Introduction to Electromagnetic Interference with VFDs

This is the first of two articles explaining what electromagnetic interference (EMI) is, how it affects components inside control panels, and how to reduce its impact.

Read the next EMI series article: 7 steps to reduce the negative effects of EMI.

What Is EMI?

Electromagnetic interference (EMI), also called radio frequency interference (RFI), refers to unwanted electromagnetic signals that disrupt the operation of electrical equipment. In elevator control panels, EMI can cause loss of serial communication, nuisance drive trips, and disruption of control signals.

EMI degrades equipment performance, shortens component life, and increases long-term maintenance costs.

Conducted and radiated EMI are two main types of high and low-frequency interference that must be considered when troubleshooting potential sources of EMI. Every EMI issue involves three elements: a source, a transmission path, and a receptor (or victim). This concept is illustrated below:

Conducted EMI

Conducted EMI is defined as interference that uses conductors as a path from a source to a receptor. For example, a motor encoder grounded to a noisy connection would conduct noise to the drive encoder interface. This conducted noise may distort voltage signals at the encoder interface. As a result, the drive may misinterpret motor speed or generate faults.

At first, operators may suspect incorrect parameter settings or a faulty interface board. Closer inspection reveals the culprit to be the poor grounding of the encoder cable.

Conducted EMI travels through shared power lines, grounding systems, or signal wires. Because these conductors are physically connected, electrical noise can easily move from the source to sensitive components. Proper grounding, correct shield termination, and separating high-power wiring from low-level signal wiring help reduce conducted interference.

Radiated EMI

Radiated EMI is interference that propagates via a wireless path from a source to a victim. This is commonly seen in elevator control panels, where AC motor wires are laid in parallel alongside low-voltage control wiring.

The result is a coupling between the wires, causing disturbances on the data transmission line. For example, motor wires laid close to a serial link between the PLC and the VFD can create signal coupling. This interference may corrupt the data packets transferred between the controller and drive.

Radiated EMI occurs when rapidly changing voltages and currents create electromagnetic fields around wires. Low-voltage control wiring placed too close to high-power motor leads can pick up unwanted signals from nearby electromagnetic fields.

How can you reduce the risk of interference? By implementing proper wire routing, adequate separation, and shielding.

How Is EMI Noise in VFDs Generated?

Drive Topology

EMI originates in the VFD’s input rectifier stage. A full-wave diode bridge converts incoming AC power into DC, introducing harmonic distortion. The input rectifier draws a non-sinusoidal current from the power supply during each AC voltage period.

This process creates current harmonics and voltage distortion on the power line. That distortion can then travel to other equipment connected to the same supply.

Related Article: How Pulse-Width Modulation Works in a VFD

Pulse-Width Modulation

VFD’s use pulse-width modulation (PWM) to provide a voltage to the AC motor. The VFD output transistors switch on and off at high frequency (around 8 kHz). This switching shapes the DC bus voltage into a simulated AC waveform with the desired frequency and amplitude.

A higher switching frequency produces a more sinusoidal current waveform to the motor. However, there are several trade-offs to consider.

Related Video: How Do Switching Frequencies Affect Harmonic Distortion

When an IGBT turns on, the voltage at the VFD output rapidly rises to the DC bus level. The rate of this voltage rise is known as dV/dt. High dV/dt rates create strong electrical fields around the motor cables. These rapid voltage changes can also send spikes down the cable toward the motor. Note: the VFD output is especially rich in EMI noise because of high-frequency transistor switching. This is important when considering how to lay out control and power wiring.

Common VFD EMI Problems in Elevator Systems

When EMI is not properly controlled, it can show up as performance issues inside the elevator control panel. Nearby control circuits are especially vulnerable because VFDs generate both conducted and radiated noise.

• Common VFD EMI problems include:

• Nuisance drive trip

• Encoder signal distortion

• Serial communication loss

• PLC data corruption

These symptoms often resemble parameter issues or component failures, so EMI can be overlooked during troubleshooting.

EMC Standards and VFD Installations

Electromagnetic compatibility (EMC) standards are vital when installing VFDs. They define acceptable limits for emitted interference and required immunity levels for equipment. In many regions, VFD systems must comply with emission and installation standards.

Meeting EMC standards often requires proper grounding, cable shielding, and the use of input or output filters. Understanding these standards helps ensure reliable operation, regulatory compliance, and reduced risk of interference with nearby devices.

FAQs

What devices cause electromagnetic interference?
Devices that switch current rapidly generate electromagnetic interference. That can include VFDs, inverters, radio transmitters, and more.

What is EMI in a VFD?
EMI in a VFD is unwanted electrical noise created by the drive’s rectifier and high-frequency PWM switching.

How do you reduce EMI from a VFD?
You can reduce EMI from a VFD by using proper grounding, separating power and control wiring, adding shielding, and installing filters when needed.

Do I need an EMI filter for my VFD?
An EMI filter may be necessary if your installation must meet regulatory standards or if the VFD is causing interference with other sensitive equipment.

EMI – Knowing Is Half the Battle

EMI refers to electromagnetic signals that interfere with the normal operation of electrical equipment. EMI problems often lead to poor equipment performance and increased maintenance costs throughout the system’s lifetime.

EMI is transmitted from a source to a receptor device via conduction and radiation. EMI problems in elevator VFD systems often stem from the drive’s non-linear components. High-frequency PWM switching of the output transistors further increases noise generation.

Understanding where EMI originates and how it travels is the first step toward controlling it. In the next article, we’ll outline seven practical steps for reducing EMI in VFD installations.