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5.2.4 Effect of EMI
While Electromagnetic Interference (EMI) related disturbances to adjustable frequency drive op-
eration are uncommon, the following detrimental EMI effects may be seen:
Motor speed fluctuations
Serial communication transmission errors
Drive CPU exception faults
Unexplained adjustable frequency drive trips
A disturbance resulting from other nearby equipment is more common. Generally, other industrial
control equipment has a high level of EMI immunity. However, non-industrial, commercial, and
consumer equipment is often susceptible to lower levels of EMI. Detrimental effects to these sys-
tems may include the following:
Pressure/flow/temperature signal transmitter signal distortion or aberrant behavior
Radio and TV interference
Telephone interference
Computer network data loss
Digital control system faults
5.2.5 Sources of EMI
Modern adjustable frequency drives (see Figure 5-1) utilize 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 adjustable frequency drive 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
Figure 5.1: Adjustable Frequency Drive Functionality Diagram
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5 Adjustable Frequency Drive and Motor Ap-
plications
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
5-7
5
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