When selecting a relay, factors such as the type of power supply, the rated voltage and current of the contacts, the rated voltage or current of the coil, the combination and number of contacts, engagement time and release time should be considered. The following are some common relay selection principles.
Select Electromagnetic Relay
(1) Current relay According to the protection function required by the load, the current relay can be divided into two types: over-current relay and over-current relay.
The T parameter selected by the overcurrent relay is the rated current and the operating current. The rated current should be greater than or equal to the rated current of the protected motor. The operating current should be set at 1.1 to 1.3 times its starting current according to the working conditions of the motor. Generally, the starting current of a wound rotor asynchronous motor is 2.5 times the rated current, and the starting current of a cage-type asynchronous motor is 5-7 times the rated current. When selecting the operating current of the overcurrent relay, there should be some room for adjustment.
Undercurrent relays are generally used for field weakening protection of DC motors and electric circuit breaker suction cups. The main parameters selected are the rated current and the tripping current. The rated current should be greater than or equal to the rated excitation current. 0.85 times. When selecting the release current of the relay under current conditions, there should be a certain adjustment space.
(2) Voltage relays According to their functions in the control circuit, voltage relays can be divided into overvoltage relays and undervoltage (zero voltage) relays.
The main parameters for relay overvoltage selection are rated voltage and operating voltage, and the operating voltage can be set to 1.1-1.5 times the system rated voltage. Electromagnetic relays or small contactors are commonly used in undervoltage relays, which can be selected as long as they meet the general requirements, and there is no special requirement for the release voltage value.
Thermal relays are mainly used for motor overload protection, and are usually selected according to motor type, working environment, starting conditions, and load properties.
(1) Thermal relay structure type When the motor winding adopts the Y connection method, a thermal relay with a two-phase structure can be selected. When the grid voltage is seriously unbalanced and the working environment is harsh, a thermal relay with a three-phase structure can be selected; when the motor winding is a delta When connecting, a three-phase structure with a phase failure protection device should be selected.
(2) Rated current of thermal relay For long-term normal operation of the motor, the rated current of the thermal element of the thermal relay is 0.95-1.05 times the rated current of the motor; 0.6 ~ 0.8 times.
For motors that are not started frequently, it is necessary to ensure that the thermal relay will not cause misoperation during the motor startup process. If the starting current of the motor is 6 times its rated current, and the starting time of the time does not exceed 6s, the thermal relay can be selected according to the rated current of the motor.
For motors with repeated short-time working system, the allowable operating frequency of the thermal relay should be determined first, which can be selected according to the starting parameters (starting time, starting current, etc.). ) and the duration of energization of the motor.
Choice of time relay
There are many types of time relays, and the following aspects should be considered when choosing:
(1) Current type and voltage level Electromagnetic damping type and air damping type are time relays, and the current type and voltage level of the coil should be the same as the control circuit; for motor and transistor type time relays, the current type and voltage level of the power supply should be the same as the control circuit The circuit is the same.
(2) Delay mode According to the requirements of the control circuit, select the delay mode, that is, the power-on delay mode and the power-off delay mode.
(3) Type and quantity of contacts The type and quantity of contacts (delayed closing or delayed opening) should be selected according to the requirements of the control circuit.
(4) Time-delay precision electromagnetic damping time relays are suitable for occasions with low precision requirements, and motor-type or electronic time relays are suitable for occasions with high time-delay precision requirements.
(5) Working frequency The working frequency should not be too high, otherwise it will affect the life of electrical appliances, and even cause delayed action obstacles.
Selection of intermediate relays
When selecting the relay in the middle, it should be noted that the current type and voltage level of the coil should be consistent with that of the control circuit. At the same time, the number, type and capacity of contacts should be selected according to the needs of the control circuit. If the number of contacts of one intermediate relay is not enough, two intermediate relays can be connected in parallel to increase the number of contacts.