Control Relays: An Overview

Control Relays Offer Multidimensional Monitoring and Protection

Commercial and industrial processes rely on control relays for monitoring and controlling nearly every one of their functions. Although the newer Programmable Logic Controllers (PLCs) have now almost entirely replaced the older relay logic, the control relay is still an important component in modern electric systems. In the industry, various types of electromechanical types of control relays coexist with their solid-state versions.


An Example of a Control Relay

Basic Functioning

The basic function of a relay is to isolate one voltage level from another. PLCs and other control logic usually operate at low voltage levels of around 1.8-24 Volts, while the breakers, motors, and other electrical equipment they control operate above 100 Volts. The relay has a coil wound over a ferromagnetic core, which generates magnetic flux when current passes through the coil. The magnetic flux pulls in a spring-loaded armature/plunger attached to a hinged contact assembly that usually has one or two sets of contacts. The poles on the hinge keep two Normally Closed (NC) contacts closed when the relay coil is not energized, but change over to connect to another two Normally Open (NO) contacts once the relay coil is energized. The relay operates in a break-before-make manner, as the NC contacts break before the NO contacts close. While the NO/NC contacts can handle high voltage and high current, they remain electrically isolated from the relay coil.

AC control relays are similar to their DC counterparts. To prevent the electromagnet of the relay vibrating at the AC current frequency, one-half of the core of an AC relay has a shading ring.

Uses of Control Relays

Control relays play an important multidimensional part in the electric systems. Apart from general purpose relays, a broad classification based on their use includes relays meant for monitoring, safety, time-delay, latching, plug-in, and machine tool.

Monitoring relays—protect expensive electronic equipment by monitoring harmful situations, and operating at preset conditions.

Safety relays—protect industrial equipment. Safety circuits operate these relays at preset conditions.

Time-delay relays—act as timers, energizing/de-energizing electric circuits after a preset time delay.

Latching relays—operate in bistable manner, offering one of two stable states (ON or OFF) that toggle only with the application of a low-voltage electric pulse.

Plug-in relays—can be plugged into sockets on a control circuit board or on DIN rails. These are usually small sized and meant for handling medium voltage/current levels.

Machine tool relays—industrial relays meeting NEMA/IEC standards. These usually have a maximum of 12 contacts of the double-break type, with two fixed contacts and one set of moveable contacts. Double-break contacts offer higher voltage ratings. Machine tool relays mount directly on to the mounting panel or on DIN rails.

Solid State Control Relays

Unlike the electromagnetic versions, solid-state relays have no moving parts. A three terminal semiconductor device or TRIAC replaces the switching contacts. An optical isolator switches the TRIAC on in the presence of a low voltage DC signal. The absence of moving parts makes the solid-state relays more reliable compared to the electromagnetic types. However, the former become exponentially more expensive as current ratings increase. They also have lower and higher contact resistances in the open and closed positions respectively.

Relays and PLCs

Programmable Logic Controllers are industrial computers programmed to execute logic and control a process. They operate with physical inputs, use internal logic, and offer physical outputs. PLCs can handle analog values and implement very sophisticated logic for controlling equipment. By themselves, the physical outputs of PLCs are limited to a few mill-amperes.

Relays interpose between the PLC output and the controlled equipment. Such relays accept the low voltage digital outputs from the PLC and transfer the action to higher voltages suitable for the electrical equipment under control.

Contact Rating of Relays

Selecting a relay depends primarily on two factors. The circuit driving the relay must be capable of supplying the level of current capable of operating the relay coil. On the output side, the relay contacts must be capable of handling the high voltage/current levels of the load. It is necessary to match the minimum load current of a relay specified in its data sheets to the load current it is required to switch. For instance, it is not advisable to use a high current rated relay to switch low current circuit loads, as this may lead to oxidizing the relay contacts, increasing its contact resistance.

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