Terminator networks for VFD
Terminator networks for VFD
(OP)
Hi,
We have a problem of voltage spikes in a VFD installation (220V) with 7 induction motor fans in parallel (about 8 amps in total).
We have measured spikes in PWM pulses (50ns rise time) about 800V peak in the farthest motor from VFD.
We are assesing what to do (add reactors, dV/dt filters etc. etc.) and I found a "termination network" discussion about using RC network between phases matching the cable impedance like this:
R about 100ohms
C about some tenths of nF
Supposedly, at the pulse rise the R matches the cable impedance so spike is "cancelled", after that C charges to line value so no permanent power is wasted.
Has any sense for you guys? any experience or supplier with this kind of terminators?
I have thought that might try with three 100ohms 47nF snubber between phases to see what happens...
We have a problem of voltage spikes in a VFD installation (220V) with 7 induction motor fans in parallel (about 8 amps in total).
We have measured spikes in PWM pulses (50ns rise time) about 800V peak in the farthest motor from VFD.
We are assesing what to do (add reactors, dV/dt filters etc. etc.) and I found a "termination network" discussion about using RC network between phases matching the cable impedance like this:
R about 100ohms
C about some tenths of nF
Supposedly, at the pulse rise the R matches the cable impedance so spike is "cancelled", after that C charges to line value so no permanent power is wasted.
Has any sense for you guys? any experience or supplier with this kind of terminators?
I have thought that might try with three 100ohms 47nF snubber between phases to see what happens...





RE: Terminator networks for VFD
That is quite fast even directly at the inverter terminals and dispersion usually makes that risetime hundreds of nanoseconds at the end of the line.
It is not very common to have any problems with voltage spikes at 220 V. Not even at 400 V. Problems with insulation usually starts at 500 V and is very common at 690 V.
Are you sure about the probe compensation? It is next to impossible to have 800 V spikes in a 220 V installation.
Anyhow, the standard remedy is to use a damped motor reactor or a du/dt filter. Try a 2 % reactor first and add a parallel resistor to fight ringing and hot core. I think that three resistors with 50 - 100 ohms and 20 - 50 W will be OK in an installation like yours.
Snubbers are not used very often to reduce reflected waves/overvoltage at the motor end.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Terminator networks for VFD
As skogs says, a reactor or dv/dt filter mounted as close to the drive output terminals as possible is preferred.
RE: Terminator networks for VFD
I have never played with vfd's, but it sounds like an interesting phenomenon. I have some questions out of curiosity.
You don't have 6 identical-length cables and the 7th one longer, do you?
What are the lengths?
What do you see at the others motors? (I assume you already measured them since you said this is the highest).
Are these cables shielded? (I don't normally see shielded lv cables, but I don't know about vfd).
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(2B)+(2B)' ?
RE: Terminator networks for VFD
when you take into consideration all motor leads in parallel, what is the total motor cable length?
RE: Terminator networks for VFD
Slide 1 shows the system model:
Source ===SourceCable===== Bus===Cable1===Motor1
===Cable2===Moto2
etc
===Cable7===Motor7
The source is a ramp increase from 0 to 1 with rise time of 50 nsec
All cables are selected to have 100 ohms characteristic impedance.
The source cable is selected very long so that the source does not produce any reflections during the transient. That simplifies the simulation.... the actual source characteristics certainly could modify these results.
Cables 1 through 6 are selected to have time delay of 201nsec through 206 nsec, which corresponds roughly to 201 through 206 feet.
Cable 7 is selected to have time delay of 400 nsec, corresponding to 400 feet.
All motors are 800 ohms (intended to simulate motor impedance much higher than cable).
The results are on slide 2.
When the pulse hits the motor, the reflection coefficient is (800-100)/(800+100) =0.778. This reflection adds with incoming wave to give magnitude 1.778, which matches the plot.
