variable frequency drive motors

Status
Not open for further replies.
CorElec said:
Is it correct to connect capacitors to variable frequency drive motors for power factor and voltage improvements?
Click to expand...
Totally incorrect.

For a capacitor C, instantaneous current i is given by:

i = Cdv/dt

with dv/dt being the rate of change of voltage.

I have measured dv/dt values of greater than 2000V/us at VFD outputs.
It needs only a small value of capacitance to result in huge instantaneous IGBT killing currents.
 
Some industrial plants have PF capacitors and no record of them. When they install a VFD
they capacitors are quickly located by the explosion.
 
CorElec said:
Is it correct to connect capacitors to variable frequency drive motors for power factor and voltage improvements?
Click to expand...
Probably not - unless they came with the drive, or maybe specified in the installation manual.

Yes I have seen drives with capacitors on the output - current source, MV. (example is AB PowerFlexf7000).

If you are trying to correct the low power factor the line side of the drive is showing to your system, yes I have also seen drives with caps on the line side. In the cases I have been involved with, the caps were selected to control the harmonics, raise the pf, and not cause too much disturbance when they were switched on with no load on the VFD.

If you are thinking putting caps on the motor side will fix the low pf on the line side - no that won't work. The DC bus between the line side and the load side stops that. You can't reflect vars across the DC bus. The output inverter has to handle whatever pf the motor (including installed caps) puts up.

I have seen a drive that automatically adjusts the output voltage to minimize the vars the motor draws, That one also had caps on the drive output.

Read the installation instructions that came with the drive.
Click to expand...
really good advice

cf
 
Cold Fusion said:
Yes I have seen drives with capacitors on the output - current source, MV. (example is AB PowerFlexf7000).[/quoted]
Yes. I had discounted the CSI. The only MV inverters I have dealt with are those we made ourselves and some ABB GTO units (ACS 1000). I wasn't aware that Rockwell made current source inverters. Appreciate the new (new to me) information, thanks.

Cold Fusion said:
If you are trying to correct the low power factor the line side of the drive is showing to your system, yes I have also seen drives with caps on the line side. In the cases I have been involved with, the caps were selected to control the harmonics, raise the pf, and not cause too much disturbance when they were switched on with no load on the VFD.
Click to expand...
On the line side of a VFD, the problem is generally distortion power factor rather than displacement and passive harmonic filters for 5th, 7th etc. are sometimes used to mitigate the effects of the harmonics. And to comply with agreed contract limits on voltage distortion. One of the constraints is that the filters are capacitive at the fundamental and, with no lagging displacement power factor to correct, there is the risk of leading power factor.
Click to expand...
 
Besoeker said:
Cold Fusion said:
Yes I have seen drives with capacitors on the output - current source, MV. (example is AB PowerFlexf7000).[/quoted]
Yes. I had discounted the CSI. The only MV inverters I have dealt with are those we made ourselves and some ABB GTO units (ACS 1000). I wasn't aware that Rockwell made current source inverters. Appreciate the new (new to me) information, thanks.


On the line side of a VFD, the problem is generally distortion power factor rather than displacement and passive harmonic filters for 5th, 7th etc. are sometimes used to mitigate the effects of the harmonics. And to comply with agreed contract limits on voltage distortion. One of the constraints is that the filters are capacitive at the fundamental and, with no lagging displacement power factor to correct, there is the risk of leading power factor.
Click to expand...

Active harmonic correction that uses both capacitos and inductors that are switched based on the harmonic 'load' is the solution. Individual capacitors, even if they are switched with the drive tend to produce overvoltages and even higher harmonics under certain cases.


ASD load power factor correction attempt is:
  • unncecessary
  • futile.
Having said that there are devices installed at the motor end that contain capacitors among other components that do help with the mitigation of both harmonics and standing wave voltages. They are useful on inverter rated motors and will save your standard, older motors. (Little known secret: you can also increase your cable length by about 30%. The manufacturer won't tell you this, but it is true.) Search for RWT at Eaton.
Click to expand...
 
weressl said:
Individual capacitors, even if they are switched with the drive tend to produce overvoltages and even higher harmonics under certain cases.
Click to expand...
(emphasis is mine)
Overvoltages match my empherical data - higher harmonics does not.


weressl said:
ASD load power factor correction attempt is:
  • unncecessary
  • futile.
Click to expand...
I didn't know that. Rockwell Engineering doesn't appear to know that either. Could you point me to any peer reviewed documentation that supports this? Sure would save me a lot of time.

cf
 
Cold Fusion said:
(emphasis is mine)
I didn't know that. Rockwell Engineering doesn't appear to know that either. Could you point me to any peer reviewed documentation that supports this? Sure would save me a lot of time.

cf
Click to expand...

