Contactors are very similar to relays in that they are electromechanical devices. Contactors can be obtained with coils designed for use on higher voltages than most relays. Most relay coils are intended to operate on voltages that range from 5 to 120 volts AC or DC. Contactors can be obtained with coils that have voltage ranges from 24 volts to 600 volts. Although these higher voltage coils can be obtained, for safety reasons, most contactors operate on voltages that generally do not exceed 120 volts. Contactors can be made to operate on different control circuit voltages by changing the coil. Manufacturers make coils to interchange with specific types of contactors. Most contain many turns of wire and are mounted in some type of molded case that can be replaced by dissembling the contactor.
It should be noted that NEMA standards require the magnetic switch device to operate properly on voltages that range from 85 percent to 110 percent of the rated coil voltage. Voltages can vary from one part of the country to another, as well as the variation of voltage that often occurs inside a plant. If coil voltage is excessive, it draws too much current causing the insulation to overheat and eventually burn out. Excessive voltage also causes the armature to slam into the stationary pole pieces with a force that can cause rapid wear of the pole pieces and shorten the life to the contactor. Another effect of too much voltage is the wear caused by the movable contacts slamming into the stationary contacts causing excessive contact bounce. Contact bounce can produce arcing that creates more heat and more wear on the contacts.
Insufficient coil voltage can produce as much, if not more, damage than excessive voltage. If the coil voltage is too low, the coil will have less current flow causing the magnetic circuit to be weaker than normal. The armature may pick up, but not completely seal against, the stationary pole pieces. This forms too much of an air gap and the coil current does not drop to its sealed value, causing excessive coil current, overheating, and coil burn out. A weak magnetic circuit can cause the movable contacts to touch the stationary contacts and provide a connection, but not have the necessary force to permit the contact springs to provide proper contact pressure. This lack of pressure can cause arcing and possible welding of the contacts. Without proper contact pressure, high currents produce excessive heat and greatly shorten the life of the contacts.
Load Contacts
The greatest difference between relays and contactors is that contactors are equipped with contacts intended to connect high current loads to the power line. These large contacts are called load contacts. Depending on size, load contacts can be rated to control several hundred amperes. Most will contain some type of arcing chamber to help extinguish the arc that is produced when heavy current loads are disconnected from the power line.
Other contacts may contain arc chutes that lengthen the path of the arc to help extinguish it. When the contacts open, the established arc rises because of the heat the arc produces. The horn of the arc chute pulls the arc farther and farther apart until it can no longer sustain itself. Another device that operates on a similar principle is the blowout coil. Blowout coils are connected in series with the load. When the contact opens, the arc is attracted to the magnetic field and rises at a rapid rate. This same basic action
causes the armature of a direct current motor to turn. Because the arc is actually a flow or current, a magnetic field exists around the arc. The arc’s magnetic field is attracted to the magnetic field produced by the blowout coil causing it to move upward. The arc is extinguished at a faster rate than is possible with an arc chute that depends on heat to draw the arc upward. Blowout coils are sometimes used on contactors that control large amounts of alternating current, but they are most often employed with contactors that control direct current loads. Alternating current turns off each half cycle when the waveform passes through zero, which helps to extinguish arcs in alternating current circuit. Direct current, however, does not turn off at periodic intervals. Once a DC arc is established, it is much more difficult to extinguish. Blowout coils are an effective means of extinguishing these arcs.
Most contactors contain auxiliary contacts as well as load contacts. The auxiliary contacts can be used in the control circuit if required. Note that a normally open auxiliary contact is used to control an amber pilot light that indicates that the heaters are turned on, and a normally closed contact controls a red pilot light that indicates that the heaters are turned off. A thermostat controls the action of HR contactor coil. In the normal deenergized state, the normally closed HR auxiliary contact provides power to the red pilot light. When the thermostat contact closes, coil HR energizes and all HR contacts change position. The three load contacts close and connect the heaters to the line. The normally closed HR auxiliary contact opens and turns off the red pilot light, and the normally open HR auxiliary contact closes and turns on the amber pilot light.
Vacuum Contactors
Vacuum contactors enclose their load contacts in a sealed vacuum chamber. A metal bellows connected to the movable contact permits it to move without breaking the seal. Sealing contacts inside a vacuum chamber permits them to switch higher voltages with a relative narrow space between the contacts without establishing an arc. Vacuum contactors are generally employed for controlling devices connected to medium voltage. Medium voltage is generally considered to be in a range from 1 kV to 35 kV.
An electric arc is established when the voltage is high enough to ionize the air molecules between stationary and movable contacts. Medium voltage contactors are generally large because they must provideenough distance between the contacts to break the arc path. Some medium voltage contactors use arc suppressers, arc shields, and oil immersion to quench or prevent an arc. Vacuum contactors operate on the principle that if there is no air surrounding the contact, there is no ionization path for the establishment of an arc. Vacuum contactors are generally smaller in size than other types of medium voltage contactors.
