Contactor

A contactor is a type of relay (electrically operated switch) with high power rating (current rating and voltage rating). Contactors usually refer to devices switching more than 15 amperes or in circuits rated more than a few kilowatts. Contactors are typically used to control electric motors (combination motor starters), lighting, heating, capacitor banks, thermal evaporators, and other electrical loads. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter on a side. Contactors usually have provision for installation of additional contact blocks, rated for pilot duty, used in motor control circuits.

A contactor is a type of relay (electrically operated switch) with high power rating (current rating and voltage rating). Contactors usually refer to devices switching more than 15 amperes or in circuits rated more than a few kilowatts. Contactors are typically used to control electric motors (combination motor starters), lighting, heating, capacitor banks, thermal evaporators, and other electrical loads. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter on a side. Contactors usually have provision for installation of additional contact blocks, rated for pilot duty, used in motor control circuits. ==Arc suppression== Without adequate contact protection, the occurrence of electric current arcing causes significant degradation of the contacts, which suffer significant damage. An electrical arc occurs between the two contact points (electrodes) when they transition from a closed to an open (break arc) or from an open to a closed (make arc). The break arc is typically more energetic and thus more destructive. The heat developed by the resulting electrical arc is very high, ultimately causing the metal on the contact to migrate with the current. The extremely high temperature of the arc (tens of thousands of degrees Celsius) cracks the surrounding gas molecules creating ozone, carbon monoxide, and other compounds. The arc energy slowly destroys the contact metal, causing some material to escape into the air as fine particulate matter. This activity causes the material in the contacts to degrade over time, ultimately resulting in device failure. For example, a properly applied contactor will have a life span of 10,000 to 100,000 operations when run under power; which is significantly less than the mechanical (non-powered) life of the same device which can be in excess of 20 million operations. Most motor control contactors at low voltages (600 volts and less) are air break contactors; air at atmospheric pressure surrounds the contacts and extinguishes the arc when interrupting the circuit. Modern medium-voltage AC motor controllers use vacuum contactors. High voltage AC contactors (greater than 1,000 volts) may use vacuum or an inert gas around the contacts. High voltage DC contactors (greater than ) still rely on air within specially designed arc-chutes to break the arc energy. High-voltage electric locomotives may be isolated from their overhead supply by roof-mounted circuit breakers actuated by compressed air; the same air supply may be used to "blow out" any arc that forms. ==Applications== Contactors are often used to provide central control of large lighting installations, such as an office building or retail building. To reduce power consumption in the contactor coils, latching contactors are used, which have two operating coils. One coil, momentarily energized, closes the power circuit contacts, which are then mechanically held closed; the second coil opens the contacts. A magnetic starter is a device designed to provide power to electric motors. It includes a contactor as an essential component, while also providing power-cutoff, under-voltage, and overload protection. Vacuum contactors utilize vacuum bottle encapsulated contacts to suppress the arc. This arc suppression allows the contacts to be much smaller and use less space than air break contacts at higher currents. As the contacts are encapsulated, vacuum contactors are used fairly extensively in dirty applications, such as mining. Vacuum contactors are also widely used at medium voltages from 1000 to 5000 volts, effectively displacing oil-filled circuit breakers in many applications. Vacuum contactors are only applicable for use in AC systems. The AC arc generated upon opening of the contacts will self-extinguish at the zero-crossing of the current waveform, with the vacuum preventing a re-strike of the arc across the open contacts. Vacuum contactors are therefore very efficient at disrupting the energy of an electric arc and are used when relatively fast switching is required, as the maximum break time is determined by the periodicity of the AC waveform. In the case of 60 Hz power (North American standard), the power will discontinue within 1/120 of a second (8.3 ms). A mercury relay, sometimes called a mercury displacement relay, or, mercury contactor, is a relay that uses the liquid-metal mercury in an insulated sealed container as the switching element. A mercury-wetted relay is a form of relay, usually a reed relay, in which the contacts are wetted with mercury. These are not considered contactors because they are not intended for currents above 15 amps. To support charging at both 400 V and the Tesla Cybertruck uses a Double-Pole Double-Throw (DPDT) contactor. When charging the battery pack of two 400 V modules are connected in either parallel or series by the contactor depending on whether the available voltage is 400 V or 800 V. At 800 V the contactor can handle 350 kW. When a series of contactors is to be operated in sequence, this may be done by a camshaft instead of by individual electromagnets. The camshaft may be driven by an electric motor or a pneumatic cylinder. Before the advent of solid-state electronics, the camshaft system was commonly used for speed control in electric locomotives.

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