LMV Series Technical Instructions
Document No. LV3-1000
SCC Inc. Page 3 Section 5
As mentioned above, VFDs switch multiple IGBTs on and off very rapidly to generate a "modified" sine
wave on all three phases going to the motor. Doing this has some tradeoffs, one of which is electrical
noise, or harmonics. This noise is typically "wire borne" instead of airborne, and can cause issues with
electronics in some situations. Thankfully, electrical noise associated with VFDs can be mitigated using
proper wiring techniques (connecting shields and grounds correctly) and by the proper application of
line reactors and / or load reactors for some applications. For difficult applications, EMC filters for the
VFD are also available.
Line Reactors
Line reactors, or "chokes", are typically used when the impedance on the input side of the drive is low.
Impedance on the input side of the drive is typically low when a relatively small VFD is being fed by a
relatively large transformer. In this situation, the supply side of the drive is "stiff", meaning that an
instantaneous current draw by the drive will be met very quickly by the large transformer (think square
wave form), causing voltage and current distortions in the power distribution system feeding the drive.
In this situation, adding a line reactor will add reactance which opposes instantaneous current draw and
"softens" the input side of the drive.
Conversely, if the transformer feeding the drive is not large relative to the drive, the impedance on the
input side of the drive is higher and the system is "softer". In this situation, an instantaneous current
draw by the drive will not be met as quickly, and the resulting voltage and current distortions in the
power distribution system feeding the drive will be smaller. An additional line reactor in this situation is
not needed.
Figure 5-1: Line Reactor Recommendation - VFD (HP) vs Transformer (kVA)
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