VFD Line and Load Reactors

[joshtrevino]

With modern VFD pulse width modulation technology are line reactors needed in any application other than extremely long VFD to motor feeders (>1000ft)?

Similarly are line reactors needed in any application other than locations where the source has significant harmonics issues? It seems to me that the practice of protecting other upstream or parallel devices from harmonic distortion caused by the VFD is no longer a real issue. Thoughts?

 

[Jraef]

It has nothing to do with how "modern" the drive is.

First off, a reactor on the LINE side is a Line reactor, if it is between the drive and motor, it's a Load Reactor. they are the same device, but they are sized differently so it's always important to make that distinction when ordering them.

A Line Reactor all by itself as a harmonics mitigation strategy is limited. Their more intrinsic value is as cheap insurance for the drive. It slows down any rapid change in current coming from line (grid) transients and lowers the amplitude to levels easier for the drive to survive. 20 years ago, grid switching was relatively infrequent, maybe a few times per week. Grid switching transients are now happening hundreds of times per day now, because of how the NIMBY (Not In My Back Yard) syndrome affects utilities. They have a hard time building new infrastructure, so they must switch power around a lot more often and every time a HV switch opens or closes, it makes a transient spike. A line reactor dampens those spikes. To that end though, one large LR ahead of several small VFDs does just as much good as a single LR ahead of each VFD, so there is no point in building them into the VFDs at extra expense.

A Load Reactor is cheap insurance for the motor. Same principle, different effects. It slows down the rise time of the individual PWM DC pulses going to the motor, so it decreases the possibility of creating Standing Waves, as well as causing capacitive coupling in the motor itself that can lead to bearing damage. It's not a solution in and of itself, it is PART of a strategy to mitigate issues, IF the conditions are right to need mitigation.

 

[ghostbuster]

It has nothing to do with how "modern" the drive is.

First off, a reactor on the LINE side is a Line reactor, if it is between the drive and motor, it's a Load Reactor. they are the same device, but they are sized differently so it's always important to make that distinction when ordering them.

A Line Reactor all by itself as a harmonics mitigation strategy is limited. Their more intrinsic value is as cheap insurance for the drive. It slows down any rapid change in current coming from line (grid) transients and lowers the amplitude to levels easier for the drive to survive. 20 years ago, grid switching was relatively infrequent, maybe a few times per week. Grid switching transients are now happening hundreds of times per day now, because of how the NIMBY (Not In My Back Yard) syndrome affects utilities. They have a hard time building new infrastructure, so they must switch power around a lot more often and every time a HV switch opens or closes, it makes a transient spike. A line reactor dampens those spikes. To that end though, one large LR ahead of several small VFDs does just as much good as a single LR ahead of each VFD, so there is no point in building them into the VFDs at extra expense.

A Load Reactor is cheap insurance for the motor. Same principle, different effects. It slows down the rise time of the individual PWM DC pulses going to the motor, so it decreases the possibility of creating Standing Waves, as well as causing capacitive coupling in the motor itself that can lead to bearing damage. It's not a solution in and of itself, it is PART of a strategy to mitigate issues, IF the conditions are right to need mitigation.

I agree with you with 100%. Also recently in our area ,utility cap bank switching is done automatically by the utility computer systems.This can occur anytime throughout the day and create havoc (High voltage transients etc.) with end users using VFD's in their processes. In the "good old Days" the utility would manually switch these caps on early in the morning (5 to 6 am.)

This would reduce the chance of VFD drive problems that only operate during the time period 7 to 6 pm.)

 

[ptonsparky]

How is the line reactor sized differently? What are the considerations?

 

[joshtrevino]

It has nothing to do with how "modern" the drive is.

First off, a reactor on the LINE side is a Line reactor, if it is between the drive and motor, it's a Load Reactor. they are the same device, but they are sized differently so it's always important to make that distinction when ordering them.

A Line Reactor all by itself as a harmonics mitigation strategy is limited. Their more intrinsic value is as cheap insurance for the drive. It slows down any rapid change in current coming from line (grid) transients and lowers the amplitude to levels easier for the drive to survive. 20 years ago, grid switching was relatively infrequent, maybe a few times per week. Grid switching transients are now happening hundreds of times per day now, because of how the NIMBY (Not In My Back Yard) syndrome affects utilities. They have a hard time building new infrastructure, so they must switch power around a lot more often and every time a HV switch opens or closes, it makes a transient spike. A line reactor dampens those spikes. To that end though, one large LR ahead of several small VFDs does just as much good as a single LR ahead of each VFD, so there is no point in building them into the VFDs at extra expense.

