The interposing relay – typically a common control relay – is often used in industrial applications. The relay is “interposed” between two systems. It is used for a variety of reasons including:
to increase the current handling capabilities of a device such as a Programmable Logic Controller (PLC).
to change voltages such as when a 24 VDC PLC output is required to drive a contactor featuring a 120 or 277 VAC coil.
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.
to increase the speed at which a contactor opens thereby minimizing arc and prolonging life.
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. From left to right we see a 0.5 A circuit breaker, terminal blocks with 4-position jumpers, Crouzet Millenium Slim PLC, control relay with integral surge suppression diode plus socket, and a contactor featuring a 24 VDC coil.
Figure 1: Picturing showing the interposing relay 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.
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.
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.
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.