Explain electrical contactor
Contactor .
A contactor is an electrically controlled switch used for
switching an electrical power circuit, similar to a relay except with higher
current ratings. A contactor is controlled by a circuit which has a much lower
power level than the switched circuit. Contactors come in many forms with
varying capacities and features.
Parts .
A contactor has
three components. The contacts are the current carrying part of the contactor.
This includes power contacts, auxiliary contacts, and contact springs. The
electromagnet (or "coil") provides the driving force to close the
contacts. The enclosure 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.
Magnetic blowouts use blowout coils to lengthen and move the
electric arc. These are especially useful in DC power circuits. AC arcs have
periods of low current, during which the arc can be extinguished with relative
ease, but DC arcs have continuous high current, so blowing them out requires
the arc to be stretched further than an AC arc of the same current. The
magnetic blowouts in the pictured Albright contactor (which is designed for DC
currents) more than double the current it can break, increasing it from 600 A
to 1,500 A.
Sometimes an economizer circuit is also installed to reduce
the power required to keep a contactor closed; an auxiliary contact reduces
coil current after the contactor closes. A somewhat greater amount of power is
required to initially close a contactor than is required to keep it closed.
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.
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 a motor the contactor is
controlling, 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 .
Operation .
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.
When current passes through the electromagnet, a magnetic
field is produced, which attracts the moving core of the contactor. The
electromagnet coil draws more current initially, until its inductance increases
when the metal core enters the coil. 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.
Because arcing and consequent damage occurs just as the
contacts are opening or closing, contactors are designed to open and close very
rapidly; there is often an internal tipping point mechanism to ensure rapid
action.
Rapid closing can, however, lead to increase contact bounce
which causes additional unwanted open-close cycles. One solution is to have
bifurcated contacts to minimize contact bounce; two contacts designed to close
simultaneously, but bounce at different times so the circuit will not be
briefly disconnected and cause an arc.
A slight variant has multiple contacts designed to engage in
rapid succession. The first to make contact and last to break will experience
the greatest contact wear and will form a high-resistance connection that would
cause excessive heating inside the contactor. However, in doing so, it will
protect the primary contact from arcing, so a low contact resistance will be
established a millisecond later.
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