Service manual

5.2.4 Sources of EMI
Modern frequency converters (see Illustration 5.1) utilize
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, and variable voltage PWM waveform. This high
rate of voltage change [dU/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.
AC Line
Motor
Filter reactor
IGBT
Filter capacitor
PWM waveform
Sine wave
InverterDC BusRectier
130BX137.10
Illustration 5.1 Frequency Converter Functionality Diagram
5.2.5
EMI Propagation
Frequency converter generated EMI is both conducted to
the mains and radiated to nearby conductors. See
Illustration 5.2.
130BC344.10
1
3
4
5
6
7
8
9
2
5
Illustration 5.2 Earth Currents
CAUTION
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 return to the 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.
130BC154.10
1
3
4
5
6
7
2
Illustration 5.3 Signal Conductor Currents
NOTE
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 in 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 5.4.
While these currents tend to travel back to the frequency
converter, imperfections in the system cause some current
Frequency Converter and Mot...
VLT
®
HVAC Basic Drive Service Manual
44 MG90X202 - VLT
®
is a registered Danfoss trademark
55