ARTICLE

Electro Magnetic Contactors

Introduction: Motor is the most widely used Electrical equipment to convert electrical energy in to mechanical energy. In day-to-day operations, Contactor accomplishes the desired switching on and off of these motors from various locations.

A contactor is a switching device capable of making, carrying and breaking currents under normal circuit conditions including operating in overload conditions. Based on the working principle, there are so many types of contactors are available, i.e., pneumatic, Hydraulic, Electromagnetic etc., But the most commonly used one is Electromagnet type because of its simple operating principle and rugged construction. Here we are discussing about Electro magnetic contactors.

Definition: Electromagnetic Contactors are power utilization electro mechanical switches, which are magnetically closed and magnetically held. In electromagnetic contactors the force for closing the main contacts is provided by an electromagnet.

They are multiple-pole air break switches capable of repetitive operations with mechanical life expectancy that may approach 10 million operations.

Operating Principle: Basic operation principle is converting electrical energy in to mechanical energy in a magnetic field. When the supply extended to the coil, it will energize and become an electromagnet, which attracts the moving core towards fixed core. While starting, coil draws more current than holding period as moving core has to overcome the initial inertia. The ratio of starting and holding currents are approximately 10. When the moving contact”Just ” touches the fixed contact, it experiences bounce. Bounce is detrimental to the life of contacts, as every bounce will generate an arc and reduce the contact material. Bounce is minimized by, reducing the speed of armature movements and improving the material property of contacts. While in “just” touch position of the main contacts, moving core of the electromagnet is still not in contact with fixed core. This gap is desirable as the further travel of the core generates the extra contact pressure on the contacts. This extra travel of the core is called “over travel”. Simultaneously auxiliary N/C contact opens first and then N/O contact closes. This design is called for in most applications. E.g. Star contactor is connected to the coil of the Delta contactor to ensure this.

Construction of contactor: A contactor is composed or the following 3 systems.

i) Contact System: This is the current carrying part of the contactor. This includes Power contacts, Auxiliary contacts & contact springs. Contactors making capacity is at 10 times the rated current of the contactor.

Power contacts: These are designed to make, carry and break the current in the power circuit. They include a fixed contact and a moving contact. During closing operation, the moving contacts, which are linked to moving cores, are pulled towards the fixed

contacts, thus closing the circuit. They are highly conductive and suitably arranged for bounce free operation.

Auxiliary contacts: They are used for signaling/interlocking purpose. They are available in different versions: Normally Open (N/O), Normally Closed (N/C). The combination of N/O and N/C depends on the usage. Contact tip: Contact tips brazed to each contacts. These tips are made of Silver Cadmium Oxide to provide high mechanical strength & resistance to welding.

Contact springs: Moving contacts and auxiliary contacts are provided with contact spring. These springs are provided to exert sufficient pressure on the contacts, so that, flow of rated current does not cause over-heating. These springs are energized during “over travel”.

ii) Electro magnet System: This is a driving unit of contact system. Force for closing of contacts is provided by t 'Electromagnet:' it consists of Fixed & Moving core & coil. Its shape varies as a function of the type of contactor, alternating or direct current. A small gap provided in the magnetic circuit in the closed condition between the central limbs of fixed & moving cores eliminates any residual magnetism (becoming a permanent magnet). This gap is generated by removal of metal or by insertion of a non-magnetic material (e., copper, brass).

Magnet gap: This is the distance between moving core and fixed core in OFF condition.

Contact gap: This is the distance between fixed contact and moving contact when the contactor is in OFF condition.

Over travel of moving contact: This is the distance between moving core & fixed core when the fixed and moving contacts “just” touch during contactor closing operation. Therefore we can say,

MAGNET GAP = CONTACT GAP + OVERTRAVEL

AC Magnetic Cores: The moving core and the fixed cores are made of riveted silicon steel laminations. Laminated cores are used in order to reduce eddy current, which are created in any metal body subjected to an alternating current flux. Grinding of pole faces is done to ensure silent operation. Pole faces are generally matched perfectly to ensure low reluctance path for magnetic core. Two shading rings (made of nonmagnetic material) are fitted on the extreme and pole faces of the fixed/moving core to create a flux out-of-phase with the main alternating flux. This out-of-phase flux prevents periodic cancellation of total flux and thereby chattering.

DC Magnetic cores: In case of DC no eddy currents are formed. So these parts can be solid/laminated cores. To avoid permanent magnetism a small gap is provided between the armature and magnet by a non-magnetic material piece.

