The coil power supply of the AC/DC universal contactor is rectified at the input position so that it can be used for both AC and DC. The DC coil and the AC coil are different. The impedance of the AC coil is different during the contactor closing and maintenance process (the change of the air gap), which can ensure that sufficient electromagnetic force is provided during startup and the power consumption is relatively small during maintenance, and it will not be burned. If the DC contactor has only one coil, it will be damaged due to the heating of the coil during operation.
Therefore, this type of contactor has a switching circuit at the same time. Commonly used are ABB's AF series, Chint NCX series (above 250A), and other models, which can be implemented in single-coil and double-coil modes.
The single-coil mode is divided into a holding coil and a starting coil. When starting, the holding coil with a large impedance is shorted by the normally closed contact point of the contactor, and there is only a starting coil with a small impedance. This coil can generate enough electromagnetic force to ensure that the contactor is closed. When the start-up is completed, the normally closed contact is opened to maintain the coil input, and the power consumption is very small at this time.
The principle of dual-coil mode is similar, but it uses two rectified power supplies.