Relay is a kind of automatic control electrical appliance which can turn on or off the small current control circuit according to some kind of input signal to realize remote control and protection. Its input can be current, voltage and other electricity, but also can be temperature, time, speed, pressure and other non electric quantity, while the output is the action of contact or the change of circuit parameters.
When the input voltage, current and frequency of the relay or the non electric quantity such as temperature, pressure and speed reach the specified value, the contact of the relay will connect or break the circuit controlled or protected.
Relay is generally composed of input sensing mechanism and output actuator. The former is used to reflect the level of input, and the latter is used to connect or cut off the circuit.
The commonly used relay has contact. The contact has two states: on and off. The change of state is driven by some mechanism.
The action of the contact is related to the action voltage of the coil, which is called voltage relay. It is used for the voltage protection and control of the electric drive system. When using, the coil of the voltage relay is paralleled with the load.
Features: it has many turns and thin wire diameter.
Classification: according to the type of coil current, it can be divided into AC and DC voltage relay, and according to the pull-in voltage, it can be divided into overvoltage, undervoltage and zero voltage relay.
Intermediate relay In order to distribute control signal and power amplification, the function of the control circuit is to increase the number of controlled lines or increase the allowable breaking capacity of contacts.
There are AC and DC relay. The characteristic of the intermediate relay is that the number of contacts is more than 3 ~ 4 pairs, and the bridge type contact is often used (the same as the auxiliary contact of the contactor).
The structure of the intermediate relay with larger action power is the same as that of the small contactor.
It is suitable for control lines with AC less than 500V. The coil voltages are 12V, 36V, 127V, 220V and 380V. The relay has eight pairs of contacts, the rated current is 5a, and the maximum operating frequency is 1200 times / h.
It is suitable for the control circuit below 110V. The rated voltage of the coil is 12V, 24V, 48V and 110V. The power consumption of the coil is not more than 3W. The relay has three normally open and three normally closed contacts with common contacts. The rated current of the contact is 3a.
Contact action and coil action current size related relay is called current relay, the current relay coil and load in series.
Features: the number of turns of the coil is small and thick.
Classification: according to the type of coil current can be divided into AC and DC current relay, according to the pull-in current size can be divided into over-current and low-current relay.
The characteristic of time relay is that when the control signal is obtained (such as the relay coil is connected or disconnected), its contact state does not change immediately, but after a period of delay, the contact is closed or disconnected. Therefore, this kind of relay is also called delay relay.
Thermal relay is a kind of protection device which uses the heat generated when the current flows through the thermal element to make the bimetallic sheet bend and push the actuator to act. It is mainly used for overload protection of AC motor, protection of phase break and current unbalance operation and control of heating state of other electrical equipment.
Selection of thermal relay:
(1) For motors with poor overload capacity, the rated current IRT of thermal element is 60% - 80% of rated current in of motor.
(2) In the case of infrequent starting, if the starting current of the motor is 6 times of its rated current and the starting time is less than 6 seconds, the thermal relay can be selected according to the rated current of the motor.
(3) When the motor is in repetitive and short-time working system, it is necessary to determine the operating frequency of thermal relay. For motors with high operating frequency, thermal relay should not be used as overload protection.