This engineering brief presents three alternatives for surge suppression in a 24 VDC contactor (relay). It specifically addresses the flyback (back EMF) voltage spike that occurs when a contactor’s coil is deenergized. This is a critical factor for long-term reliability of the associated Programmable Logic Controller (PLC) and motor starter contacts. As will be shown, there is a balance between the contactor’s opening time and the magnitude of the voltage spike.
This case study is focused on the Siemens 3RT2015-1BB41 contactor, as shown in Figure 1. This device is configured as a three-phase magnetic motor starter with matching overload block, auxiliary contacts, and a surge suppressor (items sold separately). The contactor’s surge suppressor mitigation techniques are directly related to the Siemens LOGO! PLC model 6ED10521CC080BA2 as shown in the background of Figure 1. The featured PLC has transistor outputs that could be damaged if the coil’s flyback voltage is not properly suppressed.
Figure 1: Image of a Siemens three-phase motor controller and LOGO! PLC installed on a Phase Dock trainer.
Comparison of surge suppressor solutions
This section explores three methods for handling the coil’s flyback voltage. We could do nothing, use the manufacture’s approved surge suppressor, or use a conventional diode (also known as a freewheeling or flyback diode). A picture of the suppressor configuration and an oscilloscope waveform are included for each method.
Operation without the surge suppressor
Figure 2 presents the do-nothing scenario with the absence of surge suppression. The coil’s flyback voltage spikes to -300 VDC with a fast turn-off time of about 30 ms.
In this example the turn-off time is bounded by the time it takes for the coil voltage to reach 0 VDC. This point was chosen so that we can easily compare all three techniques. The actual time is slightly less. In the Figure 2 example, we see the wiggle in the waveform at 20 ms. This is associated with the change in armature magnetic properties when the air gap is formed due to the armature returning to the relaxed (open position).
Figure 2: Image of the Siemens motor starter with the surge suppressor slot open (not installed).
Tech Tip There is an inverse relationship between the voltage spike and the time it takes to dissipate the energy stored in the coil’s magnetic field. High voltage spikes yield a fast response time while clamping the flyback to a low voltage result in a sluggish system.
While low voltages are desirable for the PLC, they are detrimental to the contactor. The sluggish opening will result in longer and hotter electrical arcs on the primary motor contacts. This extended arc time will quickly “eat” the primary contacts, resulting in additional maintenance costs. There may also be a safety hazard, as an unfortunate chain of failures could result in a sustained arc within the contactor.
Where is the coil’s energy dissipated?
We need to be very clear on this point as it holds the key to understanding the voltage spike vs time relationship. Recall that an inductor (relay coil) will do everything it can to keep the current constant. In this case we are attempting to open the contact supplying that current. The inductor “responds” by increasing the voltage to whatever value is necessary to maintain the constant current flow. The voltage will increase until it finds a path to sustain the constant current. In an ideal world, the inductor’s voltage will increase to infinity.
In the real world, the coil voltage rises until it forms an arc. In Figure 2 this arc is formed across the contacts of an interposing relay. This small Phoenix Contact control relay is just visible to the right of the Siemens motor starter. If we were to open the relay we would see a small flash. It may not look impressive, but it is a hot plasma that quickly dissipates the coil’s energy, resulting in the measured 30 ms turn-off time.
In this example, the term interposing implies a relay that is placed between the PLC and the motor starter. This is necessary as the featured LOGO! PLC has semiconductor outputs that could be damaged by the high-voltage spike.
Tech Tip: The LOGO! PLC is available with relay outputs. In that case, the interposing relay may be eliminated. However, we should move to the next section to explore an improved installation using the Siemens surge suppressor.
Operation with the matching surge suppressor
Figure 3 presents the waveform for the contactor’s coil when the Siemens 3RT2916-1BB00 surge suppressor is installed. The magnitude of the voltage spike is reduced with a slightly longer turn-off time of 40 ms. The surge suppressor clip-in module clearly visible when we compare Figures 2 and 3.
Figure 3: Image of the Siemens motor starter with the 3RT2916-1BB00 surge suppressor installed.
Where is the coil’s energy dissipated?
In Figure 3, we see a negative 110 VDC spike. This motor starter’s coil energy is dissipated inside the surge suppressor. Instead of a hot arc, the energy is now dissipated across a varistor. By comparison, this is gentle operation, as seen by the slower voltage waveform.
Once again, we stress the inverse relationship between the voltage spike and turn-off time. Because Figure 3 has a lower voltage spike, it takes longer to dissipate the energy resulting in a slightly longer turn-off time.
All things considered, this is a near-optimal solution for the Figure 1 trainer. We have an interposing relay to withstand the higher voltage with a relatively fast turn-off speed resulting in a snappy response from the motor starter. We could eliminate the interposing relay if we switched to a LOGO! PLC with relay outputs.
Tech Tip: Most PLCs are available with either relay or semiconductor (transistor-based) outputs. Both have their place. Relays are electrically robust with the ability handle high voltage and current. Yet, the transistor outputs are faster and have a longer lifetime when toggled on and off.
As a demonstration, suppose a PLC was programmed to blink a red panel lamp when the equipment was in an idle state with one cycle per second. Now suppose the equipment is placed in a blinking idle state every evening. Over a year, this horrendous program would consume a significant portion of a relay’s life. Meanwhile, the transistor-based outputs could run the same program for decades (billions of cycles) without any problems.
Operation with a conventional diode
The last surge suppressor configuration is shown in Figure 4. In this example, a conventional diode is temporarily placed across the contactor’s coil. Obviously, this is a temporary (poor) installation as there is no mechanical clamp holding the diode in place.
The turn-off time has increased from 40 ms to 170 ms. In fact, we can hear and see the difference in the contactor’s response. It sounds slow. As mentioned in a Tech Tip, this is bad situation as the contactor will be slow to open. This leads to longer arc time between the motor contacts.
Figure 4: Poor (not recommended) method of suppressing the coils flyback voltage.
Where is the coil’s energy dissipated?
In this example, the energy is primarily dissipated in the intrinsic resistance of the contactor’s coil. The voltage is small resulting in a significant increase in the turn-off speed.
Tech Tip: Figure 4 is the classic textbook method of dissipating a relay’s coil energy with a diode placed in parallel with the coil. While it is very effective for suppressing the turn-off spike it is detrimental for turn-off speed. Please have this conversation with a colleague as we need to clearly differentiate between the ideal textbook circuit and those that work in the real world.
Parting thoughts
We could summarize this article in a simple sentence, “use the manufacturer’s recommended components.” The purpose-built Siemens surge suppressor is the clear winner.
That said, we have gained a deeper understanding of the coil’s flyback voltage and associated component. We now understand the need for the interposing relay and the distinction between a PLC with relay outputs as compared to a PLC with semiconductor outputs.
Please share your thoughts and experiences regarding this textbook to real-world application.
Oh, did we mention that the interposing relay has its own integrated surge suppression module?
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.