Motor Lengths:

In this “Did You Know” article, we will continue touching on Long Leads for motors. 

In our Load Reactor article, we learned that once the VFD and motor distance becomes greater than 75ft, it’s time to consider a load reactor.  But what about when it’s much greater than 75ft, are there other choices?  Yes, there are, generically known as “dv/dt” filters.  dv/dt filters are nothing more than some extra circuitry added to the load reactors. 

These are generally good for up to 400ft or so.  So what about beyond that?  Well, then comes in the generically known, “sinewave filter” which is meant for super long leads up to 10,000 ft (that’s about 2 miles of cables).  This is a highly engineered product that makes the VFD output look like a near-perfect sinewave.  It is quite larger than the load reactor and dv/dt filter and as you might guess quite a bit of cost difference as well.

So here are some general rules of thumb, and if you ask other folks the lengths may change around somewhat, depending on which marketing team got to them first.

One question that is asked a lot is “can I use a filter even though the length may be shorter than what is required?” 
Yes, you can, but most won’t spend the extra money.  Some OEMs simply supply a V1K filter because they know it will be “a few hundred feet” to the motor.  That’s a good insurance policy to avoid breakdowns and possible liabilities.

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

For electrical consulting needs, NPQC can be contacted at