AC contactors VS DC contactors: What’s their difference?

Eddy current loss and hysteresis loss are caused by the iron core of the AC contactor. The iron core is compounded with silicon steel sheet, and the magnetic field of the iron core is changed to reduce the eddy current and hysteresis loss and prevent the iron core from overheating. Therefore, the iron core of the AC contactor is generally e-type. When alternating current passes through the electromagnetic coil, the coil generates alternating current driving force on the armature. When the alternating current is zero, the magnetic current of the coil and the actuation force on the armature are both zero.

In the spring return, the armature will show chemotaxis release potential. This makes the driving force between the moving and static iron cores change with the change of the alternating current, resulting in changes and noise, thereby accelerating the contact wear between the moving and static iron cores and causing poor contact. In more severe cases, it may also cause burnout of contacts. Insert a copper ring at the end of the valve stem, called a short-circuit ring, to eliminate contact burnout. This short circuit is equivalent to the secondary winding of the transformer.

When the coil is connected to an AC power source, the coil will generate magnetic current and induced current in the short circuit. At this time, the short circuit is equivalent to a pure inductive circuit. According to the phase of the pure inductance circuit, the magnetic flux caused by the coil current and the magnetic flux caused by the induced current of the short circuit cannot be zero at the same time.

When the current provided by the power supply is zero, the induced current of the short-circuit loop cannot be zero. Its magnetic current attracts the armature pair, thereby overcoming the release tendency of the armature and ensuring that the armature is always driven when it is opened. This greatly reduces noise and vibration, so the short-circuit ring is also called the vibration absorbing ring. The iron core in the DC contactor coil does not produce eddy currents, and the DC iron core does not have a heating problem, so the iron core can be made of all cast steel or cast iron, usually u-shaped.

The coil of AC contactor has few turns and low resistance, but the coil will also generate heat, so the coil is usually made into a thicker short cylindrical shape. In order to prevent the coil from burning, there is a gap which allows heat to escape. The coil of the DC circuit has no inductance, so the number of turns of the coil is large, and the resistance and copper loss are greater. The coil is usually made into a thin cylindrical shape to maintain good heat dissipation of the coil.

Contactors have grid extinguishers; DC contactors have magnetic extinguishers.

The starting current of the AC contactor is very large, and the maximum operating frequency is about 600 times per hour. The maximum operating frequency of the DC contactor is 1200 times/hour.

In emergency situations, AC contactors can be used instead of AC contactors. But the action time cannot exceed 2 hours (because the heat dissipation performance of the AC coil is worse than that of the DC coil, which depends on the structure of the AC coil). If you need to use it for a long time, it is best to connect the resistor in series with the AC coil. On the contrary, the AC contactor cannot replace the DC contactor.

The number of coils can distinguish between AC contactors and DC contactors. The number of coils of the DC contactor is greater than the number of coils of the AC contactor. If the primary loop current is too large (ie> 250A), the contactor should use two-phase winding coils in series. The coil reactance of the DC relay is huge, but the power consumption of the current is less, or even smaller.

From a structural point of view, the DC contactor uses a free-flowing diode to release the electromagnetic force accumulated in the inductance when the coil is de-energized. The AC contactor does not use a free-wheel diode structure. Instead, it uses laminated iron cores to prevent heat loss, and shade coils to keep the power in the equipment running efficiently.

Differentiating this device from other AC contactors, a shading coil on the AC contactor allows the device to be placed almost anywhere, as long as there is room for operation. To ensure proper operation, there must be enough clearance around the DC contactor during the assembly process.

In an AC circuit, the arc generated when the contact is opened is automatically extinguished at the zero crossing point. In a DC circuit, the arc can be maintained for a long time.


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