An interposing relay reduces the opening time for a DC solenoid or contactor. Contactors with snappy response are faster to extinguish arc reducing contact wear. The interposing relay also shields the PLC from the high voltage inductive transients.
At a Glance
- Large DC motor contactors open slowly.
- Adding a flyback diode such as the 1N4002 makes them even slower.
- Slow opening extends the arc time reducing the contact’s life.
- Most PLCs with solid-state outputs have, or require, an integral flyback protection diode. This makes the system slow.
- An Interposing relay protects the PLC while providing snappy opening for the contactor.
Problem Statements
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Problem 1: The coil of a large DC contactor is an inductor. Energy stored in the magnetic field needs somewhere to go when the coil is turned off. Without flyback protection, energy is dissipated in the high voltage inductive spike. This will damage unprotected drive transistors.
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Problem 2: Simple low-voltage diode flyback protection (0.7 v) is sluggish as the coil’s magnetic field energy is dissipated in the coil’s own resistance as opposed to the plasma of an arc or high voltage of a TVS diode. In all cases, energy must be burned outside the coil’s own resistance for snappy opening.
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Problem 3: PLCs with solid-state outputs are delicate when compared to a control relay. All OEMs provide clamping protection in the form of a conventional diode (Problem #2). Many require additional protection such as a 1N4002 diode. Failure to address this problem will destroy the PLC’s output transistors.
What is an interposing relay?
An interposing relay is “interposed” between two systems. They are used to:
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Increase the current handling capabilities of the PLC.
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Cross (“level shift”) voltages such as when a 24 VDC PLC output is required to drive a contactor featuring a 120 or 277 VAC coil.
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Increase the speed at which a contactor opens thereby minimizing arc and prolonging life.
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Provided additional safety via panel segregation. This application provides exclusive 24 VDC (blue wires) on the face of a PLC with higher AC voltages located elsewhere in a control panel. This deliberate segregation is a troubleshooting aid as a clean panel layout with dedicated space for each type of device is easier to troubleshoot.
Demonstration Hardware
This article is concerned with the last point regarding the speed at which a contactor operates. The hardware pictured in Figure 1 is used to illustrate the principles. The essential component are the Crouzet Millenium Slim PLC, control relay and a contactor featuring a 24 VDC coil.
Figure 1: The interposing relay is sandwiched between a Millenium Slim PLC and a 3-phase contactor.
From the onset, we must acknowledge that the Crouzet PLC features solid state outputs. It is certainly capable of directly powering the large contactor directly. The contactor’s 2.4 W coil consumes 100 mA which is well within the PLC’s design max of 500 mA. However, the PLC is not well matched to the contactor when we consider the dynamics associated with dissipating the contactor’s inductive energy.
Tech Tip: Turning off a large DC contactor or relay can be a stressful event. Recall that the coil stores energy in a magnetic field. Also recall that an inductor “wants” to keep the current constant. The result is known as inductive kick where the inductor does whatever is necessary to keep the current constant at the moment the coil is deenergized. With no associated protection, the voltage will rise to many hundreds of volts causing an arc to maintain the current. This will destroy any semiconductor switch used to control the coil. Surge suppression diodes are often incorporated to provide an alternative route for the current.
Reason for an interposing relay with solid state outputs
The reason for the PLC to contactor mismatch is associated with the necessary surge suppressing diode. While the Crouzet PLC datasheet does not include specific overvoltage design maximums, it does show that a diode is required in the presence of an inductive load as shown in Figure 2. This is a common configuration in industrial control systems. So common, in fact, that many control relays integrate the diode into the socket. Such is the case for the Finder control relay pictured in Figure 1.
Figure 2: Excerpt from the Crouzet Millenium Slim PLC datasheet showing the use of a surge suppressing diode for inductive loads.
Know that the Schneider contactor does feature a surge suppressing diode but of a different type. Instead of using a simple diode it features a “bidirectional diode.” This diode clamps with a voltage of about 48 VDC as will be demonstrated later in this article. A PLC with semiconductor outputs such as a Millenium Slim is not expected to allow or survive this higher clamped voltage.
This single diode (0.6 VDC) vs clamp (48 VDC) is not a trivial difference when we consider the dynamic of the contactor. Understand that the clamped inductive dissipation voltage is directly associated with speed as outlined in this article describing the opening speed of a relay. This is especially true with the DC coil of the Schneider contactor as it takes over 100 ms to open when hobbled by a single diode surge suppressor as required by the PLC.
