Dc contactor is mainly used for long distance switching on and off dc circuit and frequently start, stop, reverse and reverse brake dc motor, also used for frequently connecting and disconnecting lifting electromagnets, solenoid valve, clutch electromagnetic coil, etc . There are two kinds of structures with three-dimensional layout and plane layout of the DC contactor, Some products are derived from ac contactors. Therefore, the structure and working principle of dc contactor are basically the same as that of ac contactor, It is mainly composed of electromagnetic mechanism, contact system and arc extinguishing device.
( 1 ) electromagnetic mechanism
The electromagnetic mechanism of dc contactor is composed of iron core, coil and armature. The coil is connected by direct current, In normal operation, There is no eddy current in the iron core, Iron core does not heat, no iron loss, therefore, the core can be made of whole cast iron or cast steel. The dc contactor has many turns of coil, In order to make the coil heat well, the coil is usually wound into a long, thin cylindrical shape. Because the magnetic flux in the iron core is constant, short circuit ring is not needed on the pole surface of the iron core. To ensure the armature is released reliably, It is often necessary to put non-magnetic gaskets between the core and armature to reduce the influence of remanence. Above 250A, dc contactors often use tandem double winding coils.
Therefore, the core can be made of whole cast iron or cast steel.
1----- starting coil
2------ holding coil
Coil l is the starting coil and coil 2 is the holding coil, a normally closed contact of the contactor is in parallel with the holding coil. At the moment the circuit is connected, coil 2 is short-circuited by normally closed contact, the coil l can obtain large current and suction. When the contactor is operation, the normally closed contact is disconnected, and coil 1 and coil 2 are energized in series connection. The voltage is constant, so the current is small, but the armature can still be held together, It can save electricity and prolong the service life of electromagnetic coil.
(2) Contact system
Dc contactor has main contact and auxiliary contact. The main contact is usually unipolar or bipolar, Due to the contact connected or disconnected the large current, therefore, adopt the rolling contact of the finger-type contact.
Due to DC arc extinguishing current has no natural zero-crossing point，When the main contact of the dc contactor breaks a large current（direct-current circuit）, arc extinguishing is more difficult, strong arcs are often produced, easy to burn contacts and delay power cuts. In order to extinguish the arc quickly, the dc contactor usually uses the magnetic blowing device, and equipped with partition and ceramic arc extinguishing cover.