A relay is an electrical appliance that works automatically. When the amount of electricity such as voltage, current, and frequency or non-electricity such as temperature, pressure, and rotational speed is input to the relay, and when the specified value is reached, the contacts of the relay connect or cut off the circuit that controls or protects it. Relays are widely used in motor control and power system protection systems.
The relay is generally composed of two parts: the input sensing mechanism and the output actuator. The former is used to reflect the level of input, and the latter is used to open or cut off the circuit.
Electromagnetic relays, also known as contact relays, have roughly the same structure and operating principle as contactors. However, electromagnetic relays are small in structure, sensitive to action, do not have huge arc extinguishing devices, and there are many types and numbers of contact points
1. Current relay.
A current relay is a device that reflects the change of the current flow of the circuit, and its coil is connected in series with the circuit to reflect the change of the circuit current. In order not to affect the working condition of the circuit, its coil is less, the wire is thick, and the coil impedance is small
The current relay determines whether to work according to the magnitude of the current change in the control circuit, and the current relay can be divided into overcurrent relay and undercurrent relay.
When the current in the circuit exceeds a certain amount, the current relay action is used to cut off the circuit and play the role of overload protection in the circuit, usually the current action range of the current relay can be adjusted to between 110% and 400% of the rated current of the circuit, and the action range of the DC current relay can be adjusted to between 70% and 300% of the rated current of the circuit
When the current in the circuit is less than a certain value, the current relay will not operate enough, cut off the circuit, and protect the circuit that requires a certain current. For example, in the armature excitation circuit of a DC motor, if the excitation current decreases, its rotational speed rises according to the mechanical characteristics of the DC-motor, and when the excitation current decreases to zero, according to theoretical analysis, its rotational speed, the rotational speed of the DC-based motor increases sharply, that is, the phenomenon of flying
In order to prevent the occurrence of this phenomenon, an undercurrent relay is inserted into the hub excitation circuit of the DC motor, and when the current drops to a certain value in the hub excitation circuit, the undercurrent relay acts, cuts off the power supply, and plays a protective role of insufficient current.
In general, the pick-up current of the undercurrent relay is about 30%-65% of the rated current of the coil, and the release current is between 10%-20% of the rated current of the coil. Therefore, when the current in the control line is reduced to 10%-20% of the rated current of the undercurrent relay coil, the action of the undercurrent relay plays the role of undercurrent protection. In the electrical control circuit, J214 series AC and DC current relays are mainly used.
The selection requirements for current relays are as follows
(1) The selection of overcurrent relay, used for the overcurrent protection of wound asynchronous motor and small-capacity DC motor, the rated current of the coil of the overcurrent relay can choose the size of the rated working current of the motor, for the motor with frequent start or frequent forward and reverse start, the rated current of the current relay coil should be selected upward.
(2) Selection of insufficient current relay. The selection of insufficient current relay coil current should be selected according to the minimum current value required by the circuit. That is to say, the minimum current value required by the circuit must be equal to 10%-20% of the rated current of the undercurrent relay coil
2. Voltage relay.
The structure of the voltage relay is similar to that of the current relay, but the difference is that the coil of the voltage relay is a voltage coil connected in parallel, with a large number of turns, thin wires and large impedance. Voltage relays are mainly used for voltage rise, fall and automatic control of power input lines, overvoltage and loss of voltage protection in machine tool lines
According to the operating voltage value, the difference between overvoltage, undervoltage and zero-voltage relay of voltage relay. The main function of the overvoltage relay is that when the voltage exceeds a certain upper voltage value, the relay works, so as to achieve the protection of overvoltage. Generally speaking, the relay will work when the voltage is 105%-120% of the rated voltage of the line. In case of insufficient voltage relay.
The main function is that when the voltage in the circuit is reduced to below a certain value, the voltage relay works insufficiently, cuts off the circuit, the power supply voltage decreases, and the current rises sharply, which will not damage the power supply equipment. Generally speaking, when the voltage is less than 40%-70% of the rated voltage of the line, the voltage relay is insufficient. Zero-voltage relay when the line supply voltage drops close to zero (generally 10%-35% of the rated voltage)
AC contactors and intermediate relays themselves also have the function of insufficient pressure and insufficient pressure protection, so in the electrical control circuit, AC contactors and intermediate relays are commonly used to replace insufficient pressure and insufficient pressure relays for insufficient pressure or insufficient pressure protection.
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