On-site, significant disturbance problems often occur due to Variable Frequency Drives (VFDs), which can even render the control system unusable. It has always been a headache-causing problem. Today, we will discuss how to deal with VFD disturbance problems!

COMMON PHENOMENA OF VFD DISTURBANCES:

When the VFD is turned on for the heat exchange station, the pressure transmitter behaves irregularly.
When using a VFD for water supply control, the pressure transducer, serving as a pressure sensor, is affected by VFD disturbances.
During motor startup by the VFD, the signal of the pressure transducer becomes unstable and fluctuates significantly.
The pressure transducer (4-20mA) randomly jumps after the VFD starts, while the nearby integrated resistance thermometer (4-20mA) remains unaffected, even though the signal lines are not shielded.

All these phenomena are caused by VFD disturbances.

WHY DOES A VFD GENERATE DISTURBANCES?

First and foremost, it is important to know that VFDs are used to change frequencies.

A VFD consists of rectifier circuits and inverter circuits. The input AC voltage passes through the rectifier circuits and smoothing circuit, converting it into DC voltage. Then, the inverter converts the DC voltage into pulsed voltage with variable width.

This PWM voltage drives the motor, allowing adjustment of the motor’s torque and speed.

This working principle results in three types of electromagnetic disturbances:

Harmonic disturbances:

The rectifier circuits generate harmonic currents, which cause voltage drops across the impedance of the power supply system and distort the voltage waveform.

This distorted voltage disturbs many instruments, and a common distortion is flattening the peak of the sine wave. When the harmonic current is constant, voltage distortion is more pronounced in weak power sources.

This disturbance characteristic affects devices using the same power grid, regardless of the distance between the device and the VFD.

Radiated emissions from high-frequency lines:

Since the load voltage is in a pulsating form, the VFD draws current from the power grid in a pulsating manner.

This pulsating current contains a significant amount of high-frequency components, leading to radio frequency interference.

This disturbance characteristic affects instruments using the same power grid, regardless of the distance between the instrument and the VFD.

Radiated emissions from high-frequency radiation:

High-frequency radiation interference originates from the input and output cables of the VFD.

When high-frequency interference currents occur on the input and output cables of the VFD, the cables act as antennas, generating electromagnetic wave radiation that causes radiated disturbances.

The PWM voltage transmitted through the output cables of the VFD also contains abundant high-frequency components, resulting in electromagnetic wave radiation and radiated disturbances.

The characteristic of radiated disturbances is that the disturbance phenomena become more severe when other electronic devices are near the VFD.

HOW TO SOLVE THE DISTURBANCE PROBLEM?

Methods for coping with VFD disturbances:

The VFD should be grounded at a single point, preferably using a short and thick grounding cable.
For sensor signal lines, shielded cables with double shielding should be used, with the shielding layer grounded using cable clamps.
Install power filters, magnetic filter rings, or isolation amplifiers on the sensor’s power supply to achieve separation.
To suppress the harmonics generated by the VFD, optional filter products such as VFD input filters, VFD output filters, VFD input reactors, VFD output reactors, etc., can be used. Inserting a reactor in the input circuit is an effective method to suppress lower harmonic currents.
Additionally, to avoid VFD interference signals and control loops, separate isolated power supplies should be used to power control devices, instruments, and industrial computers.