A detail introduction to Contactors
2008-08-14

                                                                                                A detail introduction to Contactors         Contactor’definition is very simple; it is only an electrical relay which used to control the flow of power in a circuit.         Contactor is widely being used in automatic control circuit, its main target is controlling electric motor , it can also being used to control other electrical load, such as electric heating, lighting, electric welding machine, electric capacitors and so on.          A contactor is composed of three different systems. The contact system is the current carrying part of the contactor. This includes Power Contacts, Auxiliary Contacts, and Contact Springs. The electromagnet system provides the driving force to close the contacts. The enclosure system is a frame housing the contact and the electromagnet. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and to provide some measure of protection against personnel touching the contacts. Open-frame contactors may have a further enclosure to protect against dust, oil, explosion hazards and weather.          Contactors used for starting electric motors are commonly fitted with overload protection to prevent damage to their loads. When an overload is detected the contactor is tripped, removing power downstream from the contactor.          High voltage contactors (greater than 1000 volts) often have arc suppression systems fitted (such as a vacuum or an inert gas surrounding the contacts). Magnetic blowouts are sometimes used to increase the amount of current a contactor can successfully break. The magnetic field produced by the blowout coils force the electric arc to lengthen and move away from the contacts. The magnetic blowouts in the pictured Albright contactor more than double the current it can break from 600 Amps to 1500 Amps.          Sometimes an Economizer circuit is also installed to reduce the power required to keep a contactor closed. A somewhat greater amount of power is required to initially close a contactor than is required to keep it closed thereafter. Such a circuit can save a substantial amount of power and allow the energized coil to stay cooler. Economizer circuits are nearly always applied on direct-current contactor coils and on large alternating current contactor coils.          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 basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design). The coil may be energized at the same voltage as the motor, or may be separately controlled with a lower coil voltage better suited to control by programmable controllers and lower-voltage pilot devices. Certain contactors have series coils connected in the motor circuit; these are used, for example, for automatic acceleration control, where the next stage of resistance is not cut out until the motor current has dropped.         Compare contactors to relays, contactors have some own advantages.         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 contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents.         Contactor also has its own operating principles.         When current passes through the electromagnet, a magnetic field is produced which attracts ferrous objects, in this case the moving core of the contactor is attracted to the stationary core. Since there is an air gap initially, the electromagnet coil draws more current initially until the cores meet and reduce the gap, increasing the inductive impedance of the circuit. The moving contact is propelled by the moving core; 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.         For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency.        Most motor control contactors at low voltages (600 volts and less) are "air break" contactors, since ordinary air surrounds the contacts and extinguishes the arc when interrupting the circuit. Modern medium-voltage motor controllers use vacuum contactors.        Motor controls contactors can be fitted with short-circuit protection (fuses or circuit breakers), disconnecting means, overload relays and an enclosure to make a combination starter. In large industrial plants many contactors may be assembled in motor control centers.        Contactors commonly have several typically features: 1. Strong disjunction ability: disjunction current could be 10 to 20 times to rating current; 2. Long life: electrical life could be hundreds of thousands of times; mechanical life can up to 1 million times; 3. Small size, light weight, non-arc-distance, safe and reliable; 4. Maintenance is simple, the main contact with no maintenance, small operation noise, running out of bad environmental impact; 5. Could frequent operations.       When you choose a contactor, there are several attentions you should pay to as the followings: (1) Choose the type of contact:       According to the types of the load currents of the circuits, Load sharing should choose exchanges contacts, the direct load should use DC-DC contact, if the main control system of exchange is negative Upload, DC motor or a DC load capacity is smaller, it could also choose the exchanges contacts to control, but the contacts should be rated at a large number of elections. (2) Select the main contact for the rated voltage:       It should be equal to or greater than the rated load voltage. (3) Select the rated current for the main contact:       The selected current rating of the main contact should not be less than the load circuit rated current. It is also be chooses under the control of the motor's maximum power. If contact is used to control the motor's launch frequent, positive and negative or braking trig or other occasions, the main contact's rated current of the contact should be used lower, generally a lower grade is already ok. (4) Determine the coil voltage and the capacity of the assistance-contact according to the control circuit's requirements:       If the control line is relatively simple, the number of contacts would also be smaller, and the rated voltage of exchanges contacts coil is generally 380 V or 220 V. If the control line is little complex, the demand of the electrical appliances would be more, for the sake of safety, it could choose a low voltage for the coil , then it also need to increase a control transformer.