Chapter 2 How To Prevent Emi; Types Of Emi: Common-Mode And Differential Mode Noise; How Does Emi Transmit? (Noise Transmission) - Delta C2000 Series User Manual

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.
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.
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.

Chapter 2 How to prevent EMI

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