: you need an AC current to generate flux in a coil.
So the CLP thinks that there is an unbalance and shuts of the VFD but there is no real leak. It's an imaginary leak.
Some questions now to look if someone is able to help me and has some experience in this special case.
Most questions are already answered by my son - it was he who found together with the experienced VFD tech I spoke with in the beginning of this thread the solutions - but I think many can learn from these problems which cost me al ot of my hear and some days of my life .
We measure at standstill no earth-leak at all (megger), nowhere...how can we measure then that there's a leak? (we have +/-50mA/VFD now, measured this morning)
Remember, the LCP is jumping as soon the VFD's are starting.
Do the members know how the measure this sort of earth leak (we don't measure any fault with a megger and in some cases like a VFD, it is even forbidden to megger)
We installed special EMI filters from Schaffner - the biggest European company located in Germany - and this didn't solve anything.
It was even worse.
What can be done to prevent this? We have no 2 working solutions, rather expensive; perhaps there are more we don't know yet. Eager to learn these.
Does someone know a good source for this theory, Fourrier analysis, the theory behind the conversion of some of the AC in DC which is then transposed on the main sine wave, the theory that a sine wave we all know consists in fact of many sine waves in a mathematical row (saw this long ago in school but forgot most of it), why distortion on the net has something to do with the uneven ‘members’ or factors in this mathematical row (if this is correct of course)
What's the difference between an EMI and an EMC filter and the theory behind it?
So, I want to learn about all this a little bit more and I think some of us can also benefit from this.
End of the day: earth leak is now 1mA/VFD :D :D
