Difference Between AC Contactor and DC Contactor

The AC contactor can be replaced by DC contactor in case of emergency, but the actuation time shall not be more than 2 hours (because the heat dissipation performance of AC coil is poorer than that of DC coil, which is determined by their different structures). For long term use, a resistor had better to be connected in series in the AC coil. In contrary, the DC contactor can't be replaced by the AC contactor.

AC contactor has a few coils while the DC contactor has more, they can be distinguished from the size of coil. In case the current of main circuit is too large (Ie>250A), the contactor shall adopt a series duplex winding winding. Coil of the DC relay has a large reactance, but small current.

Iron core of the AC contactor is usually laminated by silicon steel sheet, so as to reduce the eddy current and hysteresis loss generated by alternating magnetic field in iron core, avoid the iron core overheating. However, iron core in the DC contactor's coil would not generate eddy current and there is no heating problem in DC core, so the iron core can be made of a complete cast steel or cast iron. Coil of DC circuit has no inductance, so the coil has more turns, thus leading to large resistance and copper loss. In order to keep a good heat dissipation for the coil, the coil is generally made into a thin and cylindrical shape. Coil of AC contactor has less turns and small resistance, but the coil also gives heat, so the coil is generally made into a fat and short cylindrical shape. Meanwhile, a clearance is left to facilitate heat dissipation while avoiding the coils from being burned out. In order to eliminate vibration and noise generated by electromagnet, a short circuit ring is embedded on the surface of static iron core of the AC contactor, while the DC contactor doesn't need a short circuit ring.

Iron core of AC contactor is laminated by silicon steel sheet, thus reducing the eddy current and hysteresis loss generated by alternating magnetic field in iron core. When the AC passes through the magnetic coil, the coil generates alternating actuation force on the armature. When the AC is at zero, the magnetic flow in the coil also becomes zero, so is the actuation force on the armature. Armature will show a release trend under the action of reset spring.

This makes the actuation force between dynamic and static core to change along with the AC change, thus generating change and noise, accelerating the contact wear between dynamic and static core, leading to poor contact. Seriously, it may also lead to burn out of contacts. In order to eliminate this drawback, a copper ring is embedded in partial end of the core column and it's called a short circuited ring. This short circuited ring is equivalent to the secondary winding of the transformer. When the coil is connected to AC power, not only the coil generates magnetic flow, so does the induced current in the short circuited ring. At this time, the short circuited ring is equivalent to a pure inductance circuit.

According to the phase position of pure inductance circuit, we know that magnetic flow caused by coil current and magnetic flow caused by induced current of the short circuited ring can't be zero at a same time. Namely, when the current provided by power supply is at zero, the induced current of the short circuited ring can't be zero, its magnetic flow attracts the armature pair and thus overcoming the release tendency of armature, and guaranteeing the armature is always actuated when it's powered on. As a result, the noise and vibration are obviously reduced, so the short circuited ring is also called vibration elimination ring.

Main differences between AC contactor and DC contactor:

1. Iron core of AC contactor would generate eddy current and hysteresis loss, while the DC contactor has no core loss. Therefore, iron core of AC contactor is made of laminated silicon steel sheets, which is generally in E shape, while the iron core of DC contactor is made of complete soft steel, which is generally in U shape.
2. As single-phase alternating current passes through AC contactor, a short circuit ring is embedded on the end of the static core to eliminate vibration and noise generated by electromagnet. However, the DC contactor doesn't need the short circuit ring.
3. AC contactor adopts a grid arc extinguishing device while the DC contactor adopts a magnetic quenching arc extinguishing device.
4. AC contactor has a large starting current, its maximum operation frequency is about 600 times an hour, while that of DC contactor can be 1200 times an hour.

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