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Line reactor for VFD

Line reactor for VFD

Line reactor for VFD

Hello Everybody

I have 5 HP 1730 RPM, 5.1amps, 600 Volts motor for an airlock which I want to connect to VFD. I have 9 KVA 1336 A.B. VFD in stock. My question is that do I need to connect line reactor? If yes, then what should be its rating? Also it would be good to know for future how to calculate these ratings (amps and henry).

RE: Line reactor for VFD

Typically you just ask for a 5A, 5% reactor. If 5A is not available then jump up to the next available size.


RE: Line reactor for VFD

Unless you are in business of manufacturing, as a user/sepcifier I woule leave it to the VFD mfrs. It is typically selected by the VFD mfrs to match with the characteristics of the VFD. I typically specify that provide a reactor to limit the voltage THD on line side to be limited to a certain percetage , say 3% to 5%. This is a much more meaningful and useful spec.

RE: Line reactor for VFD

I always use input line reactors on Inverter installations.  They are cheap insurance. Check out MTE Corp.  They have a user freindly chart for selecting reactor sizes.  A 5% usually will do the trick.  Depending on how far away you put your motor from your drive, an output line reactor may also be helpful protect your motor.

RE: Line reactor for VFD

Before we can responsibly recommend any reactor, we need to know the motor insulation class and the wire length from the drive to the motor.

You never want to use a reactor when it is not needed because reactors drop voltage and can cause low voltage problems.

RE: Line reactor for VFD

Motor is having  Class F insulation and cable length will be around 700 feet.  

RE: Line reactor for VFD

It is advisable to have a dv/dt or sinewave filter between the VFD and motor at a voltage of 600vAC. This will reduce the peak-peak voltages from the VFD to the motor and protect it, especially at 700ft. Even with class F insulation the life of the motor windings will come under severe impact with a PWM drive on this voltage.
I assume your reference to a line reactor is between the VFD and AC supply. In which case ensure the line reactor is sized to the VFD rating rather than the motor rating.  

RE: Line reactor for VFD

With that length of 700ft between the motor and the drive you would most likely benefit from a line reactor installed on both line and load side of the VFD.

The line side reactor would help protect the drive from harmful power conditions.  It would also cut down on the harmonics introduced onto your power system by your drive.  

The load side reactor is a cheap way to help protect the motor windings from the spikes caused by the long distance of the motor cabling.

Also, with that distance of motor leads, I would recommend a shielded cable.  LAPP/Olflex makes a nice cable for the job.  I believe it is called Olflex VFD Slim Cable.

Sizing the Reactor to the load is important.  Line reactors do not perform as well underloaded.  However, you don't want to go over the ratings of the reactor either

Here is the link to the MTE selection chart:

MTE has a lot of useful information on their website.

RE: Line reactor for VFD

Are you asking about a reactor to put between the line and the VFD or between the VFD and the load?

When you ask about line reactor I assume you mean between the line and the VFD because that is the line side, hence line reactor.

If you were asking about a reactor for between the VFD and the load I would expect you to ask about a load reactor or motor reactor, being that side of the VFD.

But that's just me I guess.


RE: Line reactor for VFD

The nominal impedance of a line reactor is the percent impedance it represents as a source impedance. Calculate the voltage drop across the inductance in one phase and divide that by the line to neutral source voltage. For the 14 mH reactor that you have at its rated 4 amps, 4 X 2pi X 60 X 0.014 = 21.1 V. On a 600 V source, 21.1 / 600 / sqrt3 = 0.02, so it is a 2% reactor at its rated current on a 600 V line.

That may be enough to reduce the harmonic current to the point that the VFD input current is equal to the 5.1 amp output current, but it isn't going to reduce it to the 4 amp reactor rating.

RE: Line reactor for VFD

I certainly agree with ozmosis on the sine filter.  Seven hundred feet at 5hp 600VAC is extreme length and anything less than a full sine filter will not be adequate to protect the motor insulation.

Whether a reactor is installed in front of the drive is a separate issue entirely having to do with supply power network harmonics.  Do not use a reactor if the input network harmonics are at an acceptable level.

RE: Line reactor for VFD

I used to design plant systems with 30 to 40 VFD units in each installation -- I used 5% line reactors on all of the VFD's... they typically will be either 3 or 5% if spec'd with the original VFD... they will do 2 things:  a) reduce harmonics reflected back onto the system and b) let the VFD ride out systems disturbances such as capacitor bank switching, etc.

Be sure to spec inverter duty motors (I think mine were spec'd for insulation to withstand 1600v in 2 microseconds / standard is less) [it's been awhile and my specs are at work]...

I haven't used shielded cable - cable lengths were typically less than 300ft and little opportunity for interference to parallel cabling...

Used load filters on small hp (1/2hp or less if cabling exceeded 100ft) or for larger hp if cabling exceeded 700 to 1000' -- voltage spikes would exceed the insulation withstand ratings and the load reactors kept this within the ratings...

work with your motor/VFD suppliers to make sure all components are compatible for the application...

RE: Line reactor for VFD

A sinewave filter is overkill at 700ft. dvdt filters like KLC and V1K (even better) are applied up to 2000 ft regularly and sometimes higher. They will limit peak voltage to about 150% of drive input rms voltage.

I would only recommend a sinewave filter under the following applications:

True extreme lead length, greater than 3000ft. ie downhole.
HVAC where noise is an issue. The filter will transfer the noise from the motor to the filter.
Very old motors that would be difficult/costly to replace.
Shipboard where mixed and unknown loads are present.


RE: Line reactor for VFD

Magtiger, a sine filter at 600VAC, 5hp, 700ft is definitely not overkill.

Nineteen years of VFD experience and doing it wrong enough times to have paid my dues allows me to say that with a good level of confidence.

On the other hand, the comment above about "always using a 5% line reactor" is not good practice either.  A line reactor operating at its rated current drops the same percent voltage as its impedance % figure.  So, on a 460V service running at 460V, a 5% line reactor will drop the voltage to the drive by 5% which results in 437V.  Since drives cannot generally make voltage, that's all the motor is going to get at full speed.  Now, just add another 5% motor lead reactor "just to be sure" and there goes another 5% of the voltage at full load.  Clearly, not a good idea!

Line reactors are good things when you need them but to use them indiscriminately is a waste of money and can make other problems.

Better to use them when needed and that's all.

RE: Line reactor for VFD

My turn to agree with DickDV.
As a standard on any VFD driven motor at voltages >500Vac you will need a minimum of dv/dt filters. 700ft is certainly needing sinewave filter on a standard VFD when you are looking at P-P voltages on the PWM waveform of minimum 1200v, then you have a long cable length.The motor will cook if not.
Line reactors will provide some protection on slow, low energy surges but a spike of high energy and fast acting will go straight through the AC line reactor, maybe taking out the rectifier, IGBT or if you are lucky the drive will trip.
A number of VFD's have built in DC link reactors and so the harmonic performance is as good as fitting a 5% AC reactor except you do not get the voltage drop effect that DickDV mentions. Fitting an AC line reactor to a VFD that has DC link reactors does not improve the harmonic performance that much and the downside of voltage drop and lowering the efficiency of the VFD system is an important aspect.

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