Introduction
VFD-related electromagnetic interference can appear in multiple forms. Each type has distinct characteristics, propagation paths, and mitigation strategies.
This article explains the three main types of VFD EMI encountered in industrial automation systems.
Harmonic Interference
Harmonic interference originates from the VFD’s input rectifier stage. Harmonic currents flowing through the system impedance cause voltage waveform distortion, often visible as flattened voltage peaks.
This type of interference:
Affects all equipment connected to the same power network
Is independent of physical distance
Becomes more severe in weak or lightly regulated power systems
Sensitive electronic devices, such as PLCs and instrumentation, may malfunction when exposed to distorted supply voltage.
Power Quality Disturbances Affecting VFDs
In many facilities, VFDs share the same supply with large switching loads or capacitor banks used for power factor correction. During capacitor switching or thyristor commutation, transient overvoltages and voltage notches may occur.
If these disturbances are not properly suppressed, they can stress the VFD input rectifier and, in extreme cases, lead to diode failure or unexpected shutdowns.
Conducted Radio-Frequency Interference
Because the VFD output voltage is PWM-based, the input current drawn from the grid is also discontinuous and rich in high-frequency components.
These high-frequency currents travel along power cables and enter other devices through their power supplies, creating conducted RF interference.
This interference typically causes:
Radiated Interference
When high-frequency currents flow through VFD input or motor cables, the cables act as unintended antennas and radiate electromagnetic energy.
Radiated interference is characterized by:
Stronger effects at short distances
High sensitivity of nearby control equipment
Increased severity with long motor cable runs
PWM motor cables are among the most significant sources of radiated EMI in VFD systems.
The Three Elements of EMI
All electromagnetic interference requires:
A source of interference
A coupling path
A susceptible system
Effective EMI control must address all three elements simultaneously rather than focusing on a single countermeasure.
Implications for VFD Applications
Understanding the type of interference present helps engineers determine whether grounding improvements, filtering, shielding, or isolation measures are required.
VFDs that support flexible EMI mitigation accessories allow system designers to adapt to different installation conditions without changing the drive itself.
