Emi Propagation; Signal Conductor Currents - Danfoss VLT Series Service Manual

EMI Propagation

Drive generated EMI is both conducted to the AC line and
radiated to nearby conductors. See Figures 4-2 and 4-3
for illustrations.
AC Line
Ground
Stray capacitance between the motor conductors, equipment ground,
and other nearby conductors 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 drive's DC
bus via the ground circuit and a High Frequency (HF) bypass
network within the drive itself. However, imperfections in
the drive grounding or the equipment ground system can
cause some of the currents to travel out to the power
network.
AC Line
Unprotected or poorly routed signal conductors located close to or in
parallel to motor and AC line conductors are susceptible to EMI.
4-6
VLT is a registered Danfoss trademark
VFD
Motor cable
VLT
Potential 1 Potential 2
Figure 4-2. Ground Currents
VFD
Motor cable
VLT
Figure 4-3. Signal Conductor Currents
Motor
Stray capacitance
Potential 3
Motor
Stray capacitance
Signal wiring
Stray capacitance
to BMS