The reflections of the 1st 6 all reach back to the bus at the same time (400 to 450 seconds). By looking at the response of the 7th line after it's time delay elapses, we can say roughly that the reflection from the 1st 6 upon hitting the bus tend to travel toward the 7th motor. It is as if the 6 motor cables act together like one cable with lower impedance 100/6.... when they get back to the bus they have nowhere to go but the higher impedance cable 6 (100ohms), so there is amplification in this path.
In the end, we get close to 3 times the original pulse showing up at motor 7, even though it has the same cable impedance and motor impedance as all the others (just a longer cable). Normally we think of 2 as the highest, but this involves multiple reflections.
Again, it is just an exercise in applying textbook transmission line theory, but would require a lot of further study and discussion to get any feeling for the extent that it would reflect a given real installation.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
Another note - I did not model the trailing/decreasing edge of the pulse. If pulse duration were shorter than about 500nsec, then this model would need some changing.... the original incident pulse on motor 7 would stop dropping before the reflected pulses from the others reached it.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
If anyone cares, I resolved my hand calc error. One thing I forgot was the source cable attached. It has source impedance 100/7=14.87, selected so that the source pulse passes to the 7 output lines without any change in magnitude. So when that pulse reflects back along the 6 lines, it transitions not to the single line of motor 7, but to the parallel combination of motor 7 cable and the source cable (labeled T7_Para_Tsource below).
Below is sequence of signals parameters that matches the observed increase of 1.18 (2.96-1.78) at motor 7 when the reflection from the other 6 arrives there:
Parameter Value Formula
ReflectionAtMotor 0.777777778 =(800-100)/(800+100)
RefPlusTransAtMotor 1.777777778 =ReflectionAtMotor+1
T7_Para_Tsource 12.50098436 =100*14.287/(100+14.287)
TransmCoefAtBus 0.857181427 =2*T7_Para_Tsource/(T7_Para_Tsource+100/6)
TransmittedPastBus 0.666696665 =TransmCoefAtBus*ReflectionAtMotor
Increase 1.185238516 =TransmittedPastBus*RefPlusTransAtMotor
The 1.18 calculated increase matches the program's displayed 1.18 increase.
I feel better now.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
diagram (see pdf attached for actual waveforms)
VFD--MCBs for each fan------(fan1)
---------------(fan2)
------------------------(fan ...)
----------------------------------(fan 7)
length to fan7 is about 20m. The remaining 6 proportional.
Pulse rise time was wrong, is 100ns. See waveforms.
Probe is differential I only have the measurements for fan7. I will check compensation (Actually I don´t remember to see the compensation screw, it is a Chauvin Arnoux diff. probe).
DickV "...motor terminal protector... while it may protect the motor adequately, it does nothing to quiet the motor leads."
Sorry for sure I´m uninformed but I think that I don´t understand this statment, then, what is not protected with terminal network?
Regards!
RE: Terminator networks for VFD
The recordings shown are not only reflected waves. Looks more like ringing and that, in combination with reflected wave, can explain the unusually high voltage. Pure reflection cannot get higher than twice the DC link voltage, 1.8 is a common value, and in this recording, the factor is 2.5
There are MCCs in the motor paths. They may contribute if the coils are inductive with very little R.
Dick probably means to say that emission from your cables (noisy cables) will not be affected by RC termination. There will be some reduction, but not much.
How is the 800 V affecting the motors? Do you have premature isolation failures? Or bearing problems?
I have recordings from motors with pure reflection. Shall dig and look for them.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Terminator networks for VFD
Attached are two more simulations.
1st simulation as shown on slide 1 is same as previous except:
* I have put in the actual cable lengths
* I got rid of all the other motors except #8.
* I set the source cable length to 1nsec and got rid of the delay... now the source acts to have zero source impedance. That may not be realistic, but if I put in a source impedance, it will tend to drag the final voltage below 1. Have to think about that some more.
Results are on slide 2. What is seen on motor 7 is a peak of 1.78 which we calculated before as 1+ (800-100)/(800+100). The period is about 4 travel times ... 4 * 60 = 240. This makes sense since the pulse will bounce off motor (OC) with same polarity, bounce off bus (SC due to source model) with opposite polarity, return to motor with opposite polarity have completed ½ cycle in 2 travel times.
