DC contactor structure and working principle
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DC contactors are mainly used to remotely connect and disconnect DC circuits, and frequently start, stop, reverse and reverse brake DC motors. They are also used to frequently turn on and off lifting solenoids, solenoid valves, and clutches. Electromagnetic coils and so on. DC contactors have two structures, three-dimensional arrangement and planar arrangement, and some products are derived from AC contactors. Therefore, the structure and working principle of the DC contactor are basically the same as those of the AC contactor, and are mainly composed of three parts: an electromagnetic mechanism, a contact system, and an arc extinguishing device.
(1) Electromagnetic mechanism
Electromagnetic mechanism of DC contactor is composed of iron core, coil and armature, etc. Most of them use snap-type structure that rotates around corners. Since the DC power is passed through the coil, no eddy currents are generated in the iron core during normal operation. The iron core does not generate heat and there is no iron loss. Therefore, the iron core can be made of cast iron or cast steel. The number of turns of the DC contactor coil is large. In order to make the coil radiate well, the coil is usually wound into a long and thin cylindrical shape. Because the magnetic flux in the core is constant, no short circuit ring is required on the core pole face. In order to ensure the reliable release of the armature, it is often necessary to provide a nonmagnetic gasket between the core and the armature to reduce the effect of remanence. DC contactors above 250A often use series-connected double-winding coils, as shown in the figure below.
In the figure, the coil 1 is the starting coil, the coil 2 is the holding coil, and one normally closed contact of the contactor is connected in parallel with the holding coil. Immediately after the circuit is turned on, the coil 2 is short-circuited by the normally closed contact, so that the coil 1 can obtain a larger current and suction. When the contactor is actuated, the normally closed contact is opened, and coil 1 and coil 2 are energized in series. Since the voltage is constant, the current is small, but the armature can still be pulled in, saving power and extending the service life of the solenoid. the goal of.
(2) Contact system
The DC contactor has a main contact and an auxiliary contact. The main contact is generally made unipolar or bipolar. Because the contact has a large current to make or break, a finger contact with rolling contact is used. Auxiliary contacts have smaller on-off currents, often using point-contact double breakpoint bridge contacts.
(3) Arc extinguishing device
Since the DC arc does not have a natural zero-crossing point unlike the AC arc, when the main contact of the DC contactor breaks a large current (DC circuit), arc extinguishing is more difficult, and a strong arc tends to be generated, which easily burns contacts and delays Electricity. In order to rapidly extinguish the arc, the DC contactor generally uses a magnetic blow-type arc extinguishing device and is equipped with a partition plate and a clay arc extinguishing cover.