One viable solution is to use an interposing relay. This allows room for the embedded bidirectional diode assembly to do its job. The interposing relay’s Normally Open (N.O.) will easily accommodate the higher voltage. At the same time the small coil of the interposing relay is more compatible with the PLC. Experiments suggest that the smaller control relay’s operating opening speed is not as sensitive as the larger contactor.
Demonstration of the interposing relay
The schematic for this experiment is presented as Figure 3. The interposing control relay is driven by the PLC. A N.O. contact of the interposing relay is then used to drive the coil of the large contactor. A circuit breaker is included as suggested in the PLC datasheet to minimize damage when something goes wrong.
Figure 3: Wire diagram showing the PLC, interposing relay (CR 1), and contactor.
The waveform associated with the relay opening are included as Figure 4. The event begins on the falling edge of the PLC output (orange). The interposing relay relaxes about 8 ms later as evident by the falling edge of the contactor’s coil (blue). The contactor opens approximately 37 ms later as evident by the discontinuity in the contactor’s voltage waveform. This is caused by a change in inductance due to a change in the armature-to-coil proximity. The total time for this operation is about 45 ms. However, that is not the focus of this article. Instead, we need to focus on the contactor’s waveform.
The central argument of this article is that the contactor’s coil is allowed to swing negative to 48 VDC. This would not have been allowed without the use of the interposing relay. A single surge suppression diode would have been required. The consequence of a direct PLC to contactor would be a sluggish slow to open contactor as described in the previously mentioned article.
Figure 4: Voltage waveforms associated with the PLC, Finder Control relay, and the Schneider contactor.
Additional Information about the motor starter
This article is part of a larger work that introduces the motor starter and explores the important and at time subtle applications. Please refer to these related posts for more information:
- Introduction to the Magnetic Motor Starter
- Introduction to the 3-Wire Start-Stop Circuit
- Permanent Magnets as an Energy Saving Feature of a 3-phase Motor Starter
- How Does a Motor Starter’s Thermal Overload Block Work?
Conclusion
The critical dynamic is the voltage waveform associated with the contactor. With the interposing relay installed, it is free to increase up to the limit of the bidirectional diode clamp. Most of the inductive energy is quickly dissipated across this clamp as a relatively high voltage P = IE as opposed to the much slower P = I^2R where R is the contactor’s internal resistance. This is a subtle point between the steady state and the dynamic aspects. Steady state analysis suggest that the contactor and PLC are a good match. Dynamically, the required single diode clamp reduces system performance causing a problematic slowdown.
Your comments and suggestions are welcomed. Information about the dynamics associated with contact arc and longevity are especially desirable.
Don’t make the PLC eat the flyback voltage
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 (partially interwoven with military experience). 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 educational articles about electronics and automation.
Highlighted Experience
Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 141 unique topic posts with an additional 466 forum posts answering user questions. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.
Questions
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Define the term interposing.
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Provide at least 4 reasons to use an interposing relay.
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Skitch the ladder diagram for an interposing relay. Assume a 24 VDC PLC with sourcing outputs and a large contactor with a 120 VAC coil.
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What is the significance of using blue and white with blue stripe wires?
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A contactor has a 24 V coil that consumes 0.25 A current. Calculate the coil’s power.
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What is a TVS diode? Hint: An answer is contained in this article.
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Describe the cause of a relay coil’s flyback voltage (inductive kick).
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What is meant when we describe a PLC with “semiconductor” outputs?
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Why is a conventional diode a poor choice for a large contactor with a DC coil.
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Is an interposing relay applicable to a PLC featuring relay outputs? Hint: Did you consider all the reasons a designer may use an interposing relay?
Critical thinking questions
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With respect to Figure 4, what causes the dip in the current waveform that occurs at 45 ms. Hint: An answer is contained in this article.
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Energy is stored in a relay’s coil. Describe the relationship between the stored energy and the magnetic field. Also, describe where the final destination for this stored energy when the relay coil is deactivated.
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From a system level perspective describe the impact of a slow opening of a contactor driving an inductive load such as a motor. Hint: Describe the immediate problem and long-term implication on maintenance.
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This article briefly describes panel segregation as an important use of the interposing relay. Elaborate by including a sketch of a segregated panel with separation based on voltage. Also identify the color of the various control wires. Hint: There are several competing standards, as a starting point, recommend IEC 60204-1. A good description is located in this Control Panel Reference Manual courtesy of Siemens.