2nd simulation is on slide 3. Here we have simply added in the other motors.
Results are on slide 4. Addition of the other motors caused motor 7 peak to 1.85, which exceeds previous calc'd values, but still not more than 2. Period still 240 nsec which does not match measured 1mhz ó 1000nsec period. If we wanted to describe this result in qualitative terms, we could say the interaction between motors is minimal, the longer cables tend to have higher peak because the the cable is approaching closer to the half the length of the pulse, where the theoretical reflection coefficient (up to doubling) applies. There is some interaction as evidenced by the fact that motor 7 is slightly higher with others present, but it does not seem to be a big effect in this particular model.
None of this proves anything related to your motor. It is just a wander through the world of transmission line behavior and still a long way away from recreating your results or having confidence in any model (I doubt we will ever get there).
I'm going to think some more about the source model. If anyone has suggestions on that or other aspects of the model, feel free to chime in.
Can you describe the physical configuration of the cables.. what type of cable and is it shielded and which end.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
what type of cable and is it shielded and which end.
should've been
what type of cable and is it shielded and which end of shield is grounded.
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RE: Terminator networks for VFD
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(2B)+(2B)' ?
RE: Terminator networks for VFD
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(2B)+(2B)' ?
RE: Terminator networks for VFD
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Terminator networks for VFD
The MCBs are just near VFD in the same cabinet (0,5m?).
It is a new installation so hasn´t been detected any problem (yet).
Fans are EBM-PAPST and we have been told (before making the voltage measurement) by EBM representative that should be installed a filter like schematic adjoint. He refers to that should be verified that rated motor temperature is not overcome due to VFD duty, because it could cause insulation problems (maybe he is mixing several concepts here).
The filter is a kind of low pass plus isolation transformer. Do you think that really is needed something like that?. I don´t see the point of the isolation transformer (unless it helps with bearing currents?).
How long (usually) insulation fails are noticed with this kind of spikes?
Regards!
RE: Terminator networks for VFD
Very unusual. And very expensive. ABB have delivered a few of them for large machines. Never seen that used on 1 kW machines.
I think you should reconsider the whole thing. Do you really need that kind of fan? Is this a very special installation? Like clean room for semiconductors or a medical lab?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Terminator networks for VFD
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?
======New subject:====
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?
======New subject:====
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)' ?
RE: Terminator networks for VFD
RE: Terminator networks for VFD
The transformer is usually there to step up voltage after the sine filter, where you lose so much voltage that the motor doesn't work well. ABB has (as always) made a virtue out of necessity and given the combination a name: 'Step Up Filter'
Patrick: I see, I have one of those in my ceiling. I haven't tried with a VFD. And will not. These motors usually have a high rotor resistance and can be easily controlled with a triac. Cheaper and easier than a VFD. And better for the windings.
xj: Was the VFD recommended by the fan/motor vendor?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Terminator networks for VFD
A mildy interesting article here:
http://cit
Interaction of drive modulation and cable parameters on AC motor transients
by Kerkman, R.J.; Leggate, D.; Skibinski, G.L.;
It addresses some of the things we have been talking about here:
1 -oscillation
2: reflection at > factor of 2 amplification
This author discusses oscilation on page 2 in a manner which I would say is very similar to what I described previously. They identify that the oscillating period of the cable corresponds to 4 times the cable length. The example figure is figure 1 where it can be seen tp is the travel time – first observed between inverter pulse and motor response..... then 4tp is the period of the resulting oscillation. Why does this frequency not hold for the waveforms posted in this thread? Beats me.... There's got to be more to the story somewhere.
They discuss two possibilities for reflection magnitude > twice the inverter pulse
First: IV.A on page 5 discusses "double pulsing effect" which corresponds to the case of figure 3 (a few pages earlier) where the incoming pulse changes direction at the same time that the reflected negative wave hits the motor. In this case, we would expect to see the final voltage end up zero (due to short pulse), but in the waveforms posted in this thread it ends up at 310volt, so this double-pulsing phenomenon does not apply.
