Relays play an important role in daily work production. Relays are typically used in electrical control circuits to amplify the contact capacity of miniature or small relays to drive larger loads. If the contacts of the relay can be used to turn the coil of the contactor on or off. Generally, relays have more open and close contacts. Of course, relays can also implement certain special functions, such as logic operations, through proper connection. Common relays mainly include thermal overload relays (JR series relays), time relays (JS series relays) and intermediate relays (JZ series relays). Let's discuss some of the points of attention for relay selection.
To use the relay well, it is very important to choose the correct type. First, you must have a thorough understanding of the nature, characteristics and application requirements of the controlled object, and carefully consider it. The principle, use, technical parameters, structural features, specifications and models of the selected relays should be mastered and analyzed. On this basis, the relay should be correctly selected according to the actual conditions of the project and the specific conditions. Can refer to the following points:
1. Insulation grade and structure of the motor
Due to the different insulation levels of the motor, its allowable temperature rise and ability to withstand overload are also different. Under the same conditions, the higher the insulation level, the stronger the overload capability. Even if the insulation materials used are the same, the motor structure is different and should be different when using relays. For example, a closed motor has a lower heat dissipation than an open motor, and its overload capability is lower than that of an open motor. The set current of the relay should be selected to be 60 to 80% of the rated current of the motor.
2. Starting current and starting time of the motor machine
The starting current of the motor is generally 5 to 7 times the rated current. For motors that are infrequently activated and continuously operated, the relay can be selected according to the rated current of the motor when the starting time does not exceed 6 s.
3. If the thermal relay is used as the motor phase loss protection, the connection of the motor should be considered.
In particular, when the thermal insulation device is selected, for the Y-connected motor, when a phase is broken, the current of the remaining unphased windings is the same as the current of the flow superheat relay. The general three-phase thermal relay can realize phase failure protection for the Y-connected motor as long as the tuning current regulation is reasonable. For a Δ-connected motor, when the phase is broken, the current flowing through the uninterrupted phase winding and the current increase ratio of the flow superheat relay are different. That is to say, the current flowing through the superheat relay cannot reflect the overload current of the winding after the phase failure. Therefore, the general thermal relay, even the three-phase type, cannot provide sufficient phase-breaking operation of the three-phase asynchronous motor of the delta connection. protection. In this case, a thermal relay with a differential phase-break protection mechanism such as JR20 or T series should be used.
4. Understanding of the junction
The static and dynamic contacts that are in the off state when the relay coil is not energized are called "normally open contacts", otherwise, they are called "normally closed contacts". A moving contact is normally closed with one static contact and normally open with another, and they are called "switching contacts". In the same relay, you can have one or several pairs of normally open contacts or normally closed contacts (both can also have both), or you can have a set or array of conversion contacts.