VFD's & Line Reactors

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shoon

Member
Location
Canada
I work at a liquid nitrogen plant where we have 6x outdoor ariel glycol cooling fans controlled by 6x VFD's respectively.

We are using WEG 'CFW-11' VFD's to drive 25HP 480v 28.5Amp motors running at a maximum rated speed of 1770 RPM.
The issue we have, is that we constantly get slight power blips and anomolies that cause the VFDs to trip on a "DCBUS Overvoltage" fault. It's just enough to trip the VFD's, but not enough to trip any of our motor protection relays feeding our 4160v 4000HP compressor motors, or anything else out of our 480v MCC.

I was looking into installing line reactors to each of the VFD's, but upon reading through the product manual, these VFD's (despite their small size) apparently already have line reactors built into them, and they specifiy "external line reactors not required". So my question is, would an additional line reactor provide improved isolation / surge supression? Or is installing another line reactor to a drive with a built in reactor a bad idea? I am thinking that if the impedance of multiple reactors is too great, it will result in a voltage drop / reduced torque output of the motor.

Thanks in advanced!

-Shaun
 

kwired

Electron manager
Location
NE Nebraska
I work at a liquid nitrogen plant where we have 6x outdoor ariel glycol cooling fans controlled by 6x VFD's respectively.

We are using WEG 'CFW-11' VFD's to drive 25HP 480v 28.5Amp motors running at a maximum rated speed of 1770 RPM.
The issue we have, is that we constantly get slight power blips and anomolies that cause the VFDs to trip on a "DCBUS Overvoltage" fault. It's just enough to trip the VFD's, but not enough to trip any of our motor protection relays feeding our 4160v 4000HP compressor motors, or anything else out of our 480v MCC.

I was looking into installing line reactors to each of the VFD's, but upon reading through the product manual, these VFD's (despite their small size) apparently already have line reactors built into them, and they specifiy "external line reactors not required". So my question is, would an additional line reactor provide improved isolation / surge supression? Or is installing another line reactor to a drive with a built in reactor a bad idea? I am thinking that if the impedance of multiple reactors is too great, it will result in a voltage drop / reduced torque output of the motor.

Thanks in advanced!

-Shaun
The problem may likely be the fact that the motor is running when the power "blip" happens and the drive can not synchronize its output to the already spinning motor when it restarts. However there may be some parameters you can change that will allow it to detect the already spinning motor and resume operation at the correct voltage and frequency. I am drawing a blank at the moment on what this parameter is commonly called.

The drive is trying to restart a motor with whatever output values it normally would for a stopped motor, problem is the motor is already spinning - possibly near full speed and is going to try to dump inertia energy back into the drive so the drive output and motor speed will match (basically it will try to decelerate the load) and will ultimately trip on overvoltage on the DC bus because the deceleration rate is too great for how things are otherwise set up.
 
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Besoeker

Senior Member
Location
UK
I work at a liquid nitrogen plant where we have 6x outdoor ariel glycol cooling fans controlled by 6x VFD's respectively.

We are using WEG 'CFW-11' VFD's to drive 25HP 480v 28.5Amp motors running at a maximum rated speed of 1770 RPM.
The issue we have, is that we constantly get slight power blips and anomolies that cause the VFDs to trip on a "DCBUS Overvoltage" fault. It's just enough to trip the VFD's, but not enough to trip any of our motor protection relays feeding our 4160v 4000HP compressor motors, or anything else out of our 480v MCC.

I was looking into installing line reactors to each of the VFD's, but upon reading through the product manual, these VFD's (despite their small size) apparently already have line reactors built into them, and they specifiy "external line reactors not required". So my question is, would an additional line reactor provide improved isolation / surge supression? Or is installing another line reactor to a drive with a built in reactor a bad idea? I am thinking that if the impedance of multiple reactors is too great, it will result in a voltage drop / reduced torque output of the motor.

Thanks in advanced!

-Shaun
You didn't mention if they were input or output line reactors. No matter.
The fans are probably high inertia related to the motor rating. If you lose the power, the motors can remain self excited and become generators back into the DC link via the inverse diodes even if the IGBTs are shut off.That could account for your "DCBUS Overvoltage" fault.
Reactors won't change that.
 

shoon

Member
Location
Canada
Besoeker and kwired,

Thanks for the input!

