Let's talk about the intermediate relay first.
Because the intermediate relay has more contacts and a wider voltage range of the coil, it is generally used in the circuit to control various solenoid valves to expand the signal, or to transmit the signal to several control components at the same time.
The basic structure and working principle of the intermediate relay are exactly the same as that of the contactor. The difference is that the intermediate relay has a large number of contact groups, and there is no distinction between main contacts and auxiliary contacts. The current allowed by each group of contacts is identical. Because of its small contact capacity, it is generally not used in the main circuit in the control circuit of the motor, but uses its small current to control the coil of the contactor, so as to achieve the effect of small current controlling large current.
The intermediate relay needs to be used in conjunction with the relay base. The coil is energized, the normally closed point is disconnected, and the normally open point is closed.
Each group of control contacts of the intermediate relay is divided into common terminal, normally closed point and normally open point. The one in the same direction as the coil inlet end is the contact common end, and the outlet end is divided into normally open and normally closed. The method of judging normally open and normally closed is to plug the relay into the relay base and use the on-off gear of a multimeter to measure the on-off from the common terminal to the two outlet terminals. The one that is on is normally closed, and the one that is not is normally open. If there is no multimeter, I have a little experience to tell you that generally the contacts of the intermediate relay are open and closed, that is, the upper row is the normally open point, and the lower row is the normally closed point.
Generally, when the auxiliary contacts of the contactor are not enough or the control voltage does not match the voltage of the contactor coil, we can use the intermediate relay to make a transfer point to complete the connection of the entire circuit.
The main function of the thermal relay is to protect the motor.
The new electronic thermal relay has protections against overload, overcurrent, locked rotor and various open phases, current imbalance, short circuit between phases, short circuit between turns or overvoltage and undervoltage.
We should pay attention to the following points in the process of using thermal relay.
First, when the starting time of the motor is too long or the number of operations is too frequent, the thermal relay will malfunction or burn out the electrical appliances. Therefore, in this case, the thermal relay is generally not used for overload protection.
Second, when the thermal relay is installed with other electrical appliances, it should be installed below other electrical appliances to prevent its action characteristics from being affected by the heating of other electrical appliances.
Third, the wiring wire at the outlet end of the thermal relay should be selected appropriately. If the wire is too thin, it will affect the relay may act in advance; if the wire is too thick, the thermal relay may lag its action.
Fourth, generally start at light load and work for a short time. Two-phase thermal relays can be selected. When the power supply voltage balance and working environment are poor or the power difference of multiple motors is significant, three-phase thermal relays can be selected. For relays, for delta-connected motors, thermal relays with phase-failure protection devices should be used.
Fifth, the rated current of the thermal relay should be greater than the rated current of the motor.
Sixth, generally adjust the setting current to be equal to the rated current of the motor. For motors with poor overload capacity, the thermal relay setting can be adjusted to 0.6 to 0.8 times the rated current of the motor. For longer start-up times, drag impact loads or For motors that are not allowed to stop, the setting current of the thermal relay should be adjusted to 1.1 to 1.15 times the rated current of the motor.
Seventh, under normal circumstances, we use the normally closed point of the thermal relay to control the power supply of the contactor coil, and use the normally open point of the thermal relay as the fault signal power supply.
