Construction and Operating Principle
– The contacts are made of high-conductivity metals like tungsten and copper.
– The electromagnet or coil provides the driving force to close the contacts.
– The enclosure is a frame made of insulating materials like Bakelite and thermosetting plastics.
– Magnetic blowouts use blowout coils to lengthen and move the electric arc.
– An economizer circuit can be installed to reduce power required to keep a contactor closed.
– When current passes through the electromagnet, a magnetic field is produced, attracting the moving core of the contactor.
– The force developed by the electromagnet holds the moving and fixed contacts together.
– When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts.
– Shading coils are used to average out the alternating pull of the magnetic field and prevent buzzing at twice line frequency.
– Rapid closing can lead to contact bounce, which can be minimized by bifurcated contacts or contact wipe.
Arc Suppression
– Electric current arcing causes degradation of contacts and device failure.
– Break arcs are more energetic and destructive than make arcs.
– The high temperature of the arc cracks gas molecules, creating ozone, carbon monoxide, and other compounds.
– Contact degradation occurs over time due to arc energy, causing material to escape into the air as fine particulate matter.
– Motor control contactors at low voltages are air break contactors, while medium-voltage AC motor controllers use vacuum contactors.
Applications
– Contactors are used to control electric motors, lighting, heating, capacitor banks, and thermal evaporators.
– They are also used in process industries as vessels where two streams interact, such as air and liquid.
– Contactors are suitable for switching high-current load devices, typically above 15 amperes or in circuits rated more than a few kilowatts.
– They come in various forms with different capacities, ranging from small devices to large ones approximately a meter in size.
– Contactors can handle breaking currents from several amperes to thousands of amperes and operate at different voltage levels.
Types and Features
– Contactors can have normally open (form A) contacts and optional auxiliary low-current contacts.
– They are designed with features to control and suppress the arc produced when interrupting heavy motor currents.
– Some contactors have series coils connected in the motor circuit for automatic acceleration control.
– Universal coils are available, driven by both AC and DC supplies.
– Contactors are not intended to interrupt short circuit currents like circuit breakers.
Ratings and Utilization Categories
– Contactors are rated by designed load current per contact (pole), maximum fault withstand current, duty cycle, design life expectancy, voltage, and coil voltage.
– IEC utilization categories include AC-1, AC-2, AC-3, AC-4, AC-15, and DC-13, specifying different load types and control purposes.
– NEMA contactors for low-voltage motors are rated according to NEMA size and horsepower ratings based on voltage, motor characteristics, and duty cycle.
– Contactors for medium-voltage motors are rated by voltage and current capacity.
– Relays and auxiliary contact blocks are also rated according to IEC 60947-5-1. Source: https://en.wikipedia.org/wiki/Contactors
A contactor is an electrically controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.
Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low-current contacts, contactors are almost exclusively fitted with normally open ("form A") contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents.
Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current. Contactors range from those having a breaking current of several amperes to thousands of amperes and 24 V DC to many kilovolts. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.
Contactors are used to control electric motors, lighting, heating, capacitor banks, thermal evaporators, and other electrical loads.
