GE Industrial Solutions AF-600 FP High Power (460V_575V 150HP and above) - Manual (Page 66)

5.2.5 Sources of EMI
Modern frequency converters (see Illustration 5-1) utilise Insulated-Gate Bipolar Transistors (IGBTs) to provide an efficient and cost effective means to create the
Pulse Width Modulated (PWM) output waveform necessary for accurate motor control. These devices rapidly switch the fixed DC bus voltage creating a variable
frequency, variable voltage PWM waveform. This high rate of voltage change [dV/dt] is the primary source of the frequency converter generated EMI.
The high rate of voltage change caused by the IGBT switching creates high frequency EMI.
130BX137.10
AC Line
Rectifier DC Bus Inverter
Motor
Filter reactor
IGBT
Filter capacitor
PWM waveform Sine wave
Illustration 5.1: Frequency Converter Functionality Diagram
5.2.6 EMI Propagation
Frequency converter generated EMI is both conducted to the mains and radiated to nearby conductors. See Illustrations 5-2 and 5-3.
130BX138.11
e l b a c r o t o M e n i L C A
Ground Potential 1 Potential 2 Potential 3
VFD
Motor
Stray capacitance Stray capacitance
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.
AF-650 GP and AF-600 FP High Power Service Manual
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