There are a variety of relays, which can be divided into voltage relays, current relays, time relays, speed relays, and pressure relays according to the input. And based on the working principles of the relays, they can be divided into electromagnetic relays, inductive relays, protective relays and so on. However, according to the input variable, the relays can be divided into non-relays and measurement relays.
Non-relays and Measurement Relays
The non-relays are classified based on whether the relay actions with the input or not. The relays do not work if there is no input coming in, while they work when there is input, such as intermediate relays, general relays, time relays and so on.
The measurement relays operate according to the change of input variable. The input always exists when it works, while the relay can only operate when the input reaches to a certain value, such as current relay, voltage relay, thermal relay, speed relay, pressure relay, liquid level relay, and so on.
Electromagnetic Relay
The electromagnetic relays are considered to be the most widely used ones in the control circuits. The electromagnetic relays have the advantages such as simple structure, low price, convenient operation and maintenance, small contact capacity which is generally under SA, large contact points and no main and auxiliary differences, no arc extinguishing device, small volume, rapid and accurate action, sensitivity and reliability. The electromagnetic relays are widely applied to the low voltage control system. The commonly used electromagnetic relays include current relays, voltage relays, intermediate relays and various small general purpose relays.
The structure and working principle of the electromagnetic relays is similar to that of contact, which is mainly composed of electromagnetic mechanism and contact.There are two types of electromagnetic relays, one type with DC and another type with AC. When the electromagnetic force is greater than the spring reaction force, the armature is drawn to make the normally open and closed contact move; when the voltage or current of the coil drops or disappears, the armature is released, contact reset.
Thermal relay
Thermal relays are mainly used for electrical equipment (mainly motor) overload protection. Thermal relay is a kind of electrical appliance that makes use of the principle of current thermal effect. It has the inverse time action characteristic which is similar to the allowable overload characteristic of the motor, it is used to protect three-phase asynchronous motor from over-load and off-phase. The phenomenon of over-current (over-load and off-phase) caused by electrical or mechanical reasons is often encountered in the actual operation of three-phase asynchronous motor. If the over-current is not serious, the duration is short, and the winding does not exceed the allowable temperature rise, the over-current is allowed; if the over-current is serious and the duration is long, the insulation aging of the motor will be accelerated, even burn motor. Therefore, the motor protection device should be set in the motor circuit. There are many kinds of motor protection devices in common use. The most widely used is the bimetallic thermal relay. The double metal plate type of thermal relays are all three-phase type, which have two kinds, that is, open-phase protection and non-open-phase protection.
Time Relay
Time relays are used in a control circuit for time control. According to the action principle, it can be divided into electromagnetic type, air damping type, electric type and electronic type, and so on. The air-damping time relay is made up of electromagnetic mechanism, time-delay mechanism and contact system. The electromagnetic mechanism is a direct-acting double-e-type iron core, the contact system borrows the I-X5 type fretting switch, and the time-delay mechanism adopts the air-bag damper.
Post time: Jul-09-2022
