Appendix A. EMC Standard Installation GuideVFD-ED
A-2 How to Prevent EMC
A-2-1 Types of EMC: 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.
A-2-2 How does EMC Transmit (Noise Transmission)
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 Figure A-1.
Noi se
Unshi el ded cabl e
Ground
Cst r ay
Send
L oad
Recei v e
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