What are the major types of electromechanical relays?

General Purpose Relays

General purpose relays are cheap, easily replaceable, and are used for troubleshooting and other low priority switching applications. These relays follow a design like that in Figure 1 below. General purpose relays are designed for commercial and industrial applications where price and replaceability are high priorities. Most general-purpose relays have a plug-in feature that makes for quick replacement and simple troubleshooting.

Figure 1: A diagram of the components of a general-purpose electromechanical relay.

Machine Tool Relays

Machine tool relays are standardized for the industrial control of machine tools, transfer machines, and other forms of sequential control. The designation of “machine tool relay” is synonymous with both the NEMA (National Electrical Manufacturers Association) style relay and the IEC (International Electrotechnical Commission) style relay.

These relays are specifically engineered to withstand the harsh operating conditions and heavy loads often encountered in machine tool applications. They are characterized by their robust construction, high current and voltage ratings, and ability to handle the inductive loads typically associated with machine tools. Machine tool relays are easily convertible from normally open (NO/Form A) to normally closed (NC/Form B) status. Physically, they have easily replaceable coils and a form factor that allows for multiple relays to be installed in a control panel.

Figure 2: Several different types and sizes of machine tool relays.

Reed Relays

Reed relays use reed switches as a core component. Reed switches are made up of two thin, flexible metal reeds that are sealed within a glass envelope or a hermetically sealed enclosure. The reeds are positioned close to each other but do not touch when the relay is in its normal state. Reed relays are particularly useful when converting analog signals to digital signals.

Reed relays consist of a coil wrapped around a sealed glass tube that contains the reeds and contacts. Applying power to a reed relay coil forms a magnetic field that causes the reeds to move and the contacts to close. Compared to general-purpose relays, reed relays have a much longer mechanical and electrical life, and feature significantly faster switching times. Reed relays have a low contact resistance, which means they introduce minimal electrical resistance when closed, making them good for use in applications that require low signal loss.

Figure 3: A close up view of a reed switch.