Load Reactors:

In this “Did You Know” article, we will cover Load Rectors and their uses for the output section of a VFD.

As you can recall, Line Reactors are used to extend the life of the drive and reduce harmonics.  Load Reactors are placed after the drive and perform some perks for the motor.  VFDs do not produce a sinewave like you would get directly from the power company.  They produce a distorted, almost square-like waveform.  The motor however works okay with this distorted waveform due to its inductive electrical characteristics.  But when the length of the motor leads start exceeding 75ft, then a phenomenon known as “voltage ring-up” or “voltage doubling” occurs.  That means a 480V motor would see close to 1000V spikes.   The insulation of the motor cannot handle this and will sooner or later breakdown and destroy the motor.  These spikes can also lead to bearing failures as the larger voltage can arc thru the bearing.  This is called “bearing pitting” or “bearing fluting”.  If you get voltage ring-up, a short circuit in the motor will occur.  With bearing failures, you get excessive noise and finally bearings lock up. 

This is where Load reactors come into play.  Adding a load reactor to motors above 75ft will reduce this phenomenon.  It also smooths out the VFD waveform somewhat, thus providing a cleaner sinewave.  This cleaner sinewave produces less losses in the motor, which leads to less heat in the motor.  The result?  An extended life for the motor.

The product used for Load reactors is the KLR and KDR series from TCI.

So, does the KDR fix all long motor lead problems? 
No, it doesn’t, but we have a solution for that too.  We’ll save that one for another “Did You Know” article, Motor Lengths.

Special thanks to National Power Quality and
Consulting for providing these Did You Know articles.

For electrical consulting needs, NPQC can be contacted at www.natpowerquality.com