Relays come in a wide variety of types. Based on the input quantity, they can be classified into voltage relays, current relays, time relays, speed relays, pressure relays, and others. Based on their operating principles, they can be categorized as electromagnetic relays, induction relays, motor-driven relays, electronic relays, and so forth. Based on their application, they are classified as control relays, protective relays, etc. Finally, based on the nature of the input signal variation, they are divided into "presence/absence" relays and "measuring" relays.
"Presence/absence" relays operate based on the mere presence or absence of an input signal; the relay remains inactive when there is no input and activates only when an input signal is present. Examples include intermediate relays, general-purpose relays, and time relays.
"Measuring" relays operate in response to changes in the magnitude of the input signal. During operation, the input signal is continuously present; the relay activates only when the input signal reaches a specific threshold value. Examples include current relays, voltage relays, thermal relays, speed relays, pressure relays, and liquid-level relays.
Electromagnetic Relays
The majority of relays utilized in control circuits are electromagnetic relays. Electromagnetic relays are characterized by their simple structure, low cost, and ease of use and maintenance. They typically feature a low contact capacity (generally below 5A), a large number of contacts with no distinction between main and auxiliary contacts, an absence of arc-suppression devices, and a compact size. Furthermore, they offer rapid and precise operation, as well as sensitive and reliable control, making them widely applicable in low-voltage control systems. Commonly used electromagnetic relays include current relays, voltage relays, intermediate relays, and various types of small general-purpose relays.
The structure and operating principles of electromagnetic relays are similar to those of contactors; they primarily consist of an electromagnetic mechanism and a set of contacts. Electromagnetic relays are available in both DC and AC versions. When a voltage is applied across-or a current is passed through-the coil, an electromagnetic force is generated. When this electromagnetic force exceeds the opposing force of the return spring, it attracts the armature, thereby actuating the normally open and normally closed contacts. Conversely, when the voltage or current in the coil decreases or is interrupted, the armature is released, and the contacts return to their original (reset) positions.
Thermal Relays
Thermal relays are primarily utilized to provide overload protection for electrical equipment, particularly electric motors. A thermal overload relay is an electrical device that operates based on the principle of the thermal effect of electric current. It possesses an inverse time-delay characteristic-meaning its response time decreases as the magnitude of the overload increases-that closely matches the permissible overload characteristics of an electric motor. Primarily used in conjunction with contactors, it serves to provide overload and phase-loss protection for three-phase asynchronous motors. During actual operation, three-phase asynchronous motors frequently encounter overcurrent phenomena (including overloads and phase losses) caused by various electrical or mechanical factors. If the overcurrent is not severe and is of short duration-such that the temperature rise in the motor windings does not exceed the permissible limit-such an overcurrent condition is considered tolerable. However, if the overcurrent is severe and persists for an extended period, it will accelerate the aging of the motor's insulation and may even result in the motor burning out; therefore, motor protection devices must be installed within the motor circuit. There are many types of commonly used motor protection devices; among them, the bimetallic thermal overload relay is the most widely and universally utilized. Bimetallic thermal overload relays are invariably designed as three-phase units and are available in two variants: those equipped with phase-loss protection and those without.
Time Relays
Time relays are utilized within control circuits to manage timing functions. They exist in numerous varieties; based on their operating principles, they can be classified into electromagnetic, air-damped, motor-driven, and electronic types, among others. Furthermore, based on their delay mechanism, they can be categorized into "on-delay" (delay-on-energization) and "off-delay" (delay-on-de-energization) types. The air-damped time relay achieves its time delay by harnessing the principle of air damping; it consists of three primary components: an electromagnetic mechanism, a delay mechanism, and a contact system. The electromagnetic mechanism features a direct-acting, double E-core design; the contact system utilizes an I-X5 type microswitch; and the delay mechanism employs a bellows-type damper. [8]
