Line Reactors for AC inverters

[PLCRookie]

Just curious, how many of you out there specify/install line reactors when you use an AC inverter? I am installing a 2hp AC inverter and have purchased a line reactor for the incoming and outgoing power (Per the reccommendation of the manufacturer). My coworkers think I am crazy to do this."We aint never had them there things before".

I would like to know others opinions.

Thanks

 

[jimbo3123]

Just curious, how many of you out there specify/install line reactors when you use an AC inverter? I am installing a 2hp AC inverter and have purchased a line reactor for the incoming and outgoing power (Per the reccommendation of the manufacturer). My coworkers think I am crazy to do this."We aint never had them there things before".

I would like to know others opinions.

Thanks

For me, it depends on a lot of things (wire run lengths, etc.)
However, I ALWAYS follow the mfg's reccomendations, especially with something like this. A line reactor is A LOT cheaper and easier to install than a new drive or motor.

Besides, the reactors can only improve power quality and motor lifespan.

 

[Tom Jenkins]

Many VFDs have line reactors on the input as standard. You have to make sure you always specify line side (input power) or load side (motor power) when you are talking about protection and filtering.

Normally output reactors or filters aren't necessary if the motor to VFD power wiring is short. Normally you don't need special precautions for filtering on the line side of the VFD. However, if the VFD is a large part of the total system power draw or if your utility has restrictions on distrotion you may need to take extra precautions. As jimbo said, throwing a VFD in isn't as simple as a starter, and the drive supplier should be made aware of the details of the installation.

 

[nswu1]

As a standard I use front and back end reactors.

 

[jraef]

.... A line reactor is A LOT cheaper and easier to install than a new drive or motor. ...

To me, that's the key statement right there. I have seen way too many VFDs let their smoke out because of a short on the load side. The VFDs advertize that they have short circuit protection, but it is designed to protect DOWNSTREAM equipment from colateral damage. It typically is NOT fast enough to protect the transistors themselves. An output reactor slows down the fault rise time to where it can. As to input reactors, if the VFD already has them (or a DC bus choke) I usually don't add them, but as jimbo3123 said, it depends. Some VFDs put in line chokes only to reduce EMI emmissions, which may or may not address other problems. Nothing beats a thorough investigation.

 

[Tom Jenkins]

As a standard I use front and back end reactors.

I personally dislike that kind of thinking. You not only tend to waste money, but you may have a warm fuzzy feeling of security that isn't justified because the "standard" isn't doing for you what you think it is.

For example, line reactors aren't intended for motor protection. They are intended to to minimize harmonics. On the line side of the drive that means they will reduce the amount of electrcal noise and in some cases transformer heating.

Now, it is true that the harmonics and dV/dt effects on the load side can cause some heating in a motor's windings. But, depending on the length of wiring and the carrier frequency and so on, a reactor may not help. Or a reflective wave trap may be both more economical and more effective. Or the installation and motor selection may be such that nothing whatever is required to provide adequate motor life.

Dick DV can certainly add more detail and the engineering basis for the above, and correct any misstatements I may have made.

However, in my opinion, standards should never be used as a substitute for good engineering.

 

[DickDV]

It will never be as simple as "I specify input and load reactors on all drives".

Let's just think this thru for a minute. If you have dual voltage motors and are running them on the lower voltage, 230V or 190V, load side reactors give you absolutely nothing in terms of protection and actually waste some of the system energy as heat. To say nothing of taking up space and costing real money.

Next, if you have spent your precious cash for Insulation Class F motors with MG1 Part 31 endorsement, you are good for 60 feet motor lead length at 10hp and 250 feet motor lead length at 100hp. You can extrapolate between those points linearly. Why spend the money for a good motor and then spend it again for a reactor? Further, motor lead reactors make it much harder to do speed and torque control in sensorless vector mode so they are to be avoided as much as possible for those applications.

Further, if you really have longgggggggggg motor leads or the potential for common mode noise, a reactor may make you feel all warm and fuzzy but, in the end, it will still keep you up nights with false faulting, ground noise and motor insulation damage. Unfortunately, in this field, you actually have to think, analyze, and then respond appropriately sometimes.

