Line Reactors for VFD's

At what point are the use of line reactors recommended for vfd's? Is it size of drive, length of run? any help is appreciated.

thanks,
scott

The size of the drive is the deciding factor. All you are trying to do is to reduce the surges and harmonic distortion on your incoming supply. However, using a transformer for the drive is the best option.

the length of the load side wiring does not matter?

Line Reactors

I will start out by saying there are members on this site with much more expertise on VFDs than I have. Hopfully they will also post a response. I can give an example where Line Reactors and terminators were a workable solution.

We have two AB 1336 30 Hp drives that are placed aproximately 500 feet from where the motors are mounted. We had allready installed three DB (dynamic Brake modules rated for drive and motor Hp) for each drive. The load is a slat boot sorter (one for each drive). Slow ramp starting and slow decell rates. The problems we were experiencing related to motor failures due to pin holes in the winding insulation. I had contacted Rockwell's drive specialists and we discussed standing wave ratio, wire lenth, wire size, load inertia, motor types and other pitfalls that are experienced with VFDs and motors. I had checked the wires going to the motors and found them to be 10 gauge. The formula I had used showed the wire size needed to be 6 gauge for current handling and wire length. We replaced the wiring but the problem persisted in the months to come. The finale solution was to install Line Reactors rated for these drives in the same panel that the drives were in (this panel had plenty of space to accommodate the reactors and has very good fan ventilation) and terminators at the motor locations. This may sound like overkill but my quest is to eliminate down time (If we are not shipping product we are not making money). We have not experienced any of the problems we had experienced before in the last three and half years. So this solution was completely satisfactory to my managers.

PS: yes the length of the load side wires does matter and falls under the standing wave ratio scenario.

I hope this helps

Thanx Dan

I still believe, the length of load wiring matters only for standing wave and protects the motor and the load-side.
However, the line reactors only reduce the noise in inoming supply and load distance has no effect on them.
(Again we may need a feedback from a VFD specialist)

Maybe The terminators

shahn4,

As I said before I am not a drive specialist either and most of the VFD applications I have worked with didn't have any more severe problems that a simple Dynamic Brake could address. If I remember correctly it was the terminator at the motor end that attenuated the standing wave ratio problem. But I think I will wait to see if someone will more expertise on VFD's and a better understanding of the standing wave thing will post. I hope to learn more also.

Thanx Dan

I think some of the dispute comes from terminology. "Line reactor" is simply a series inductor and can be placed upstream or downstream from a drive. Purposes to doing so are independent and different.

Some have chosen to call reactors ahead of drives line reactors and those in the motor lead, load reactors. I'm ok with either, it really doesn't matter.

What does matter is understanding how to use them. As far as input side reactors go, their primary purpose is the provide high frequency isolation between the drive and the power supply. Most of the time, you are trying to protect the rest of the power network from the harmonics the drive generates backward into the supply network. Less frequently, they are used to protect the drive from pulses and noise in the AC supply.

Some drive manufacturers provide some input reactance standard in their drives. I usually see 3%. Of course, the motive for not having any standard is cost and you can draw your own conclusions there!

On the other hand, load reactors are placed in the motor leads primarily to block high frequency components in the drive output pulses from traveling down the leads and causing motor insulation failures. Note that load reactors are pretty much only used to protect motors, not drives. If you have a motor rated Class F insulation, and better, meets the testing requirements of IEEE MG1 Part 31 which is a 1600V withstand test, then you will not need load reactors for most applications. In general, the longer the motor leads, and the higher the output carrier frequency from the drive increases the chance you'll need a load reactor.

In my experience, I end up needing to add input reactance about 10% of the time and motor lead reactance only about 1% of the time. (The input reactance figure of 10% is somewhat lower for me because I'm almost always dealing with a brand of drives that have a 3% input reactor included standard).

Just a couple of notes about motor lead reactors: First, they are almost never used in 230V applications and almost always in 575V. Second, on precision sensorless vector drives, motor lead reactors can damage the ability of the drive to manage motor torque slightly. And, on input side reactors, the old rule about if a little is good a lot must be better definitely does NOT apply. Input reactors drop AC voltage and, in systems that are already on the low side, they can cause low voltage problems. In those cases, you may have to pony up to a wave trap or other more elegant (expensive) solution. A comment was made earlier about an input transformer which is an expensive option in higher horsepowers. They do work good and, if equipt with taps, can even correct for low voltage. They also have the added benefit of helping reduce common mode noise which is getting to be a bigger and bigger problem, it seems to me.

I am by no means a "motor expert"

In trying to find out the "mystery" of the line reactor, I have asked various people (some so called "experts") and I have heard:

A line rector will always help.
A line reactor won't always help.
A line reactor will hurt your situation.
A line reactor always goes on the line side of the drive.
A line reactor always goes on the load side, between the drive and the motor.
A line reactor sometimes goes on the line side other times it would be better on the load side, it depends on the application.
It depends on the distance from the motor to the drive.


I was always trying to get a "rule of thumb" as for the use of line reactors, and never could. All of the so called "experts" I asked, could not answer the 'why' type questions I had.

I do however take the "writings of DickDV" to be correct and true. This has been more of a help than any of the other "motor experts" answers.

Most of my motor installations are <5HP. My final conclusion has been to install the motor/VFD's as I always have and if a problem surfaces tackle it then, and so far I have seen no need for a line reactor.

/agree DickDV.