A Load Reactor is cheap insurance for the motor. Same principle, different effects. It slows down the rise time of the individual PWM DC pulses going to the motor, so it decreases the possibility of creating Standing Waves, as well as causing capacitive coupling in the motor itself that can lead to bearing damage. It's not a solution in and of itself, it is PART of a strategy to mitigate issues, IF the conditions are right to need mitigation.

I meant to address load reactors in the first paragraph of my question.

I appreciate this input, especially about the effects of utility switching, however, most of the utility transients should be sufficiently mitigated by transformer, feeder, etc impedances between the utility connection and the drives, correct?

Also, it seems to me that the "modern" drive does have something to do with the need for load reactors because pulse width modulation is so much more refined than it was 15 years ago. The dV/dT that contributes to "long lead effect" is much less and inverter duty motors are designed more appropriately for VFD applications. It just seems to me that the 300-400 foot motor lead rule of thumb is not really applicable anymore. Which leads to the question that I originally meant to ask:

(Questions addressed to all reading the post) When are load reactors really NEEDED? I know that they are cheap insurance and won't do any harm if not needed, but wasted money is wasted money.

Similarly, would anyone agree that load reactors are only really needed if significant transients or harmonics from the utility or local generation are measured at the drive level?

 

[ptonsparky]

AB has a short paper about sizing and when they may be needed. I read it this morning so now I know everything.:happysad: I don't know if it can be linked to without permission or you may just have to register with them.

 

[Jraef]

AB has a short paper about sizing and when they may be needed. I read it this morning so now I know everything.:happysad: I don't know if it can be linked to without permission or you may just have to register with them.

In a nutshell, anything on the LOAD side of the drive will have to be sized to include the reactive current in addition to the active current, essentially you have the motor power factor to contend with, so the reactor has to be sized for the actual total current going through it.

If the reactor is on the line side, the motor power factor has already been corrected by the drive, so the current through the reactor is lower and the reactor can be smaller.

 

[Jraef]

I meant to address load reactors in the first paragraph of my question.

I appreciate this input, especially about the effects of utility switching, however, most of the utility transients should be sufficiently mitigated by transformer, feeder, etc impedances between the utility connection and the drives, correct?

Also, it seems to me that the "modern" drive does have something to do with the need for load reactors because pulse width modulation is so much more refined than it was 15 years ago. The dV/dT that contributes to "long lead effect" is much less and inverter duty motors are designed more appropriately for VFD applications. It just seems to me that the 300-400 foot motor lead rule of thumb is not really applicable anymore. Which leads to the question that I originally meant to ask:

(Questions addressed to all reading the post) When are load reactors really NEEDED? I know that they are cheap insurance and won't do any harm if not needed, but wasted money is wasted money.

Similarly, would anyone agree that load reactors are only really needed if significant transients or harmonics from the utility or local generation are measured at the drive level?

As to transients being mitigated by transformers etc., you are partially correct, but it depends on the size of the transformer. The quick-and-dirty rule is that if your supply transformer is more than 10x the kVA of the VFD down stream of it, the VFD can see very damaging current spikes. So for example if your facility has a 1,000 kVA transformer, and you have a 25HP drive, put in a line reactor. We did a study at one of our facilities using that example. With no reactor, the VFD, which has diodes rated around 60A, saw current spikes of 805A dozens of times per day from grid transients. Putting in a simple 3% line reactor brought that down to 55A max. as seen below the reactor. Conversely though if you have a 100kVA xfmr ahead of a 25HP drive, you probably don't need to worry as much.

You are actually flipped on the dV/dt issue. The newer the drive, the faster the transistors are turning on (upwards of 5x faster than the previous generation of transistor on the latest versions), which has INCREASED the dV/dt issue because the leading edge of each pulse is now a LOT steeper than it used to be. But to combat that, most large mfrs of drives (as opposed to cloners who are after the cheap-o market) have developed firing techniques that mitigate some of the effects. But that tends to be done in the more expensive versions of their drives, not the "component class" small drives, because those are intended to compete against the cloners. In addition, at 10HP and below, even "inverter duty" motors cannot use all of the tricks to mitigate the effects of standing waves. One of those tricks is to use added "phase paper" in the slots. In smaller frames, there is just no room for it. So right where the cheaper drives that lack mitigation techniques play a larger role in the market is also right where even inverter motors are more susceptible to damage.