To clarify, the built in line reactors would be on the input side. Besoeker, I think you might have hit the nail on the head with that explanation. These ariel fans will free spin if the wind hits them the right way. I remember trying to megger from the MCC out to the motor and kept getting screwed up readings- its because as you mentioned those motors were basically acting as generators.

kwire, you are also most correct. The motors are always running. The VFD's are fed with a 4-20mA signal based off of a temperature controller used to regulate the temperature of the glycol. I have a textbook sized manual of settings. The manufacturer was supposed to have gone through and programmed the VFDs to our application, but it seems like they missed a few settings. I have spent quite a bit of time (without much success) trying to change the settings to see if I can somehow configure the VFD's to not trip on "DCBUS overvoltage" Please let me know if you happen to remember which settings I should look into a little more!

So basically a line reactor would probably be of little use to me right now, as the issue is likely related to the inertia of the fan during a power blip, and the VFD's inability to synch the output quickly enough.

Thanks again!
 

mgookin

Senior Member
Location
Fort Myers, FL
The manufacturer who sold it to you may be able to help as well. They'd much rather give you some of their time in the interest of it working well for you in lieu of you using another company's product. Give them a call and ask if you can speak with one of their engineers.
 

Besoeker

Senior Member
Location
UK
Besoeker and kwired,

Thanks for the input!

To clarify, the built in line reactors would be on the input side. Besoeker, I think you might have hit the nail on the head with that explanation. These ariel fans will free spin if the wind hits them the right way. I remember trying to megger from the MCC out to the motor and kept getting screwed up readings- its because as you mentioned those motors were basically acting as generators.

kwire, you are also most correct. The motors are always running. The VFD's are fed with a 4-20mA signal based off of a temperature controller used to regulate the temperature of the glycol. I have a textbook sized manual of settings. The manufacturer was supposed to have gone through and programmed the VFDs to our application, but it seems like they missed a few settings. I have spent quite a bit of time (without much success) trying to change the settings to see if I can somehow configure the VFD's to not trip on "DCBUS overvoltage" Please let me know if you happen to remember which settings I should look into a little more!

So basically a line reactor would probably be of little use to me right now, as the issue is likely related to the inertia of the fan during a power blip, and the VFD's inability to synch the output quickly enough.

Thanks again!

If I have been of assistance that's thanks enough.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
...if I can somehow configure the VFD's to not trip on "DCBUS overvoltage"

It is extremely unlikely you would ever find that setting. Things will go boom if the DC bus rises too high.

You might want to look into 'braking resistors'. These are often switched into the system to provide addition loading in order to lower a DC bus overvoltage.
 

Besoeker

Senior Member
Location
UK
It is extremely unlikely you would ever find that setting. Things will go boom if the DC bus rises too high.
Totally agree.

You might want to look into 'braking resistors'. These are often switched into the system to provide addition loading in order to lower a DC bus overvoltage.
Yes. I did think about that. Some inverters have brake chopper transistors built in as standard. They still need an external resistor.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
It is extremely unlikely you would ever find that setting. Things will go boom if the DC bus rises too high.

You might want to look into 'braking resistors'. These are often switched into the system to provide addition loading in order to lower a DC bus overvoltage.

Translate "not trip on over voltage" to "sync differently so that no over voltage is generated."
Braking resistors are a good idea as long as OP is willing to have the fans stop momentarily and restart instead of riding through the glitch.

Sent from my XT1080 using Tapatalk
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Good Thread

Good Thread

Its always great to read the knowedge that presents in this forum. A lot of great minds are always there for the subject matter.
I would think Dynamic braking resistors might be well suited to that application.
They have been spec'd on retrofit of another type of machine I worked on in the past, also with a high inertia spinning load involved.
Sometimes the VFD tech support can help, but in at least one case I found a mystery setting in the manual that did nto come up in our discussion.
Those things have so many levels of menus anymore. Complicated they are.

all the best
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
It looks like Hitachi calls it 'resume after frequency matching' in their sj200 series manual, and Rockwell calls it 'flying start' in their PFLEX manual. Hope that helps you find the correct parameters in your drive manuals.

I agree that braking resistors are not the correct solution for this problem. If you add braking resistors you might prevent DC bus overvoltage, only to have the drive trip on excessive output current.

-Jon
 

Besoeker

Senior Member
Location
UK
It looks like Hitachi calls it 'resume after frequency matching' in their sj200 series manual, and Rockwell calls it 'flying start' in their PFLEX manual. Hope that helps you find the correct parameters in your drive manuals.

I agree that braking resistors are not the correct solution for this problem. If you add braking resistors you might prevent DC bus overvoltage, only to have the drive trip on excessive output current.

-Jon
So what do you think causes DCBUS Overvoltage?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
My guess would be reverse current from the motor (therefore unregulated) which is gated onto the DC bus as the VFD tries to synchronize speed to or brake the spinning motor. Either one could do it, depending on VFD programming, IMHO.

