The difference between AC contactors and DC contactors
- The iron core of the AC contactor produces eddy current loss and hysteresis loss. Laminating the iron core with silicon steel plates reduces the eddy current and hysteresis loss by alternating the iron core’s magnetic field to prevent overheating the iron core. Therefore, the iron core of the AC contactor is usually E-shaped. When an alternating current passes through the electromagnetic coil, the coil generates an alternating driving force on the armature. When the alternating current is zero, the coil’s magnetic current and the actuation force on the armature are both in zero states. In the reset action of the spring, the armature will exhibit chemotactic release potential. This makes the actuation force between the moving and static iron cores change with alternating current changes, resulting in changes and noise, thereby accelerating the contact wear between the moving and stationary iron cores, resulting in poor contact. In more severe cases, it may also cause burnout of the contacts. A copper ring, known as a short-circuit ring, is inserted into the end of the stem to eliminate contact burnout. This short-circuit loop is equivalent to the secondary winding of the transformer. When the coil is connected to an AC power source, the coil will generate a magnetic current and the induced current in the short-circuit loop. At this time, the short-circuit loop is equivalent to a purely inductive circuit. According to the purely inductive circuit phase, we know that the magnetic flux caused by the coil current and the magnetic flux generated by the short-circuit loop’s induced current cannot be simultaneously zero. When the current provided by the power supply is zero, the induced current of the short-circuit ring cannot be zero. Its magnetic current attracts the armature pair, thereby overcoming the armature’s release trend and ensuring that the armature is always actuated when it is turned on. As a result, noise and vibration have been significantly reduced, so the short-circuit ring is also called a vibration elimination ring. The iron core in the DC contactor coil does not generate eddy currents, and there is no heat generation problem in the DC iron core so that the iron core can be made of complete cast steel or cast iron, usually U-shaped.
- The coil of an AC contactor has few turns and low resistance, but the coil also generates heat, so the coil is usually made into a thicker short cylindrical shape. While avoiding the burning of the coil, there is a gap to facilitate heat dissipation. The coil of the DC circuit has no inductance, so the coil has more turns, which leads to more significant resistance and copper loss. The coil is usually made into a thin and cylindrical shape to maintain the coil’s good heat dissipation.
- AC contactor uses grid arc extinguishing device; DC contactor uses magnetic arc extinguishing device.
- The AC contactor’s starting current is vast, and its maximum operating frequency is about 600 times per hour. In comparison, the maximum operating frequency of the DC contactor is 1200 times per hour.
- In emergencies, AC contactors can be used instead of AC contactors. However, the action time cannot exceed 2 hours (because the AC coils’ heat dissipation performance is worse than that of DC coils, depending on their structure). If you need to use it for a long time, it is best to connect a resistor with the AC coil series. On the contrary, the DC contactor cannot be replaced by an AC contactor.
- The number of coils distinguishes the difference between an AC contactor and a DC contactor. A DC contactor’s coils are more than the number of coils of the AC contactor. If the primary circuit current is too large (Ie> 250A), the contactor should use a series-connected two-phase winding coil. The DC relay’s coil reactance is gigantic, but the current power use is less, even smaller than that.
- From the construction point of view, the DC contactor uses a freewheeling diode, which releases the electromagnetic force accumulated in the inductance when the coil is de-energized. The AC contactor does not use the freewheeling diode structure. Instead, it uses laminated iron cores to prevent heat loss and shading coils to keep the electricity running efficiently in the device.
- In terms of differences, due to the AC contactor’s shading coil, the device can be placed in any position as long as it has room for operation. The DC contactor needs air during the process, so there must be enough clearance around the equipment to operate normally.
- In an AC circuit, the arc drawn when the contacts are disconnected is automatically extinguished at the zero crossings. In a DC circuit, the arc can be maintained for a longer time.