Emi Propagation - GE AF-650 GP Operating Instructions Manual

5.3.3 EMI Propagation

Frequency converter generated EMI is both conducted to the mains and radiated to nearby conductors. See illustrations below.
A C L n i e
Ground
Illustration 5.2: Earth Currents
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.
Illustration 5.3: Signal Conductor Currents
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 the following illustration.
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.
Drive
M t o r o c a b
Stray capacitance
Potential 1 Potential 2
High Power Service Manual for Unit Sizes 6x
Motor
e l
Stray capacitance
Potential 3
5
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