Generally, here, we always put reactors on the line side, just in front of the drives. The line side reactors are there to help reduce noise coupled back up the line, and also to provide at least some impedance before the drive. Input impedance can be extremely important if you have a very 'stiff' feed, as in a lightly loaded sub-station. Many input-circuit failures on AC drives can be traced to stiff feeders. This is valid both with AC and DC drives, by the way.

We install reactors on the output side of drives in many cases, even if using inverter graded motors. The cases we specify output reactors are:
A) Long lead lengths to the motor
B) Multi-paralleled motors
C) PWM drive running a NON inverter grade motor no matter what the lead length
D) High carrier frequency PWM drives no matter what the lead length.

The older '6-step' VFD's didn't generally need output reactors, but newer PWM drives very often do. The reason is the nature of the pseudo-sine wave generation. In PWM drives, you have high frequency full on / full off pulses, which can cause severe voltage spikes at various points along the conduction path (leads to motor and in the motor windings). The higher the base carrier frequency, the worse the 'transmission lead' transformer effect becomes.

In some extreme cases, or where it is required to minimize as much HF switching noise as possible, we'll install sine filters on drive outputs, but due to the cost, that is very special cased.

Just two notes I forgot:

I've never had a problem with a reactor where it wasn't actually needed, so my rule of thumb is 'when in doubt, put one in'

And, I generally use 5% impedance reactors, as they seem to only cost a couple dollars more than 3% ones, but in most cases, 5% is overkill.

One More Question

I would like to thank SLaubach for asking this question and shahn4, DickDV, 93lt1 and rdrast for their input on this thread.

I often install AB VFD's using GD1 modules (the 1336 plus can be ordered with the card installed) and treat the drive as a remote I/O device to the PLC. So this information at least for me can be PLC related. The programming can be very solid but I can still run into a problem with the motor performance. Usually adjustments to the parameter settings, Accell and decell rates and adding DBs take care of the problems. Once in a while though I run into problems like the example I posted above (these motors were and are inverter rated) and would like to know as 93lt1 posted above some rule of thumb and rdrast post seems to fit that bill. I too find that terminology can be a PITA in respect to the jargon used by people from different locals. Often we can be discussing devices by different names, but talking about the exact same thing.

The device I was calling a Line Reactor is basically 3 large inductors. This was installed in series on the output or load side of the VFD in the panel with the VFDs. The device I was calling the terminator was installed in it's own small panel close to the motors and wired in parrallel with the motor leads. I'm not sure if it was a capacitance device or what or if this is what DickDV was referring to as a Load Reactor?

I have worked with RF transmitters and receivers and understand the standing wave problem as it pertains to RF tramsmissions. To my understanding the high frequency PWM can cause the same type of problem in long load side lead lengths form the VFD to the motor. Could this be what was causing the pin holes in the motor insulation?

Thanx Dan

Hi Dan-
Like you and DickDV said, people refer to the same thing in several different ways so it can certainly be confusing.
If someone says 'load reactor' they usually mean a reactor installed between the drive and the motor. This is usually done to help with standing wave (also refered to as 'reflected wave') problems. These problems usually show up as breakdowns in the motor insulation, typically in the first few turns on the ends.
A terminator is just a resistor network that goes from phase-to-phase on the motor. It performs the same function as a terminating resistor on a communication cable. The terminators act to correct the same standing wave problems the inductors do, just through a different physical mechanism.
So installing both load reactors and terminators address the same problem and can be viewed as complementary solutions. Our machines are pretty compact so we don't usually see a problem with standing wave. However, I have been told that terminators are the most effective method of reducing standing wave issues.
As stated above, line side reactors (between the drive and the 3-phase supply) will reduce the effects of noise either coming from the drive or going into the drive. They also work to make the supply line appear 'softer' to the drive, which will help reduce instantaneous current spikes going into the drive.

Hope this helps,
Keith

Thanks

Thank you kamenges for clearing up the terminology stuff for me and thanks to shahn4, DickDV, 93lt1 and rdrast for your participation in this post. I just hope that SLaubach was able derive an answer to his question also.

Thanx Dan

Use a reactor:

To add Line Impedance.

To provide some light buffering against low magnitude line spikes.

To reduce Harmonics (When no link choke is present).

To compensate for a low inductance motor.

Only as part of a filter for reflected wave reduction.

93lt1, thanks for the compliment! In answer to your list of questions, reactors should be used to solve specific problems. That implies that we have a problem or expect one.

As mentioned above, if you have a drive with a 3% input reactor built in or a DC bus choke (does nearly the same thing), then I normally install without anything additional and see what happens.

Now of course, if you've got an old Insulation Class B motor and 500 ft motor lead length and a drive output carrier up in the ozone at 15-18KHz, then good judgement dictates heading off the problem beforehand. But that's extreme and normally nothing further is needed.

By the way, I've never encountered a drive application that I couldn't do just as well at 4 or 8 KHz rather than the ridiculously high carriers (18-20KHz) some manufacturers are pushing. Sure its great for the drive but it's a drive/motor system, if you please. If the motor can't survive, what good is a crafty high frequency drive?

We have over three hundred VFD's installed in my facility. I have always required either Line Reactors or an isolation transformer on the input (always 460 VAC). I also require Output Line Filters from TCI if motor lead length is over 60 feet. New motors are required to have at least 1600 V insulation. We have not expereinced any problems using this setup in motors or other equipment that can be affected by VFD's. It does add cost to the job but it is easier to fix issues on the front end than once production starts.