Second: IV.B on page 6 discusses "Polarity Reversal" – this is where the pulse doesn't change to zero, but changes to an opposite polarity again at about the same timeframe when the first negative reflection hits the motor. There is no evidence of this behavior in the one inverter pulse that was zoomed in, so we might be inclined to discount this possibility. BUT, there are some interesting clues that this might be occurring.
First clue: the data posted 14 May 11 4:47 in this thread on page 5/7 seems to show a very regular pattern of alternating magnitudes of the peaks at the motor: 800 / 620 / 800 / 620 / 780 / 620 / 760. It is a distinct enough pattern that we suspect it may not be coincidence (if it is just a characteristic of system reacting to the same pulse every time, we expect roughly the same result every time). So maybe there is some slight difference among pulses .... What difference in pulse can cause this.... Something like the oddball reversal of figure 9.
Second clue: This is probably a bigger clue. look at figure 9 of the linked paper and see there is an interesting feature that the waveform first goes in one direction and then reaches the high peak in the opposite direction. That feature is shared by all of the high peaks on page 5/7 and by none of the low peaks.
So, there is at least a few clues that this "polarity reversal" of the inverter pulse might possibly be going on. It doesn't particularly show anything like that evident on page 2/7, although there are a few tiny positive peaks evident above zero on the right side of that figure where everything should be at/below zero. Since it is so far zoomed out, I don't particularly trust it to show a small polarity reversal. Perhaps further inspection zooming in on a few pulses might show some clues. You showed us one.... Perhaps look at a few more... maybe only 2 in a row are required if some anomaly is occurring every other one.
Just a thought.
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I'll be interested to hear like if this is a normal induction motor or a ceiling fan type motor.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
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(2B)+(2B)' ?
RE: Terminator networks for VFD
Would you have to take into account motor winding capacitances, or are they sufficiently decoupled from the cable capacitance by the series inductance of the winding that they wouldn't contribute much?
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If we learn from our mistakes I'm getting a great education!
RE: Terminator networks for VFD
The motor power is about 300-400W at current conditions (near nominal).
It is a ceiling axial fan type, and I think it is external rotor type.
Insulation class I don´t know but EBM seems worried about overheating due to working with VFD, so maybe is not very good. I will check on catalogue on monday and will ask them details about the filter.
I will check also details about cable type installed (should be 1.5/3kV insulation, halogen free type).
Regards
RE: Terminator networks for VFD
While we're talking about the model of the motor, I'd like to backtrack to my statement about the model and whether it includes any L to create any ringing. (. 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). I do remember now reading several places L-C ringing associated with the TRV accross the contacts during circuit interruption. But I still am pretty sure it does not end up playing any role in surge seen at the motor terminals under the EPRI model. I'm not sure why it would be that L-C ringing applies in one case and not the other.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
1 - add capacitance into motor model.
2 - use a reversing pulse.
Slides 1 and 2 show the model. As noted on slide 1, I did not pick the capacitance value based on any empircal data.... I picked it based on the value I thought would recreate the measured waveform best (that might be considered cheating, depends how you look at it).
Slide 3 shows the reversing pulse that I used. The negative peak is -0.5 with 100nsec rise time, followed after a delay of 500nsec by change of 1.5 to value of +1 with 100nsec rise time. The 500nsec was again chosen to try to recreate the measurements. 500nsec is roughly worst case because the opposite polarity reflection of initial negative pulse arrives at motor 7 at the same time as the delayed positive pulse (for motor 7).... all of this under the assumption that I will create a 1usec period, which I did (it was a result of changing the capacitance, not changing the cable parameters).
Slide 4 and 5 are results. Slide 4 shows that motor 7 is the worst of all... probably because time delay was selected to create worst case at motor 7. Slide 5 shows chracteristic of the waveform. It matches xj25's waveform in the following respects:
* Peak of 2.85 times initial pulse.
* Oscillation frequency of 1 Mhz (adding the caps slowed down the oscillation frequency... did not have to resort to the Hypalon cable assumption).