Sent from my XT1080 using Tapatalk
 

Besoeker

Senior Member
Location
UK
My guess would be reverse current from the motor (therefore unregulated) which is gated onto the DC bus as the VFD tries to synchronize speed to or brake the spinning motor. Either one could do it, depending on VFD programming, IMHO.

Sent from my XT1080 using Tapatalk
Well, I've explained what I think. And I've seen it happen. But, of course, there are other possibilities and combinations thereof.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
So what do you think causes DCBUS Overvoltage?

My guess is more inline with kwire's. The inverter trips out when the supply voltage drops, and then attempts to restart. The inverter is assuming that the motor is at zero speed, and restarts its output at a low frequency. This puts the motor into a regenerating state, trying to slow down the fans. Kinetic energy gets dumped into the DC bus, raising its voltage.

I don't see how the backemf of the motor would produce a voltage greater than the current DC bus voltage without the inverter providing magnetizing current. I can see motor backemf supplying power to the DC bus as the bus voltage goes down, but can't see it boosting the voltage higher.

-Jon
 

Besoeker

Senior Member
Location
UK
My guess is more inline with kwire's. The inverter trips out when the supply voltage drops, and then attempts to restart. The inverter is assuming that the motor is at zero speed, and restarts its output at a low frequency. This puts the motor into a regenerating state, trying to slow down the fans. Kinetic energy gets dumped into the DC bus, raising its voltage.
Most modern inverters are capable of restarting a spinning load. The applied frequency has to be within a fairly narrow band in relation to motor speed for that to work.


I don't see how the backemf of the motor would produce a voltage greater than the current DC bus voltage without the inverter providing magnetizing current. I can see motor backemf supplying power to the DC bus as the bus voltage goes down, but can't see it boosting the voltage higher.
If the IGBTs shut off on loss of power the motor can remain excited and generate back into the DC link via the inverse diodes. Since there is nowhere for that energy to be dissipated the 0.5CV2 increases. With no change in the value of C........
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the IGBTs shut off on loss of power the motor can remain excited and generate back into the DC link via the inverse diodes. Since there is nowhere for that energy to be dissipated the 0.5CV2 increases. With no change in the value of C........

I've had the misfortune of blowing up several experimental inverters due to that pesky C remaining constant :)

What I am not getting: during operation as a motor, the applied terminal voltage must exceed the motor back EMF in order to supply power _to_ the motor.

If the IGBTs simply shut off, the motor back EMF is clearly applied to the back diodes, but I don't see how the _voltage_ of the back EMF can exceed the DC bus voltage, so I don't see why these diodes conduct.

Does the phasing of the diode conduction current act as a source of capacitive VARs and provide the necessary excitation? Is there some other characteristic that would cause the back EMF to exceed the DC rail voltage?

-Jon
 

Besoeker

Senior Member
Location
UK
I've had the misfortune of blowing up several experimental inverters due to that pesky C remaining constant :)

What I am not getting: during operation as a motor, the applied terminal voltage must exceed the motor back EMF in order to supply power _to_ the motor.

If the IGBTs simply shut off, the motor back EMF is clearly applied to the back diodes, but I don't see how the _voltage_ of the back EMF can exceed the DC bus voltage, so I don't see why these diodes conduct.

Does the phasing of the diode conduction current act as a source of capacitive VARs and provide the necessary excitation? Is there some other characteristic that would cause the back EMF to exceed the DC rail voltage?

-Jon

I don't know the answers to your questions.
I played around with using asynchronous motors as a possible means of inexpensive power generation for remote communities. Getting them to self excite wasn't a big problem - a few VARS did the trick.
Regulating the voltage was another matter.

I didn't pursue it. Other business pressures pushed it off the agenda.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I too think this sounds like a DC bus regen problem, only indirectly related to your power glitches. That drive does include a Dynamic Braking chopper, so you could set it up to be used as a way to dump off any excess DC bus energy from regen by simply hooking up a resistor.

You might also want to look for a feature called (something like) "Flying Restart" which provides the drive with the ability to detect that the motor is spinning and catch it on the fly to re-accelerate it. With that, you might avoid some of the regen in the first place. If you find it and it is not enabled, enable it and try it before buying the resistor.

Just to clear up a misconception though. That drive does NOT include a "line reactor", it includes a DC bus choke (inductor). Functionally they both provide a small amount of harmonic mitigation, so from that standpoint you don't need to add another line reactor. But that is not the ONLY reason to have a Line Reactor, in fact it's not even the best reason. A real Line Reactor ahead of the drive is like cheap insurance for the DRIVE. It slows down the rise time of line transients caused by OTHER external events and helps the drive rectifier components survive better. A DC bus choke is ZERO help in that regard, because it is BEHIND the rectifier, not ahead of it. So your vendor was an ignoramus.
 
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