DC Contactor Structure

DC contactor is mainly used for remotely connecting and disconnecting the DC circuit, as well as frequently starting, stopping, reversing and plug braking the DC motors. It's also widely used for connecting and disconnecting the lifting electromagnet, solenoid valve, clutch's magnetic coils. DC contactor has two structure of stereoscopic arrangement and plane arrangement, some products are derived on the basis of AC contactor. Therefore, structure and working principle of the DC contactor are basically the same as that of AC contactor, which mainly consists of three parts, electromagnetic mechanism, contact system and arc-extinguishing equipment.
DC contactor basic structure

• Electromagnetic mechanism: Electromagnetic mechanism Electromagnetic mechanism of the DC contactor consists of iron core, coil and armature, it mainly adopts a clapper-type structure that rotates by the edges. As the coil is connected with direct current (DC), there would be no eddy current in the iron core when the coil operates, nor the iron core generates heat, thus there’s no iron loss, the iron core can be made from block cast iron or cast steel. The DC contactor has many coil turns, which would be made into long and thin cylindric shape, so as to maintain good heat dissipation. Because the magnetic flux in the iron core is constant, the polo surfaces need no short-circuit rings. In order to ensure the reliable release of armature, a non magnetic pad would often be added between the iron core and armature to reduce the effect of residual magnetism.
  The DC contactor above 250A generally applies the series-wound duplex winding coils, take ATO-ASW400 as example, of which coil 1 serves as the starting coil and coil 2 serves as the holding coil. A normally closed contact of the contactor would parallel with the holding coil. At the moment when the circuit is connected, coil 2 is shorted by the normally closed contact, thus coil 1 gets a large current and suction. When the contactor operates, the normally closed contact disconnects, coil 1 and coil 2 power on in series. Because the voltage keeps constant, the current will be relatively small, but it still ensures the armature to be pulled in, thus realizing the purpose of saving electricity and prolonging the service life of magnetic coil.
• Contact system: DC contactor has the main contact and auxiliary contact. The main contact is used to connect and disconnect the larger current circuit, that is, the main circuit, so the main contact has a larger sectional area and it's generally in a planar type. The auxiliary contact has small sectional area and it's generally in a spherical type. Auxiliary contact is mainly used to connect and disconnect the control circuit and signal circuit. Main contact of the DC contactor is generally a normally open contact while auxiliary contact includes two types, namely the normally open contact and normally closed contact. The main contact is usually made into monopole or bipolar. As the current is relatively large when the contact connects or disconnects, a finger contact is usually applied. On-off current of auxiliary contact is small, and double break bridge contact is generally adopted.
• Arc-extinguishing equipment: DC arc doesn’t have the natural zero point as the AC arc, when the main contact of DC contactor disconnects larger current (DC circuit), the arc will be difficult to be extinguished, thus producing a strong arc, burning the contact and delaying the outage. In order to extinguish the arc quickly, DC contactor usually adopts magnetic blowing type arc-extinguishing equipment, a partition and clay arc-extinguishing cover will also be equipped. Some SMV contactors adopt the magnetic blowout force produced by permanent magnet, medium sized and high-capacity DC contactors usually adopt the narrow slot arc-extinguishing and magnetic quenching. Arc-extinguishing equipment for DC contactors include splitter arc-extinguishing, electrodynamic arc-extinguishing, narrow slot arc-extinguishing and magnetic quenching.

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