There are many types of relays. According to the input quantity, it can be divided into voltage relay, current relay, time relay, speed relay, pressure relay and so on. According to the working principle, it can be divided into electromagnetic relays, inductive relays, electrical relays, and electronic relays. According to the purpose, it can be divided into control relays, protection relays, etc., according to the input change form can be divided into whether there is a relay and a measuring relay.
The presence or absence of the relay functions according to the presence or absence of the input value. When there is no input value, the relay does not work, and when there is an input value, the relay works, such as intermediate relays, general relays and time relays.
The measuring relay acts according to the change of the input quantity. During work, the input quantity always exists. The relay only works when the input quantity reaches a certain value, such as current relay, voltage relay, thermal relay, speed relay, pressure relay, liquid level relay, etc.
Most relays used in control circuits are electromagnetic relays. Electromagnetic relay has simple structure, low price, convenient use and maintenance, small contact capacity (generally lower than SA), large number of contacts, no distinction between main and auxiliary, no arc extinguishing device, small size, rapid and accurate action, convenient and sensitive control, and reliable And so on, widely used in low-voltage control systems. Commonly used electromagnetic relays include current relays, voltage relays, intermediate relays and various small general-purpose relays.
The structure and working principle of an electromagnetic relay are similar to that of a contactor, and it is mainly composed of an electromagnetic mechanism and contacts. There are two types of electromagnetic relays: DC and AC. A voltage or current is applied to both ends of the coil to generate electromagnetic force. When the electromagnetic force is greater than the spring reaction force, the armature is sucked in to make the normally open and normally closed contacts work; when the electromagnetic force is greater than the spring reaction force, the armature is attracted. When the voltage or current of the coil drops or disappears, the armature is released and the contacts are reset.
Thermal relays are mainly used for overload protection of electrical equipment (mainly motors). A thermal relay is an electrical appliance that uses the principle of the thermal effect of current. It has an inverse time action characteristic, which is similar to the allowable overload characteristic of a motor. It is mainly used in conjunction with a contactor to protect the three-phase asynchronous motor from overload and phase failure. In actual operation, asynchronous motors often encounter overcurrent (overload and phase failure) caused by electrical or mechanical reasons. If the overcurrent is not serious, the duration is short, and the winding does not exceed the allowable temperature rise, the overcurrent is allowed; otherwise, the overcurrent is allowed. If the overcurrent is serious and lasts for a long time, it will accelerate the aging of the motor insulation and even burn the motor. Therefore, a motor protection device should be installed in the motor circuit. There are many types of commonly used motor protection devices, the most commonly used and the most commonly used is the bimetal thermal relay. Bimetal thermal relays are all three-phase, and there are two types, with phase protection and without phase protection.
The time relay is used for time control in the control circuit. There are many types. According to its working principle, it can be divided into electromagnetic type, air damping type, electric type and electronic type. According to the delay mode, it can be divided into power-on delay type and power-off delay type. The air damping time relay uses the principle of air damping to obtain the time delay. It consists of three parts: electromagnetic mechanism, delay mechanism and contact system. The electromagnetic mechanism is a direct-acting double E core, the contact system uses I-X5 micro switches, and the delay mechanism uses airbag shock absorbers.