The Good, the Bad, and the Ugly: Using Frequency Filters for Control Systems

What is Electromagnetic Interference (EMI)?

With today’s factories utilizing automation to manufacture components faster and of higher quality, the amount of electromagnetic interference (EMI) has grown within the factory and within the equipment. The problem with EMI is that it can cause damage to equipment or make sensors report incorrect data, and when decisions are based on that data, people might get hurt or equipment might do unexpected tasks as a result.

 

Figure 1. Electromagnetic interference is all around us, caused by everything from motors to cell phone signals. Image used courtesy of Bruno Oliveira

 

EMI can come from man-made sources such as welding equipment, handheld radios, or even servo motors. Another source of EMI is from the environment itself, through solar storms or lightning strikes. It is for these reasons that adding frequency filters to electrical installations is very important.

 

Sources of EMI and its Effects

In industrial automation equipment, there are many sources of EMI, but the most common are servo and variable frequency drives (VFDs). These components produce signals with high frequency on/off cycles to provide precise control over the motor speed and torque. On average, these drives can produce signals at a frequency of 8 kHz - 20 kHz—that is up to 20,000 on/off cycles per second. 

This high frequency can induce voltages into other equipment. If there were an analog sensor nearby, its output signal might pick up some of this induced voltage causing an inaccurate reading. 

Communication devices are also susceptible to induced low voltage, so if unshielded Ethernet cables are running in parallel to servo cables, communications could be disrupted to important devices. Handheld or wireless network devices rely on frequencies to transmit and receive signals, but if these devices are subjected to high frequency-induced voltage their ability to transmit and receive signals might be disrupted which could cause an accident if that communication is required for operator safety. 

 

Figure 2. Motor protection filters remove harmonics generated by heavy inductive loading caused by motor coils. Image used courtesy of MTE Corporation

 

EMI Mitigation

There are few options when trying to mitigate EMI. Some are costly and require expensive equipment—others are simple design practices. 

 

Proper Cable Placement and Routing

Separate high-frequency cables. During the design phase, identify cables that might use high-frequency signals and separate them from equipment that might be susceptible to induced voltage or high-frequency. If possible, run the cables in a different cable tray/track. If it's not possible, then try to avoid long parallel runs of the two cable types

 

Shielded Cable

Use shielded cables. Whenever possible, try to use shielded cables with a proper grounding or drain wire. This ensures that any EMI that the cable is exposed to will be properly disposed of through the ground terminal. For Ethernet, always use high-quality, industrial shielded cables, and ensure your Ethernet switches are grounded on the PE/chassis terminal.

 

Minimize Cable Length

Avoid unnecessary cable lengths. While the cable itself is not a source of EMI, it does act as an antenna, so by avoiding unnecessary cable lengths, you can reduce the amount of EMI that is transported around your equipment. 

 

Frequency Filter

Install frequency filters. Frequency filters are devices that are installed inside the electrical cabinet in series with the source voltage of sensitive equipment such as VFDs or servo drives. Their job is to filter out the high-frequency but let lower-frequency voltage pass through. 

 

Figure 3. In-line 3-phase filter. Image used courtesy of Enerdoor

 

Filtering Noisy Industrial Signals

Frequency filters are used in a number of applications. They come in three main categories with many other variations and are suitable for a variety of different voltages. Frequency filters will typically have specifications such as the pass-band region or the stop-band region. These are the regions of the frequency where the filter will allow the signal to pass through or send the signal to the ground. The cut-off frequency is where the stop-band region effectively begins and the pass-bass region ends.

 

Low Pass Filter

This design filters out high-frequency signals and allows low-end frequencies to pass through. They can be designed to filter out signals in the kHz to GHz range. Within an audio system, a low-band filter might be used to filter the signal for the subwoofer. 

 

High Pass Filter

This filter will remove low frequencies with high frequencies being allowed through the filter and can typically be found in wireless or radio frequency applications. These applications make use of high-frequency signals so any low frequency found in the signal is not useful. 

 

Band Pass Filter

This filter is used for specific tight frequency ranges. The filter will be designed for a particular frequency range. When using a band-pass filter, the signal quality for that specific range will be greatly improved. These filters can be found in mobile applications or high-definition audio systems where accurate filtering of the signal provides a high-quality signal.

 

Figure 4. Band-pass filter response, with the blue shaded region indicating the frequencies that can ‘pass’ with the center being the strongest frequency, and decreasing in strength as the frequency increases and decreases.

 

Industrial Applications of Frequency Filters

Frequency filters are a great way to improve the quality of a signal before using that signal for its intended purpose. With VFDs and servo drives, frequency filters are typically installed on the input side of the drive. The AC mains will be connected to the input of the filter and the output will be connected directly to the input of the drive. This cleans the incoming AC voltage before entering the drive, providing the drive with a high-quality source of AC voltage. 

Smaller filters can also be used with communication devices or supply voltage for analog sensors. As the number of electronic devices on a typical machine increases, so will the amount of EMI within the machine, making frequency filters even more important.