Contactor

Last updated

AC contactor for pump application Contactor.jpg
AC contactor for pump application

A contactor is an electrically controlled switch used for switching an electrical power circuit. [1] 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.

Contents

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.

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.

Construction

SPST normally-open Form-X (double make) hermetically sealed DC contactor cut-away animation showing main movable contacts and AUX feedback plunger Contactor cut-away animation with AUX.gif
SPST normally-open Form-X (double make) hermetically sealed DC contactor cut-away animation showing main movable contacts and AUX feedback plunger
Powerful DC contactor with electro-pneumatic drive Pneumatic by-passing contactors of electric locomotive VL11.jpg
Powerful DC contactor with electro-pneumatic drive

A contactor has three components:

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). Universal coils (driven by AC as well as DC) are also available in the market today. [3] 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. [4]

Operating principle

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.

Another technique for improving the life of contactors is contact wipe; the contacts move past each other after initial contact in order to wipe off any contamination.

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. [5]

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. [6]

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 600 V) 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. [7] [8]

Ratings

Contactors are rated by designed load current per contact (pole), [9] maximum fault withstand current, duty cycle, design life expectancy, voltage, and coil voltage. A general purpose motor control contactor may be suitable for heavy starting duty on large motors; so-called "definite purpose" contactors are carefully adapted to such applications as air-conditioning compressor motor starting. North American and European ratings for contactors follow different philosophies, with North American general purpose machine tool contactors generally emphasizing simplicity of application while definite purpose and European rating philosophy emphasizes design for the intended life cycle of the application.

IEC utilization categories

The current rating of the contactor depends on utilization category. Example IEC categories in standard 60947 are described as:

Relays and auxiliary contact blocks are rated according to IEC 60947-5-1:

NEMA

NEMA contactors for low-voltage motors (less than 1,000 volts) are rated according to NEMA size, which gives a maximum continuous current rating and a rating by horsepower for attached induction motors. NEMA standard contactor sizes are designated 00, 0, 1, 2, 3 to 9.

The horsepower ratings are based on voltage and on typical induction motor characteristics and duty cycle as stated in NEMA standard ICS2. Exceptional duty cycles or specialized motor types may require a different NEMA starter size than the nominal rating. Manufacturer's literature is used to guide selection for non-motor loads, for example, incandescent lighting or power factor correction capacitors. Contactors for medium-voltage motors (greater than 1,000 volts) are rated by voltage and current capacity.

Auxiliary contacts of contactors are used in control circuits and are rated with NEMA contact ratings for the pilot circuit duty required. Normally these contacts are not used in motor circuits. The nomenclature is a letter followed by a three-digit number, the letter designates the current rating of the contacts and the current type (i.e., AC or DC) and the number designates the maximum voltage design values. [10]

Applications

Lighting control

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.

Magnetic starter

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 contactor

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 (8.3ms).

Mercury relay

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.

Mercury-wetted relay

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.

Camshaft operation

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. [11]

Differences between a relay and a contactor

In addition to their current ratings and rating for motor circuit control, contactors often have other construction details not found in relays. Unlike lower-powered relays, contactors generally have special structures for arc-suppression to allow them to interrupt heavy currents, such as motor starting inrush current. Contactors usually have provision for installation of additional contact blocks, rated for pilot duty, used in motor control circuits.

Related Research Articles

<span class="mw-page-title-main">Relay</span> Electrically-operated switch

A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.

<span class="mw-page-title-main">Switch</span> Electrical component that can break an electrical circuit

In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of switch is an electromechanical device consisting of one or more sets of movable electrical contacts connected to external circuits. When a pair of contacts is touching current can pass between them, while when the contacts are separated no current can flow.

<span class="mw-page-title-main">Stepper motor</span> Electric motor for discrete partial rotations

A stepper motor, also known as step motor or stepping motor, is an electrical motor that rotates in a series of small angular steps, instead of continuously. Stepper motors are a type of digital actuator. Like other electromagnetic actuators, they convert electric energy into mechanical position can be commanded to move and hold at one of these steps without any position sensor for feedback, as long as the motor is correctly sized to the application in respect to torque and speed.

<span class="mw-page-title-main">Alternator</span> Device converting mechanical into electrical energy

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.

<span class="mw-page-title-main">Circuit breaker</span> Automatic circuit protection device

A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by current in excess of that which the equipment can safely carry (overcurrent). Its basic function is to interrupt current flow to protect equipment and to prevent fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.

