1. The type and series of relay selection
First of all, the type of relay is selected according to the work requirements of the object to be controlled or protected, and then the appropriate series is selected according to the sensitivity or accuracy requirements. When choosing a series, you should also pay attention to the compatibility of the relay with the system.
(1) When selecting voltage relays, first of all, it should be noted that the type and voltage level of the coil current should be consistent with the control circuit. It is then selected according to its role in the control circuit (whether it is overvoltage or undervoltage). Finally, the type of contacts (whether they are active or broken) and the number of contacts should be selected according to the requirements of the control circuit.
(2) When selecting the current relay, first of all, it should be noted that the type and grade of coil current should be consistent with the load circuit. The type of current relay is then selected according to its protection effect on the load (whether it is overcurrent or undercurrent). Finally, the type of contacts (whether they are active or broken) and the number of contacts should be selected according to the requirements of the control circuit.
(3) When selecting intermediate relays, first of all, we should pay attention to the type of coil current (whether it is AC or DC), and the voltage or current of the coil should meet the requirements of the circuit. In addition, the number and capacity of contacts (rated voltage and rated current) should meet the requirements of the control circuit, and attention should also be paid to whether the power supply is AC or DC.
(4) When selecting time relays, they should be selected according to system requirements (such as accuracy, delay range, operating power supply, etc.).
2. Category of use for relay selection
The typical use of relays is to control AC and DC electromagnets, such as coils used to control AC and DC contactors. Because the use category determines the load nature and on-off conditions controlled by the relay, it is the main basis for selecting the relay.
3. Rated working voltage and current for relay selection
The rated operating voltage and rated operating current of the relay contacts under the corresponding usage category, which characterizes the ability of the relay contacts to switch circuits. When selected, the maximum working voltage of the relay can be the rated insulation voltage of the relay, and the maximum working current of the relay should generally be less than the rated heating current of the relay. Usually a relay specifies several rated operating voltages, and the corresponding rated operating current is listed at the same time (control power can also be listed).
It is worth noting that some product samples or nameplates often indicate the rated heating current of the relay, rather than the rated working current, which should be distinguished when selecting, otherwise it will affect the service life of the relay and even burn out the contacts.
Overcurrent relays are mostly used as short-circuit protection for motors, and their selection parameters are mainly rated current and action current. The rated current of the overcurrent relay should be greater than or equal to the rated current of the protected motor, and its action current can be adjusted according to the working condition of the motor, according to 1.1 times of its starting current - 1.3 times. If there is no given data, the starting current of wound rotor asynchronous motor is generally considered according to 2.5 times of its rated current, and the starting current of squirrel cage type asynchronous motor can generally be considered according to 5 times - 7 times of its rated current.
4. The use environment of relay selection
The relay is generally an ordinary type, and the surrounding environmental temperature, altitude, relative humidity, pollution level, shock, vibration and other conditions of the relay installation site must be considered when selecting, so as to determine the structural characteristics and protection type of the relay. If it is used in a place with a lot of dust, a fully enclosed relay with a cover should be selected; If used in humid tropical areas, wet tropical relays should be selected to ensure that the relay works normally and reliably.
5. Working system for relay selection
Different working systems have different requirements for the overload capacity of relays. For example, when the AC voltage (or intermediate) relay is used in repeated short-time duty system, because there is a large starting current when pick-up, its burden is inversely heavier than that of long-term duty system, and this should be fully considered in relay selection. The rated operating frequency of the relay for repeated short-time duty is usually stated in the product brochure, and the actual operating frequency in use should be lower than the rated operating frequency.
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