My peers only(allowed) spend their time on subjects that are technically meaningful and/or have commercial benefits. (Try some Government sponsored study or a Socialist Counrty.) :D
 
CorElec said:
Is it correct to connect capacitors to variable frequency drive motors for power factor and voltage improvements?
Click to expand...

harmonics on the load side of a VFD are generally high enough to warrant a reactor to be installed so as to protect a motor against overheating the harmonic currents cause. installing a capacitor there would suck all these harmonics currents, since the capacitor acts like the path of least resistance. hence them capacitors blow.
 
weressl said:
My peers only(allowed) spend their time on subjects that are technically meaningful and/or have commercial benefits. (Try some Government sponsored study or a Socialist Counrty.) :D
Click to expand...
I'll translate that as a, no you don't have any data whatsoever and are just quoting the anecdotal thoughts of arrogance with no hard fact backup.

Note to Lazlo: Grin or not, your comment is still arrogant and demeaning. (no grin for you - these are for me):grin::grin:

cf
 
Cold Fusion said:
I'll translate that as a, no you don't have any data whatsoever and are just quoting the anecdotal thoughts of arrogance with no hard fact backup.

Note to Lazlo: Grin or not, your comment is still arrogant and demeaning. (no grin for you - these are for me):grin::grin:

cf
Click to expand...

Nope Tonto. I know enough about physics not to entertain flat-earth validation arguments.:roll:

I find 'anecdotal thoughts of arrogance' an especially nice English phraseology, stringing fancy worlds together in a totally gobleygook fashion. Now that WAS demeaning, and was meant to be too.
 
My peers only(allowed) spend their time on subjects that are technically meaningful and/or have commercial benefits. (Try some Government sponsored study or a Socialist Counrty.)
Click to expand...
I know enough about physics not to entertain flat-earth validation arguments.
Click to expand...

Laszlo -
When you come up with an unsupported statement that is crossways with my understanding, I'm going to ask for documentation to support your position. From my side of the monitor, the "laws of physics according to LW" do not trump the "Laws of Physics as understood by the rest of us".

Your post 14 was what my buddy, the paralegal, would call non-responsive. There was no useful information. I can't tell if you couldn't be bothered or simply couldn't support your position. Either way, without support, your post 11 is useless. Your saying so doesn't make it true.

I try my darnest to support my thinking - IEEE documentation is the top of my list. (Yes, I like the dreaded color books) Association Standards (maybe not the right term), such as those from Nema, Neta, Ansi, NFPA, API are other reputable sources. Mfg's data on their equipment is another favorite. And when I get to my empirical/anecdotal data, I say that is what it is and let the other member decide the validity. I'm certainly not demanding you do any of this, but it certainly would help some of us take your posts as serious engineering comments. Okay - maybe not, "some of us", maybe only me.:) (the smile here means the joke is on me - not on you)

weressl said:
Now that WAS demeaning, and was meant to be too.
Click to expand...
Well, I certainly responded in kind. But that doesn't matter, my response did not help. I will find a better way to phrase an answer when you are non-responsive.

Currently (minor pun) working my way through Violet, IEEE 551. This sucker is brutal.

cf
 
Cold Fusion said:
Laszlo -
When you come up with an unsupported statement that is crossways with my understanding, I'm going to ask for documentation to support your position. From my side of the monitor, the "laws of physics according to LW" do not trump the "Laws of Physics as understood by the rest of us".

Your post 14 was what my buddy, the paralegal, would call non-responsive. There was no useful information. I can't tell if you couldn't be bothered or simply couldn't support your position. Either way, without support, your post 11 is useless. Your saying so doesn't make it true.

I try my darnest to support my thinking - IEEE documentation is the top of my list. (Yes, I like the dreaded color books) Association Standards (maybe not the right term), such as those from Nema, Neta, Ansi, NFPA, API are other reputable sources. Mfg's data on their equipment is another favorite. And when I get to my empirical/anecdotal data, I say that is what it is and let the other member decide the validity. I'm certainly not demanding you do any of this, but it certainly would help some of us take your posts as serious engineering comments. Okay - maybe not, "some of us", maybe only me.:) (the smile here means the joke is on me - not on you)


Well, I certainly responded in kind. But that doesn't matter, my response did not help. I will find a better way to phrase an answer when you are non-responsive.

Currently (minor pun) working my way through Violet, IEEE 551. This sucker is brutal.

cf
Click to expand...

OK, I'll bite.

  1. What would be the value of a capacitor on the output side of an ASD?
  2. What is it intend to accomplish either from the technological or the finanacial side?
  3. Since the capacitor is not variable a resonant circuit at some frequencies, especially with varying carrier frequency is practically unavoidable.
  4. How would one surmount the resultant problems?
  5. What about the increased discharge voltages due to the harmonics?
Just for starters.........
 
Cold Fusion said:
(emphasis is mine)

Originally Posted by weressl
ASD load power factor correction attempt is:
unncecessary
futile.
Click to expand...

I didn't know that. Rockwell Engineering doesn't appear to know that either. Could you point me to any peer reviewed documentation that supports this? Sure would save me a lot of time.

cf
Click to expand...


What possible benefit would there be to putting capacitors on the load side of a VFD? It does not improve the PF the utility sees at all, and thats really the only reason for PF correction.
 
Status
Not open for further replies.
Top