When selecting a relay, factors such as the type of power supply, the rated voltage and rated current of the contacts, the rated voltage or rated current of the coil, the combination and number of contacts, the pull-in time and the release time should be mainly considered. The following describes the selection principles of electromagnetic relays.

 
Selection of electromagnetic relays
 
(1) Current relays are divided into two types: overcurrent relays and undercurrent relays according to the protection required by the load.
The main parameters of t selected by the overcurrent relay are the rated current and the operating current. The rated current should be greater than or equal to the rated current of the protected motor, and the operating current should be set according to the working conditions of the motor according to 1.1 to 1.3 times the starting current. Generally, the starting current of the wound rotor asynchronous motor is considered as 2.5 times the rated current, and the starting current of the squirrel asynchronous motor is considered as 5-7 times the rated current. When selecting the operating current of the overcurrent relay, there should be some room for adjustment.
(2) According to the function of the voltage relay in the control circuit, the voltage relay is divided into two types: over-voltage relay and under-voltage (zero voltage) relay.
The main parameters of overvoltage relay selection are rated voltage and operating voltage, and its operating voltage can be set according to 1.1-1.5 times the rated voltage of the system. The undervoltage relay is usually used as a general electromagnetic relay or a small contactor, and its selection only needs to meet the general requirements, and there is no special requirement for the release voltage value.