Analysis of Overheating Faults in AC Contactor Main Contacts:
Excessive current flow through the main contacts: The contacts of any electrical device must operate within their rated current values to avoid overheating. Excessive current flow can be caused by high or low system voltage, overload operation of electrical equipment, improper selection of contactor capacity, or faulty operation.
Increased contact resistance between the main contacts: The level of contact resistance directly affects the degree of contact heating. The increase in contact resistance can be attributed to two reasons:
Insufficient pressure is caused by the loss of elasticity in the contact pressure spring or thinning of the contact surface due to wear. In such cases, the spring or contact should be replaced.
Poor contact on the contact surface. During operation, dust, oil, and other impurities may cover the contact surface, increasing the contact resistance. Additionally, the formation of arcs during contact opening and closing can lead to surface burns and reduce the contact area, resulting in poor contact. Therefore, regular maintenance is necessary to ensure optimal contact performance. Copper contacts can be cleaned using a scraper or fine file to remove oxidation and burns. For larger or medium-capacity contacts, smoothness is not as important as maintaining a flat surface. Small-capacity contacts require a good surface quality. Silver and silver-plated contacts can be cleaned with cotton soaked in gasoline or carbon tetrachloride, as the oxidation layer does not affect their contact performance.
Prevention and Treatment of Overheating in AC Contactor Main Contacts:
If the contactor’s engagement is too slow or stagnant, and the contacts are in a semi-contact or non-contact position, refer to the “Abnormal Engagement of AC Contactor” method for proper handling.
Severe oxidation and burns on the contact surfaces can increase contact resistance and lead to overheating of the main contacts. To address this, remove the oxidation layer from the main contacts and gently file them to ensure tight contact.
Frequent starting of equipment can subject the main contacts to repeated shocks from starting currents, leading to overheating or welding. In such cases, avoid frequent starting or choose contactors that can withstand the operating frequency and continuous power supply.
Prolonged exposure to overloaded currents can cause overheating or welding of the main contacts. To prevent this, reduce the load on the transmission equipment and ensure it operates within its rated capacity. Alternatively, select a suitable contactor based on the equipment’s working current.
Asynchronous closure of the three-phase main contacts can result in significant current shocks on two of the main contacts, leading to welding. In such cases, check the closure condition of the main contacts, adjust the gap between the dynamic and static contacts, and ensure synchronous contact of the three-phase main contacts.
Some main contacts have poor resistance to welding, such as pure silver contacts. In these cases, it is recommended to replace them with main contacts made of silver-based alloys, which have better resistance to welding.
If the contact travel distance is too short, adjust the contact travel or replace the contact.
These are the complete contents of “Analysis and Prevention of Overheating Faults in AC Contactor Main Contacts.” When repairing and grinding contacts, it is important for maintenance personnel to avoid excessive scraping or filing, as it may affect the lifespan of the contacts. Additionally, do not use abrasive cloth or grinding wheels, as quartz particles may embed in the contact surfaces and affect their contact performance.
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