As for input reactors, first and foremost, reactors drop voltage at rated current, 3% of the voltage for a 3% reactor and 5% for a 5% unit. If you are in a low voltage invironment to start with and you automatically add a reactor just to "feel good", you just made your low voltage problems worse. And, how about all those specs written "5% reactor required ahead of ALL drives" when the drive is already built with 3% reactance internally. That's 8% voltage drop at full load! Let's see here. 460V times .92 is 423V. Maybe this isn't so smart after all! Did I mention that these things cost money and waste energy.

Further, let's say your drive system is to be installed on one of these miserable floating delta power sources. You can add all the reactors you want and you will just make the common mode noise problems worse. You need a drive isolation transformer with a grounded wye secondary to give the ground pulses a short path back to the DC bus. Reactors don't do that.

Further, if there are PF caps upstream somewhere (no, the customer will not tell you where) and you add reactors, you risk putting the power supply into resonance. Probably want to avoid that if you don't really need reactors to start with.

Some manufacturers are now starting to add swinging (saturable) reactor in the DC bus instead of input reactors. These will give you the harmonic suppression you need just like an input reactor but without the high load voltage losses. Very crafty! What they don't tell you is that the swinging choke doesn't protect your input against voltage spikes nearly as good so that could be a negative.

I guess I've said enough on this. Bottom line: you have to think about what you are doing and maybe even analyse or take data. The alternative is to waste money and energy and simply kid yourself about the results.

 

[PLCRookie]

Thanks for all the comments. The motor on this particular application is about 40 ft. total from the enclosure. It is in a very hard to reach place. I was told it is very old, and "would not last long" with the inverter. I would like to have a good inverter rated motor for a spare, but these guys aren't much for spare parts."When it breaks, we'll get a new one".

Obviously there are more factors to consider when applying an inverter, in this case I thought(possibly erroneously) that the line reactors could only help the situation. i guess only time will tell.

Again thanks for all the info.

 

[Logix]

Here's an Allen Bradley link that pretty much backs up what Dick and Tom said.


http://www.ab.com/drives/techpapers/Line%20Reactors%20and%20AC%20Drives1.pdf


I've tried to use input reactors in all cases in the past, more to isolate the noise that the drive creates from the rest of the electronics then for drive protection. I guess I'll have to research more before I make that decision in the future.

 

[PLCRookie]

Thanks for the link Logix. Just what I needed to help me clarify the issue somewhat.

 

[DickDV]

PLCRookie, if this is an "old" motor, it may well be an Insulation Class B unit. If it is, and you are powering the drive at 460V, a 5% motor lead reactor would be good insurance as long as you don't need super precise speed and torque regulation from the drive/motor system. If the motor is wound for dual voltage and you want to protect it even further, put a 460/230V transformer ahead of a 230V 2hp drive and rewire the motor for 230V. That and maybe even the motor lead reactor should keep the "old" motor singing along for many years as long as you don't run it so slow it can't cool itself.

Good luck!

 

[katratzi]

DickDV- what equipment would you recommend for analyzing the output of a VFD? I assume the measurements should be made as close to the motor as possible, so you are testing "what the motor sees". I run across a lot of installations where the motors are located 200-500 feet from the drive, and would like to know if looking at the voltage/current with an osilloscope or other instrument can help in determining whether a problem exists, and if something such as a reactor is necessary.

 

[Leadfoot]

DickDV- what equipment would you recommend for analyzing the output of a VFD? I assume the measurements should be made as close to the motor as possible, so you are testing "what the motor sees". I run across a lot of installations where the motors are located 200-500 feet from the drive, and would like to know if looking at the voltage/current with an osilloscope or other instrument can help in determining whether a problem exists, and if something such as a reactor is necessary.

I have used a Tek scope 2205, 710, 710A and like the 720 best with a 2.5KV 100:1 probe to look at the voltage waves at the motors. They were 575 vac motors. A current probe that attached to your scope is also good to have to see the individual motor current wave. No other specialized equipment is really necessary.

The PWM can cause rather high level voltage spikes at the motor on long runs. For this you set the carrier freq to 2K and an output reactor helps a bunch. I have seen a "snubber/RC filter" on the drive output that helps suppress the voltage spikes by causing the leading edge to roll off. These filters have NO effect on the overall function and actually help the motor insualtion last longer by eliminating the need for special insulation characterisitcs in the motor.