Marconi conducted some of the earliest experiments with radio frequencies. Perhaps you have seen his experiment involving a loop of wire with a small spark gap. When his rudimentary transmitter was activated, a spark could be seen in this loop of wire.
This may seem like an odd introduction to Electromagnetic Interference (EMI) as related to an industrial control panel. However, we can see a clear relationship if we consider two facts:
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Reciprocity: Reciprocity is a fundamental property of radio antennas. An antenna that is good at receiving is also a good transmitting antenna.
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Loops: Just like Marconi, our industrial control panels contain loops and spark gap generators. The antenna-like loops are formed when the supply and ground wires are not run side by side in the wire duct. For example, in Figure 1, the blue (24 VDC supply) wire is placed in the upper wire duct while the white with blue stripe wire (return) is in the lower wire duct. A spark generator is formed whenever we open a relay contact driving an inductive load.
Taken together, we observe that every industrial control panel has the potential to generate EMI. Most of the time, it is harmless. However, the relay (contactor) turn-off pulse can interfere with sensitive analog circuits. Also, there have been reports of EMI with enough intensity to force a step-direction motor drive to advance.
Figure 1: Image and schematic of a multifunction relay toggling a mid-sized 24 VDC contactor.
How is EMI generated in an industrial control panel?
To demonstrate the problem, the simple circuit shown in Figure 1 was constructed. A small Selec 600XU multifunction relay was used to toggle a Schneider Electric LC1D09BD three-phase contactor. To accentuate the EMI problem, the Schneider contactor’s Transient Voltage Suppression (TVS) diode-based surge suppressor was removed, as shown in Figure 2. Without this protective measure in place the contactor’s coil produces a high voltage spike when it is turned off.
Figure 2: Image of the Schneider contactor with the TVS surge suppressing diode removed.
Tech Tip: I once received a very nasty electric shock as an early attempt to understand the properties of an inductor. I had constructed a series connection between a 12 VDC battery, an ammeter, and a large inductor. Since this was a 12 VDC circuit, my naive self did not consider the safety aspects and simply held the inductor wires in the circuit.
Not a good idea!
Everything was fine, and I was able to get the reading. However, when I went to disconnect the inductor, I truly learned the electrifying nature of inductor kickback. A collapsing magnetic field generates a high voltage that causes a person to involuntarily toss the inductor across the room.
Let’s look back at Figure 1 to understand the source of the EMI-generating pulse:
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The contactor is turned on via a normally open contact from the Selec multifunction relay. The current generates a strong magnetic field within the inductor with a considerable amount of magnetic potential energy.
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The Selec multifunction control relay turns off, opening the contact.
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We have an instantaneous rise in voltage across the contactor’s coil. With the TVS diode removed, there is no local constraint on the magnitude of this voltage.
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An electrical arc is formed across the Selec relay’s opening contacts.
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The arc is sustained until the contactor’s magnetic potential energy is dissipated.
Without question, this is Marconi’s classic experiment in reverse. When we factor in antenna reciprocity, we conclude that a burst of RF energy has been generated and radiated. The intensity is influenced by the coil’s stored energy, the area of the antenna loop, the nature of the arc, and system’s RC properties.
Observing EMI in an industrial control panel
As I prepared this article, I attempted to measure the pulse amplitude.
I failed!
But let me tell you why.
Every time the system cycled, the EMI pulse would cause a fault in my USB-based oscilloscope. It didn’t matter how I connected the oscilloscope ground to the circuit. The results were the same with a loss of connectivity to the oscilloscope. The only way to keep the oscilloscope running was to reinstall the TVS diode (Figure 2).
Maybe I didn’t fail.
This little experiment shows that the EMI pulse is real. If it can cause my USB oscilloscope to fail, it may be a problem in your control panel.
EMI reduction tips for industrial control panels
As a thought experiment, consider the industrial control panel to be a very large Printed Circuit Board (PCB). This allows us to reuse the PCB EMI reduction techniques. For example:
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Minimize the loop area by keeping the supply and return paths close together or even twisting them together. With regards to Figure 1, this may result in unsightly wiring practice as the white with blue stripe wire should ideally be run in the upper wire duct side-by-side with the blue supply wire. An alternative is to move the contactor to the edge of the wire duct (far right-hand side of Figure 1). We then immediately loop the return wire up to the blue supply wire to minimizing the loop area.
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Reduce the loop area for circuits with the potential to generate EMI. For example, install the interposing relay as close as possible to the large contactor.
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Reduce the frequency and magnitude of the signal using surge suppression devices. This requires careful consideration as lowering the speed has a detrimental effect on contact opening speed and arc quenching.
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Shorten the wire length for sensitive signals and keep them separate from control and power wires. Use shielded cables if necessary and cross wires at a 90-degree angle whenever possible. This can result in undesirable wiring practices as we may need to keep the wires out of the wire duct. Bounded and grounded noise shields may help maintain the look of a professional panel.
Parting thoughts
This article provides a brief introduction to the problems of EMI within a control panel. It is focused on the pulse that is produced when a large contactor is opened. This is a gentle introduction to the wider topic of EMI. Someday, we may explore the problem associated with line-induced spikes or the challenges of Variable Frequency Drives (VFDs).
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.