A contactor is an electrically-controlled switch used to connect or disconnect power to large lighting loads and motors. Contactors consist of two basic parts: multiple normally-open power contacts and an electromagnet, or coil. When current passes through the coil, a moving contact is attracted and holds to a fixed contact. When the coil is de-energized, the contacts open, and power to the load is disconnected.
Contactors that are matched and sized as precisely to their application as possible can last for many years of continuous duty. If a contactor fails prematurely, there is often an identifiable cause. Always examine the contactor to determine why it failed before replacing it with an identical unit to avoid repeated failure. There are a number of common reasons for contactor failure.
Contactor overheating, which results in severe pitting or deformation of the contact surface, can occur if too much current is transmitted, if the contactor does not close quickly and firmly enough, or if it opens too frequently. Once this happens, operation will become erratic and failure will be swift.
Overheating of contactor coils is another cause of failure that can result from voltages that are too low or too high, or if contacts fail to open or close due to misalignment, dirt, oxidation, or rust. High ambient temperatures over a long period can cause contactor coils to fail, as well. Chattering, humming, and buzzing are common symptoms of contactor problems, and the stress of chattering can be a cause of failure in and of itself.
If a coil issue is suspected, an ohmmeter can be used to compare resistance to a nearby matching contactor nearby. The contactor should be replaced if the resistance is significantly higher or lower.
Omni Instrumentation & Electrical Services, Inc., located in New Jersey and Maryland, is a premier instrumentation and electrical contractor, providing superior total care solutions since 1986. Services include instrument installation, tubing installation, instrument calibration, control loop testing, startup and commissioning, power and lighting, process control wiring, BMS wiring, telecommunications and data wiring, fire alarm wiring, security wiring, process network wiring, and control panel fabrication. Omni Instrumentation & Electrical Services, Inc., is certified in Foundation Fieldbus, Profibus, DeviceNet and ASI-Bus installation.