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Frequency Converter and Mot...
5.3.3 EMI Propagation
Frequency converter generated EMI is both conducted to the mains and radiated to nearby conductors. See Illustration 5.3.
Stray capacitance between the motor conductors, equipment earth, and other nearby conductors results in induced high
frequency currents.
High earth circuit impedance at high frequencies results in an instantaneous voltage at points reputed to be at earth
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 frequency converter's DC bus via the earth circuit and a high frequency (HF)
bypass network within the frequency converter itself. However, imperfections in the frequency converter earthing or the
equipment earth system can cause some of the currents to travel out to the power network.
Unprotected or poorly routed signal conductors located close to or in parallel to motor and mains 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 frequency converter or the interconnected control device. See
Illustration 5.5.
While these currents will tend to travel back to the frequency converter, imperfections in the system will cause some
current to flow in undesirable paths thus exposing other locations to the EMI.
High frequency currents can be coupled into the mains supplying the frequency converter when the mains conductors are
located close to the motor cables.
A
C
L
n i
e
Ground
Illustration 5.3 Earth Currents
High Power Service Manual for Modular F Frame Drives
VFD
M
t o
r o
c
a
b
Stray capacitance
Potential 1
Potential 2
®
MG90K202 - VLT
is a registered Danfoss trademark
Motor
e l
Stray capacitance
Potential 3
5
5
81
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