Electromechanical Relays in the Age of Digitalisation

The experience of recent years has shown that relays have by no means lost their importance in the age of digitalisation. On the contrary, they are taking on ever more specialised functions

ComatReleco AG, incorporated in 1970, is a Swiss headquartered global supplier of high-quality relays and contactors of any kind. The core competencies are industrial relays, timing relays, monitoring relays and contactors. The electromechanical relay was invented in 1835 by Joseph Henry, who is mainly known as the inventor of the electromagnetic phenomenon of selfinduction and mutual induction. Henry was only really interested in the science of electricity, and the relay was a laboratory trick to entertain students.

Henry’s invention remained relatively unknown for several decades, but it became widespread in the 1860s with the development of the telegraph and telephone. In 1941, Konrad Zuse used electromechanical relays to build the world's first functioning computer Z3. Today, electromechanical relays have disappeared from computers.

Although fewer logic circuits are realised with the help of industrial relays nowadays, the relay is gaining in importance as an interface between logic and load due to the increasing use of process control systems and controllers. In their daily work, designers are mainly guided by the specified values such as 10 A at 250 VAC or 10 A at 30 VDC with a width of 15.6 mm (interface relay) or 22.5 mm or 38 mm (industrial relay). It is quite often not considered that these specifications refer to the ohmic load.

Contact load resulting from switching operation

Experience has shown that statements such as ‘we have always used the relay, but never had any problems before – why now?’ are first asked to the supplier. In most cases, however, it turns out that the type of signals to be switched has changed. This is because the levels of the signals to be switched are becoming smaller and smaller and the loads are becoming more and more inductive. Nevertheless, the proven relays with their standard contacts continue to be used.

It is not the stable current that stresses the contact, but the actual switching operation in connection with load size and load type. Therefore, the ratio of the switching current to the stable continuous current must be considered. In case of a resistive load, switching current and continuous current are approximately identical. With a lamp load, however, the switching current can be up to twenty times the continuous current due to the cold resistance of the filament. Motors, valves and contactors, on the other hand, are inductive consumers that are characterised by an inrush current that can be five to ten times higher. The switching peaks of valves can also be up to twenty times the continuous current. These inductive loads also cause strong switch-off sparks. Capacitor loads and very long cables are a special case. Here, the switching currents can reach forty times the continuous current.

If the switching contacts are opened under high DC loads, an electric arc is formed that can lead to fast wear or even welding of the contacts. Thanks to the magnetic field of a blow magnet, the Lorentz force acts on the electric arc, which is ‘blown away’ or ‘pushed away’ and thus extended as a result.

Utilisation categories

Some manufacturers specify the utilization categories for switching elements according to IEC/EN 60947. However, usually only the utilization categories AC-1 and DC-1 are specified, whereby the utilization categories AC-1 and DC-1 are described according to IEC/EN 60947-5-1 as resistive loads, precisely as ‘non-inductive or only slightly inductive loads (resistive loads)’. But in practice, the user hardly ever finds a resistive load. Here, the utilization categories for inductive loads are needed to find the right relay for the respective application.

For the utilization categories for DC voltage, the standard refers to the calculation of the load limit curve, here the time constant t = L/R is evaluated. Resistance heaters are resistive loads (t = 0.95) and are classified in class DC-1. DC-13 includes electromagnets with 6 x P (continuous load in W) and DC-14 are electromagnets with economy resistors (IE = 10 x IB; t = 15 ms).

Basically, the categories DC-1 and DC-13 are of interest for an industrial relay. The utilization category DC-13 describes the control of electromagnets, auxiliary current switches, contactors and solenoid valves where the inrush current is less than or equal to the rated current, but the cut-off voltage peak can reach fifteen times the rated voltage.

The influence of the inductances on the switching capacity of a contact can be illustrated by the typical relay power curve. A normal contact reaches its limits under inductive load even at low currents. With the knowledge of this problem, ComatReleco has created a complete product range for these applications. These special relays in the sizes of the standard industrial relays (38 and 22.5 mm) make it possible to switch even higher direct currents reliably due to their contact architecture.

The right choice

The basic condition for a satisfactory relay service life is the correct selection of the relay type. If inductive loads have to be switched, it is generally recommended to connect the load in order to eliminate the disturbance at the point of generation. For alternating current with an RC element or a VDR, for direct current with an RC element or a free-wheeling diode. This measure is rewarded by lower interference and a considerably longer contact life.

With capacitive loads, only a series resistor or a choke can help. If the designer does not have data of the connected devices, ComatReleco relays can be fitted directly with a protective circuit.

Relays have some very good properties that make them ideal components for coupling between systems. The galvanic separation between the coil and the contacts makes isolation between systems easy. The choice of coil voltages is wide and does not limit the voltage and current values of the contacts. It is relatively easy to select a relay to interface between a 24 V DC control circuit and a 230 V AC circuit. Relay coils are almost immune to overvoltage and EMC. There are also no software malfunctions. Due to the mechanical nature and simplicity of a relay, it has predictable behaviour in the event of a fault. However, there is no such thing as a one- fits-all relay. Knowing the application exactly is crucial for optimal dimensioning.

Usually, the actuation of the relay is manageable. It is more challenging when it comes to the load. It is not only important to know the voltage and current of the load, but also whether the load is resistive, inductive or capacitive. The switching interval, the switching frequency and special ambient conditions have an influence on the selection of the relay. The utilization categories according to IEC/EN 60947 help to characterise the application cases considering the operating conditions. Depending on the application and when specified correctly, relays offer reliable solutions in railway applications in many cases and are therefore state-of- the-art even today. Relay -based designs are usually simpler than electronic controls with software and this also simplifies certification.

With one of the widest product portfolios of Relays and Contactors including customized solutions ComatReleco serves customer within the segments of industrial automation and building installation technology as well as transport and railway. Built with latest solid-state technologies, using the traditional electro-mechanical design or a combination of both. For more than 20 years, ComatReleco products have found their way into rail vehicles and their subcomponents such as toilet systems, HVAC, doors, etc.

Our future

Based on customer feedback, continuous improvements, and adaptations to comply with the standards, the product range has been continuously expanded over the past years.

ComatReleco’s CMS-10R IoT messaging system, often referred to as remote relay or SMS butler, secures contact with your assets.

One of the newer additions to the product range is the remote relay, the CMS-10R monitoring and control system. Despite the connected world, there are always situations where there is no internet access, the PLC control is unavailable, or monitoring is required outside the existing IT infrastructure. For these cases, ComatReleco has developed a remote relay with an integrated eSIM for 4G, 3G, 2G communication that works worldwide. The system includes an IoT cloud and an app for smart phones.

The world today is full of uncertainties. Nevertheless, we need to look forward and continue to grow our business as you and we did during the pandemic. A workplace where you can develop, share and evolve and the satisfaction of having done a good job will help us innovate and prepare for the future.

Tel: +41 31 838 55 77

Email: info@comatreleco.com

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Address: ComatReleco AG Bernstrasse 4 3076 Worb Switzerland