Selection of relays

Selection of relays

When selecting relays, the main considerations should include the type of power source, rated voltage and current of the contacts, rated voltage or current of the coil, combination and quantity of contacts, and the pull-in and release times. Below are the selection principles for several common types of relays. Selection of Electromagnetic Relays

1. Current relays are divided into overcurrent and undercurrent types based on the protection required by the load. The key parameters for selecting an overcurrent relay are the rated current and operating current. The rated current should be greater than or equal to the rated current of the motor being protected, and the operating current should be set at 1.1 to 1.3 times the starting current of the motor according to its working conditions. Generally, the starting current for wound-rotor asynchronous motors is considered to be 2.5 times the rated current, while for squirrel-cage asynchronous motors it is between 5 to 7 times the rated current. When choosing the operating current for an overcurrent relay, some adjustment margin should be left. Undercurrent relays are generally used for weak magnetic field protection in DC motors and electromagnetic chucks. The main parameters to consider are the rated current and dropout current. The rated current should be greater than or equal to the rated excitation current, and the dropout current setting should be lower than the minimum excitation current that may occur within the normal working range of the excitation circuit, typically set at 0.85 times the minimum excitation current. When selecting the dropout current for an undercurrent relay, some adjustment margin should be left.

2. Voltage relays are classified into overvoltage and undervoltage (zero voltage) relays based on their role in control circuits. The primary parameters for selecting an overvoltage relay are the rated voltage and operating voltage, which can be set at 1.1 to 1.5 times the system's rated voltage. Undervoltage relays are often general-purpose electromagnetic relays or small contactors, and their selection only needs to meet general requirements without special demands for the dropout voltage value. Selection of Thermal Relays Thermal relays are mainly used for overload protection of motors and should be selected based on factors such as the type of motor, working environment, starting conditions, and nature of the load. For motor windings connected in star configuration, two-phase thermal relays can be chosen. If there is severe voltage imbalance in the grid or harsh working conditions, three-phase thermal relays should be selected; for delta-connected windings, a three-phase thermal relay with phase loss protection should be chosen. The rated current of the thermal element in a thermal relay for motors that operate continuously under normal conditions should be set at 0.95 to 1.05 times the motor's rated current. For motors with poor overload capacity, the rated current of the thermal element should be set at 0.6 to 0.8 times the motor's rated current. For motors that start infrequently, it is important to ensure that the thermal relay does not misoperate during startup. If the motor's starting current is six times its rated current and the startup duration does not exceed 6 seconds, the thermal relay can be selected based on the motor's rated current. For motors with a repetitive short-time duty cycle, it is first necessary to determine the allowable operating frequency of the thermal relay, which can be selected based on the motor's startup parameters (startup time, starting current, etc.) and duty cycle. Selection of Time Relays There are many types of time relays, and selection should consider the following aspects: The type of current and voltage level of electromagnetic damping and air damping time relays should match those of the control circuit; similarly, the current type and voltage level of motor and transistor time relays should match the control circuit. The delay mode should be selected based on the requirements of the control circuit, namely energizing delay or de-energizing delay. The type and number of contacts should be selected based on the requirements of the control circuit (delayed closing or delayed opening). The delay accuracy of electromagnetic damping time relays is suitable for applications with low precision requirements, while motor or electronic time relays are suitable for high delay accuracy requirements. The operating frequency should not be too high, as it may affect the electrical lifespan and even cause delay action. Selection of Intermediate Relays When selecting intermediate relays, it is important to ensure that the type of current and voltage level of the coil matches the control circuit, and the number, type, and capacity of contacts should also be selected based on the needs of the control circuit. If the number of contacts in an intermediate relay is insufficient, two intermediate relays can be used in parallel to increase the number of contacts.