5.2.6 EMI Propagation
Adjustable frequency drive generated EMI is both conducted to the line power and radiated to
nearby conductors. See figures 5-2 and 5-3.
130BX138.11
Motor Cable
AC Line
Ground
Potential 1
Potential 2
Potential 3
VFD
Motor
Stray capacitance
Stray capacitance
Figure 5.2: Ground Currents
Stray capacitance between the motor conductors, equipment ground, and other nearby conduc-
tors results in induced high frequency currents.
High ground circuit impedance at high frequencies results in an instantaneous voltage at points
reputed to be at
ground potential
. This voltage can appear throughout a system as a common
mode signal that can interfere with control signals.
Theoretically, these currents will return to the adjustable frequency drive’s DC bus via the ground
circuit and a high frequency (HF) bypass network within the adjustable frequency drive itself.
However, imperfections in the adjustable frequency drive grounding or the equipment ground
system can cause some of the currents to travel out to the power network.
130BX139.11
Motor Cable
AC Line
VFD
Motor
Stray capacitance
Signal wiring
to BMS
Figure 5.3: Signal Conductor Currents
Unprotected or poorly routed signal conductors located close to or in parallel to motor and line
power conductors are susceptible to EMI.
Signal conductors are especially vulnerable when they are run parallel to the power conductors
for any distance. EMI coupled into these conductors can affect either the adjustable frequency
drive or the interconnected control device. See Figure 5-4.
5 Adjustable Frequency Drive and Motor Ap-
plications
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5-8
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
5
Summary of Contents for VLT HVAC FC 100
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