I don´t see the point of the isolation transformer (unless it helps with bearing currents?).
Again, I’m not a vfd guy. Throwing in my 2 cents and hoping to learn. Anyone correct me if I’m wrong.
It seems the purpose of the isolation transformer would be
1 - limiting high dv/dt pulses at the motor;
2 - limiting common mode voltlage so as to limit bearing current.
Neither of these goals seem to have been accomplished.
#1 – we can see in your graphs that we have very high magnitude and high frequency content of thes pulses.
I would say #2 is not accomplished either. Here is my simple understanding of common mode voltage: we switch a given phase between + and -. The + and – are voltages with respect to ground since they derive (via rectifier) from input power which is grounded. Since we have 3 phase system, we always have 2 on + and 1 on – or 1 on + and 2 on -….. either case represents common mode voltage that can encourage zero-sequence voltage to ground. The fact that it has such high frequency means it can flow easily through capacitances to make it’s way from stator to rotor. Once in rotor, it can simply complete the path to ground through the bearings.
Since these same pulses show up so distinctly on the output and at the motor, I would say it seems like the zero sequence cannot have been reduced at all (and therefore you lose nothing in terms of bearing protection to remove it).
BUT, notice we have a ground on the secondary of this transformer. That certainly makes it easy to complete the zero sequence path.
A question for the gurus: what happens if we remove that ground on output of the transformer?…. wouldn’t that be better for the bearings?
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Looking back to your attached waveform, pdf page 6/7.
The time to rise 800 volts is 0.4 usec = 400 nanoseconds.
It almost looks as if there are two regions of the curve:
First region rises 0 to 400 volts in 100 nanoseconds
Second region rises 400 to 800 volts in 300 nanoseconds.
It is kind of bizarre to start with 100 nanosecond rise time and end up with 400 nanosecond rise time. Just brainstorming why that may be going on to cause that:
1 – multiple pulses (some possibly reflected) superimposed
2 – motor is acting something like a capacitor (surge capcitor spreads out the pulse).
3 – dispersion due to the individual frequencies seeing different impedances and speeds
Beats me… just thinking. Any other explanations?
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Note in my previous modeling, I had assumed the wave was traveling at the speed of light in a vacuum, which resulted in my calculated 240 nanosecond period of ringing from the source (by the way I don’t think viewing source as short circuit is too unrealistic with reflect to reflections from the source because the multiple lines tied together at that location in parallel result in very low impedance). But actually, the speed would likely differ from speed in a vacuum by 1/sqrt(EpsilonRelative), where EpsilonRelative reflects not just the insulation but an average of everything between conductor and ground (or conductor and other conductor). If I pick the highest conceivable number EpsilonRelative =4 (probably unrealistically high for the average), even then we end up with speed of 1 / 2 speed in vacuum, and the period would double to 480 nanoseconds…. still a long way from the 1000 nanosecond period of your measured ringing.
The source of the ringing is a curiosity to me. It is common in measurements Gunnar and others have posted (although not necessarily with overshoot exceeding 2). I have heard it referred to on the forum as LC ringing. It may be the case, but I can tell you in large volumes of surge study by EPRI, I don’t recall them every referring to anything like that. The model of the motor for purposes of predicting voltages within the system up to the terminals of the motor is an R / C circuit (See EPRI 5862 Volume 2, Section 5), and an R/C of termination of a transmission line does not create ringing.
Probably there is something I am missing. I have a completely open mind to the cause of the ringing, because nothing I know of explains it. I would be curious to know what is the frequency of the ringing occurs on motor # 3 which has about half the cable distance. If it is same frequency, then I am leaning toward this “L-C ringing” even though I don’t understand how that applies in this context. If the ring frequency is factor of 2 higher, then we expect something to do with reflections from the source (although I can’t reconcile the frequency, and I’m not sure why we wouldn’t see evidence of it on the source voltage trace).
Another question: the vfd output voltage traces you took are on the output of that transformer?
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(2B)+(2B)' ?
