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Chapter 19 EMC Standard Installation Guide

CT2000 Series
19-5
19-2 How to prevent EMI
19-2.1 Types of EMI: Common-mode and differential-mode noise
The electromagnetic noise of an AC motor drive can be distinguished into common-mode and differential-
mode noise. Differential-mode noise is caused by the stray capacitance between the conducting wires and
common-mode noise is caused by the common-mode coupling current path created by the stray capacitance
between the conducting wires and ground.
Basically, differential-mode noise has a greater impact to the AC motor drive and common-mode noise has a
greater impact to high-sensitivity electronic devices. An excessive amount of differential-mode noise may trigger
the circuit protection system of the AC motor drive. Common-mode noise affects peripheral electronic devices via
the common ground connection.
EMC problems can be more serious when the following conditions apply:

When a large horsepower AC motor drive is connected to a large horsepower motor.

The AC motor drive’s operation voltage increases.

Fast switching of the IGBTs.

When a long cable is used to connect the motor to the AC motor drive.
19-2.2 How does EMI transmit? (Noise transmission path)
Noise disturbs peripheral high-sensitivity electrical devices/systems via conduction and radiation, their
transmission paths are shown hereafter:
1. Noise current in the unshielded power cable is conducted to ground via stray capacitances into a common-
mode voltage. Whether or not other modules are capable to resist this common-mode noise depends on their
Common-Mode Rejection Ratio (CMRR), as shown in the following figure.
Noise
Unshielded cable
Ground
Cstray
Send
Load
Receive
2. Common-mode noise in the power cable is transmitted through the stray capacitance and coupled into the
adjacent signal cable, as shown in Figure 2. Several methods can be applied to reduce the effect of this
common-mode noise; for example, shield the power cable and/or the signal cables, separate the power and
signal cables, take the input and output side of the signal cable and twist them together to balance out the stray
capacitance, let power cables and signal cables cross at 90°, etc.