The composition of the power relay and its control function 5

A reed power relay composed of a coil and a reed tube is also a special electromagnetic power relay, as shown in Figure 1-7. When the coil is energized, the reed, which is made of magnetic conductive material and acts as a contact, is magnetized, and its free ends form a north pole (N) and a south pole (S) to attract each other, so that the controlled circuit is connected. After the coil is powered off, the reeds are separated under the action of their own elastic force, and the circuit is cut off, so the same control function of the above-mentioned general electromagnetic power relay can also be achieved by using it. Because it has the advantages of high sensitivity, fast action speed, very simple structure, small volume, and long life due to the contact point being sealed in the protective gas, it is more and more widely used in various automatic control systems.

Using the principle of electromagnetic induction, power relays that can respond to various signals can also be formed. For example, polarized power relays (with permanent magnets working) and pulse power relays that can respond to the current direction, that is, polarity can be formed; magnetoelectric power relays that can respond to weak currents and have high sensitivity (similar to magnetoelectric instruments); can react Frequency power relays, resonance power relays of current frequency; and power relays that can reflect power such as power impedance. They all have different control functions in production practice. The signals in the production process are not only electric, but also non-electric. Therefore, people have created many power relays that can respond to various non-electrical signals according to various principles.

For example, two metal sheets with significantly different thermal expansion coefficients are compounded together to form bimetal sheets, which can be made into temperature power relays and thermal power relays that can respond to temperature or heat. Figure 1-8a shows the structure of the bimetal temperature power relay and its function in simple constant temperature control. The temperature power relay is installed near the heating device. After closing the knife switch, the heating device is connected to the power supply to start heating. When the temperature rises to a certain value, the disc-shaped bimetal 6 will generate upward bending force due to the large expansion and elongation of the lower metal and the small expansion and elongation of the upper metal. When it reaches a certain level, the moving contact 4 and the static contact 3 on the reed are closed, so that the coil of the power relay J is connected to the power supply, and its normally closed contact is disconnected, so that the heating device stops heating. After the temperature drops to a certain value, the disc-shaped bimetal gradually returns to its original state, and the contacts 3 and 4 are disconnected under the action of the spring plate 5, so that J is powered off, and the J contact is closed, so that the heating device continues to heat again. In this way, the temperature can be maintained within a certain range. When the bimetallic sheet works by the thermal effect of the current, it can form a thermal power relay that can respond to the current signal, the principle of which is shown in Figure 18b. It is widely used as overcurrent protection of electric motors. Since the bimetallic sheet is heated and deformed to make the contact action take a certain time, it can be used to form a bimetallic sheet delay power relay. If the reed switch shown in Figure 1-7 is driven by a permanent magnet (that is, what we usually call a magnetic steel) instead of a coil (as shown in Figure 1-9), and the permanent magnet is fixed on some kind of production machinery, then It can be used to reflect mechanical quantities such as position, rotation angle, speed, pressure, and flow rate. It can be seen that it has a very wide range of applications and uses.