* Rise time in the neighborhood of 400nsec, depending on how you count it. (**)
* Initial negative deflection before large positive peak. (**)
Some factors that didn't match well:
** The initial negative deflection here is much larger magnitude than in xj25's data.
** The rise time in the simulation is relatively smooth, whereas xj25's seemed to have a distinctive kink, as if 2 slopes: one in the first 100nsec and another lower slope in the last 300nsec of the rise. The simulation did not recreate this.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
I think that don´t uderstand very well why is not used any motor winding inductance in the model. Could you just comment it?
I added to the model an inductance of 0.7mH in each branch representing the MCB coil (I made up the value from an inductance of MCB that should be something like 1cm x 1cm x 3/4turns) but then all the effect of the ringing dissappears
About referenced EPRI 5862 Volume 2, Section 5, is possible to check it somewhere?
Reading in detail the VFD manual, it explicitly says that it should be used a output inductance filter if motors are cabled in paralel with star topology (however in bus it accepts it).
Looking for info I found this document that maybe someone finds useful.
I am having a quite busy time now so I will return when having news to tell you how it ends.
Regards
RE: Terminator networks for VFD
A less expensive less-detailed article by the same authors can be obtained from IEEE here:
"Equivalent Circuits for Simulating Switching Surges at Motor Terminals" by Sharma, Gupta, Pillai, et al from IEEE Transactions on Energy Conversion, Vol 3 , No. 3 , September 1988
Attached is an excerpt from that article. Figure 2 is the model that these authors use for purposes of calculating surge at the motor terminals (the same model as the EPRI report). It has resistance and inductance at the source, a transmission line representing the cable, and resistance/capacitance at the motor. Even though both L and C are present, I don't believe this would give traditional L-C ringing because of the separation of these components by a time-delay transmission line. I tend to think in your case where there are many parallel cables connected to the source bus, then the source acts low impedance for purposes of reflections, since the parallel cables have low impedance. Therefore I have assumed that neglecting the source inductance is not a big error here. I haven't tried it out.... does your simulation show different results?
Also I will mention that the multiple series L / parallel C is sometimes used to represent a transmission line. The intent is that the discretization must be fine enough such that each L and C corresponds to a length of transmission line far less than the wavelength of interest (for example less than 10%). In that case when the transmission line model is properly applied/discretized these particular L-C elements generate no oscillation.
As to the model of the motor itself, I have seen (not necessarily understood) a wide variety of models used to represent motors for a variety of high-frequency purposes. Many of them do include inductance elements. Some of them distinguish differential mode vs common mode. I believe the EPRI authors focus on common mode. Which model is right I'm sure may be slightly subjective and certainly depends on the purpose of the analysis. Personally I tend to put a lot of weight on the EPRI report because it has more details than any other reference I have on the subject. But I'm no expert.
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(2B)+(2B)' ?
RE: Terminator networks for VFD
(Of course don't lose track of Gunnar's comment... maybe a vfd is not even needed for ceiling fans which can be controlled by other simpler means)
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(2B)+(2B)' ?
RE: Terminator networks for VFD
Of course, that document says a sinewave filter and the drawing may just be a generalization not really represting any specific filter. There are sinewave filters that don't use an isolation transformer.
RE: Terminator networks for VFD
In Pete model the small MCB inductance will flat out the transients. Here not, but not sure if it is correctly done.
The difference with actual results is the duration of the transients, the real case is about 4us. I coundl´t get to adapt it without modifying quite much the "ringing" freq.
Using a serial inductance of 2mH eliminate the transients in the simulation.
Regarding varying speed "by other means" i.e. voltage, how much is the speed variation range than can be expected without overheating too much the motor? We are working with speed(freq.) changes from 60 to 40Hz more or less.
We have been recomended to use a multistep, stepdown transformer.
Regards!
RE: Terminator networks for VFD
interesting that you are seeing turn-on & turn-off transients at the motor and not at the vfd.
good luck
RE: Terminator networks for VFD