A motor controller is a device or group of devices that can coordinate in a predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and electrical faults. Motor controllers may use electromechanical switching, or may use power electronics devices to regulate the speed and direction of a motor.

<span class="mw-page-title-main">Motor–generator</span> Device for converting electrical power to another form

A motor–generator is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line. Large motor–generators were widely used to convert industrial amounts of power while smaller motor–generators were used to convert battery power to higher DC voltages.

A snubber is a device used to suppress a phenomenon such as voltage transients in electrical systems, pressure transients in fluid systems or excess force or rapid movement in mechanical systems.

<span class="mw-page-title-main">High voltage</span> Electrical potential which is large enough to cause damage or injury

High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, high voltage refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant special safety requirements and procedures.

<span class="mw-page-title-main">Switchgear</span> Control gear of an electric power system

In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.

<span class="mw-page-title-main">Electronic component</span> Discrete device in an electronic system

An electronic component is any basic discrete electronic device or physical entity part of an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements. A datasheet for an electronic component is a technical document that provides detailed information about the component's specifications, characteristics, and performance.

This is an alphabetical list of articles pertaining specifically to electrical and electronics engineering. For a thematic list, please see List of electrical engineering topics. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.

<span class="mw-page-title-main">Galvanic isolation</span> Isolating sections of electrical systems

Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.

In Electrical Power Systems and Industrial Automation, ANSI Device Numbers can be used to identify equipment and devices in a system such as relays, circuit breakers, or instruments. The device numbers are enumerated in ANSI/IEEE Standard C37.2 "Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations".

<span class="mw-page-title-main">Vibrator (electronic)</span> Electromechanical device

A vibrator is an electromechanical device that takes a DC electrical supply and converts it into pulses that can be fed into a transformer. It is similar in purpose to the solid-state power inverter.

A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact.

<span class="mw-page-title-main">Dynamo</span> Electrical generator that produces direct current with the use of a commutator

A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.

<span class="mw-page-title-main">Applications of capacitors</span> Uses of capacitors in daily life

Capacitors have many uses in electronic and electrical systems. They are so ubiquitous that it is rare that an electrical product does not include at least one for some purpose. Capacitors allow only AC signals to pass when they are charged blocking DC signals. The main components of filters are capacitors. Capacitors have the ability to connect one circuit segment to another. Capacitors are used by Dynamic Random Access Memory (DRAM) devices to represent binary information as bits.

<span class="mw-page-title-main">Trembler coil</span> Part of early car ignition systems

A trembler coil, buzz coil or vibrator coil is a type of high-voltage ignition coil used in the ignition system of early automobiles, most notably the Benz Patent-Motorwagen and the Ford Model T. Its distinguishing feature is a vibrating magnetically-activated contact called a trembler or interrupter, which breaks the primary current, generating multiple sparks during each cylinder's power stroke. Trembler coils were first used on the 1886 Benz automobile, and were used on the Model T until 1927.

This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.

References

  1. Croft, Terrell; Summers, Wilford, eds. (1987). American Electricians' Handbook (Eleventh ed.). New York: McGraw Hill. p. 7-124. ISBN   0-07-013932-6.
  2. Fink, Donald G. (1978). Beaty, H. Wayne (ed.). Standard Handbook for Electrical Engineers (11th ed.). McGraw Hill. pp. 4–84. ISBN   0-07-020974-X.
  3. Electrical Classroom,, Contactor – Construction, Operation, Application and Selection
  4. Croft & Summers 1987 , p. 7-125
  5. Holm, Ragnar (1958). Electric Contacts Handbook (3rd ed.). Berlin / Göttingen / Heidelberg: Springer-Verlag. pp. 331–342.
  6. "Contact Life: Unsuppressed vs. Suppressed Arcing". Arc Suppression Technologies. April 2011. Lab Note #105. Retrieved February 5, 2012.
  7. Hammond, Rolt (1968). "Development of electric traction". Modern Methods of Railway Operation. London: Frederick Muller. pp. 71–73. OCLC   467723.
  8. Ransome-Wallis, Patrick (1959). "Electric motive power". Illustrated Encyclopedia of World Railway Locomotives. London: Hutchinson. p. 173. ISBN   0-486-41247-4. OCLC   2683266.
  9. "All about Circuits". All about circuits. Retrieved September 18, 2013.
  10. "General Information / Technical Data NEMA / EEMAC Ratings" (PDF). Moeller. p. 4/16. Archived from the original (PDF) on March 25, 2012. Retrieved September 17, 2013 via KMParts.com.
  11. "Electric Locomotives". Railway-Technical.com. n.d.