Differences Between AC Contactor and DC Contactor?

Figure 1: AC contactor and DC contactor.

The working principle of DC contactor is similar to that of AC contactor. They all make the main contact on or off through the actuation of electromagnetic coil. They are also similar in structure for being composed of electromagnetic mechanism, contact system and arc extinguishing device. But they are different in electromagnetic mechanism. Their main differences are reflected in the following points:

1. Iron Core

Figure 2: Iron cores.

Since eddy current and hysteresis loss will be generated in the iron core after the coil is connected with AC, the iron core of AC contactor is made of 0.5mm thick silicon steel sheets coated with insulating paint on both sides. It is usually made into E-shape; The DC contactor has no eddy current loss, so the iron core is usually made of a whole piece of mild steel, and most of them are made into U-shape.

Because the magnetic force of the iron core of the AC contactor is affected by the AC zero crossing, it will be connected in and disconnected repeatedly under the reaction force of the spring. In this way, the contactor will produce vibration and noise which will affect the work. In order to eliminate vibration and noise, we usually add a short-circuit ring to the core. Because the iron core is composed of multiple silicon steel sheets, plus the short-circuit ring is equivalent to shifting a part of the magnetic field, so that the electromagnetic force is divided into two parts with different phases which solve the problem of zero crossing of the electromagnetic force.

2. Arc Extinguishing System

AC contactor adopts grid arc extinguishing device while DC contactor adopts magnetic arc extinguishing device.

Figure 3: Grid arc extinguishing device of AC contactor.

Figure 4: The structure of magnetic arc extinguishing device.

1.Coil 2. Iron core 3. Magnetic splint 4. Arc angle 5. Arc shield
6.Magnetic field of coil 7. Magnetic field of arc current

3. Coil Turns

The impedance of AC contactor coil has inductive reactance and the resistance of wire circumference. The inductive reactance will be generated after the AC contactor wire is connected with AC. And because the inductive reactance hinders the current much more than the resistance of the coil, the number of turns of the AC contactor coil is generally less than that of DC contactor. When the coil of DC contactor is connected with DC power, the coil impedance only has its own resistance, so the number of turns is more than that of AC contactor.

Figure 5: AC contactor coil (left) and DC contactor coil (right)

4. Operating Frequency

Because the coil current of AC contactor is related to the gap of iron core, the larger the gap, the greater the current. The AC contactor frequently breaks the suction table for a long time causing the coil to overheat and burn out, so it is not suitable for frequent on-off occasions.

The coil current of the DC contactor has nothing to do with the gap of the iron core. The coil current of the DC contactor remains unchanged before and after closing, so the frequent closing and breaking of the DC contactor will not cause the coil to overheat which is suitable for frequent on-off occasions.

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