Mitigating Conducted EMI with Ferrite Rings

Electromagnetic interference (EMI) from unknown sources can significantly challenge machine manufacturers, especially when it impacts crucial components like encoders or other electronics.

EMI cannot be easily observed or directly removed to troubleshoot, unlike other common drive issues. Only its symptoms can effectively be used to identify the root cause. A common reaction to EMI issues is blindly adding complex and costly filter solutions to the system.

This blog post will explore how a simple, cost-effective solution – ferrite rings – resolved EMI problems in a real-world application.

Identifying the Problem: EMI and Encoder Signal Drift

In most troubleshooting scenarios, isolating the problem is straightforward. By observing the system, forming a hypothesis, and testing that hypothesis, one can systematically address each potential cause of an issue. But EMI poses a unique challenge.

EMI is an invisible, insidious challenge that cannot be observed or directly removed, presenting itself only through symptoms. In cases like these, EMI typically becomes the primary suspect of an issue only after other more tangible causes have been ruled out.

A machine manufacturer was using a KEB S6 drive to control a motor in a winch application. In this setup, the S6 drive sends a virtual incremental encoder signal (TTL) to a controller, which uses that signal to track the motor’s position. However, the manufacturer began to notice erratic behavior; the TTL signal would “run away” even when the motor was stationary. Since the encoder signal should remain stable when there is no movement, it became evident that some external interference was causing false encoder pulses on the encoder cable. This led to the conclusion that electromagnetic interference (EMI) was introducing phantom pulses onto the encoder cable, causing the controller to lose track of the motor’s position.

Approaching EMI Mitigation: Source, Receiver, and Transmission

When EMI is identified as the issue, it’s essential to address the issue as a wholistic system. In general, EMI mitigation is addressed in three different modes:

• Source – The origin of the EMI, which may be a noisy component or device.
• Receiver – The component affected by the EMI, like a cable or other device.
• Transmission – How EMI travels from the source to the receiver, whether via direct contact or over space.

For effective EMI mitigation, all of these should be considered together, but the approach may vary depending on the application. In this application, we consider all three modes:

1. Source: The VFD
In power systems, the drive itself often generates EMI. VFDs rely on pulse-width modulation (PWM) to generate near-sinusoidal waveforms, creating high-frequency noise that can impact nearby equipment. Additionally, harmonics generated by the VFD’s rectifier circuit contribute to EMI, with rapid dV/dt. Given that the VFD was essential to this application, removing or significantly altering is generally not an option, so alternative solutions are needed to limit its EMI.

2. Receiver: The Encoder Cable
In this case, the encoder cable was particularly vulnerable to picking up noise. High-quality encoder cables generally follow strict EMC design principles, with features like:

• Outer Shielding to block EMI.
• Twisted Pairs to equalize the signal differences.
• Shielded Pairs for double noise immunity.
• Prefabricated Connectors to ensure protection throughout the cable run and drain noise.

Ideally, a fully shielded encoder cable with prefabricated connectors on both ends would be used. The cable had to be split with a breakout board in this case. As a result, the encoder cable was left exposed to interference.

3. Transmission: Pathways of EMI
EMI can travel from the source to the receiver through several methods:

• Radiative
• Inductive
• Capacitive
• Conductive

Given that the encoder cable was exposed, it was susceptible to multiple modes of EMI transmission, creating additional challenges for mitigation.

Focusing on Conductive EMI Mitigation

With the source identified and the cable vulnerable to interference, the best approach was to limit conductive EMI transfer. Conducted EMI, which travels directly through cabling and shared ground, can be mitigated in several ways:

• Active Filters – Counter EMI signals with precise, active filtering, though often costly.
• Passive Filters – Simple inductors and capacitors to smooth out signals, a more affordable but slightly less effective solution.
• Ferrite Rings – Rings made of ferrous material that tamp down high-frequency EMI, providing simple, low-cost suppression.

Implementing Ferrite Rings: A Simple and Effective Solution

Ferrite rings were selected as the solution because they are simple and affordable. These rings help reduce common mode switching and, more importantly, leakage currents in the ground. This suppression effectively minimizes high-frequency interference. Ferrite rings are commonly used in variable frequency drive applications, where they are added to the pulse width modulation output to reduce conductive electromagnetic interference.

To apply this solution, ferrite rings were added to the encoder’s output cable, and the cable was looped around the ring for increased interference suppression. This setup stabilized the system, reducing conducted EMI from the VFD and restoring the encoder signal. The result was an immediate improvement: the encoder signal became stable, and the motor no longer exhibited erratic behavior.

Results: A Restored System

After installing the ferrite rings, the encoder drift issue was resolved, allowing the winch system to operate effectively. The rings prevented high-frequency noise from disrupting the encoder, thereby restoring the system’s accuracy. As a result, the winch provided consistent signal performance, even in environments with high EMI.

Conclusion: Ferrite Rings as an Effective EMI Solution

This case demonstrates that while ferrite rings may be a simple and low-cost approach, they can play a crucial role in EMI management. In this application, ferrite rings effectively controlled EMI, restoring encoder functionality without the need for costly redesign or advanced filters.

Common KEB part numbers for ferrite rings are:

• 0090390-5241
• 0090396-2010

However, it’s important to remember that while ferrite rings provide a valuable solution for EMI interference, they are no substitute for comprehensive EMI design. EMI should ideally be addressed during system design, with shielded cabling, properly grounded equipment, and careful component placement. For new systems, proactive EMI planning can prevent many of these issues from arising.