Coil: Coil is the driving force for the contactor operation. The coil produces the magnetic flux required to attract the moving core of the electromagnet. The coil is designed to withstand mechanical shocks caused by the closing and opening of the

moving & fixed cores. In order to reduce mechanical shocks, the coil or the core, and sometimes both, are mounted on shock absorbers. The coil is made of super enameled copper wire. The windings are wound on a Bobbin and covered with polyester tapes suitable for withstanding high temperature. Coils are generally designed to 0.8 to 1.1 of rated voltage.

Return spring: This spring is fitted in between moving core & fixed core. When the moving core closes, it closes against the fore of the turn spring. This spring is provided to avoid accidental touch of moving & fixed core in OFF condition. This helps in breaking of contacts very effectively.

Enclosure System: An external frame housing the contact and the electromagnet made of insulating materials like Bakelite, Nylon 6, thermosetting plastics etc. to constitute a protective structure, adequate insulation, protection to the components within, protection to personnel coming in contact & protection against dust & ingress of vermin.

Arc Chutes: Arc Chutes are fitted above the moving and fixed power contactor. The purpose of Arc chutes is to split the arc established at contactor tips while breaking the current and to quench the arc. Arc chutes also provides phase barriers between the phases.

Terminals: These are hold the input / output supply cables.

Selection of Contactor: Proper selection of a contactor avoids many problems and gives maximum efficiency.

The various factors to be considered while selecting a contactor are

-The type of voltage (AC / DC) and mains supply (Single / three phase). -The installed capacity. -The load characteristics. -The duty requirements -The standards required by certain organizations -The environmental conditions Application to motor control: For selection of contactors for motor application following points to be taken care of -Type of duty -Rating of the motor -Type of starting – DOL, Star-Delta, Autotransformer, etc. -Acceleration time. Various problems encountering with contactors in industries the controlled switching on and off of the motors are achieved by electromagnet contactors. One can operate these contactors from various locations, i.e. push button stations, DCS and PLC systems.

The most common problems every one facing with these contactors due to the increased number of operations and aging of the contactors are -Chattering of contactors -Frequent burning of contactor coils -Excessive pitting and welding of contacts -More hum noisy magnets -Failure to pick up and seal -Failure to drop out -Over heating of the contactor -Contactor flash over -Sluggish in operation

Chattering of contactors: The main five reasons for contactor chattering and remedies are

i) Inadequate control voltage. Due to low control voltage, sufficient magnetic force may not produce and Hence contactor starts chatter.

Ensure the coil end voltage and in case of persistent low voltage, change the coil to lower voltage rating or change the control transformer tapping.

ii) Poor contact or loose connections in control circuit. These loose contacts cause major voltage drop and hence coil gets low voltage.

Use proper lugs for termination of all control cables and tighten all control connections regularly. Replace the rusted lugs periodically.

iii) By misplacing or because of broken poles shading ring of contactor, it may get chatter at very high rate. Ex.100/per sec. In this case we have to replace the contactor as the mechanical life of the contactor is over.

iv) Smaller cross-section or long length of control cable also cause for chatter.

When long and smaller cross section control cables are used for actuating the contactor, large voltage drop may occur causing chattering of the contactor or even failure to pick up. In this case use higher size of cable or use a signal relay for the contactor.

v) If system starts from PLC or DCS due to faulty system, control relays or overload relay with auto reset may cause for chattering of contactors.

Check the control circuit and rectify the problem.

Frequent burning of contactor coils: Major share of failures with contactors are due to burning of coils. These failures are mainly due to

i) Core gap is major cause for coils burning. If core faces are not closing properly leads to sharp rise in magnetizing current. General reasons for core gaps are

-Any obstruction in operation of the contactor or foreign material in side the contactor. In this case coil tries to close the contactor and hence draw more current.

Check the contactor and remove the obstruction.

ii) -Rusting of Core face due to humidity. Corrosive atmosphere has a profound effect on the performance of the contactors. Overheating takes place due to the formation of the foreign films at the connections, on the springs and core faces. Other reasons are Dust formation between core faces or Deformation of core faces shape. Deformation of contactor tips may cause of core gap. For such applications contactors are suitably derated. Occasional spraying with Dehumidify sprays prevents reaction of the silver parts with the atmosphere and minimizes damage. Strictly maintain anti condensation of heaters in side the panel where relative humidity is more than 90%. i.e. at sea shore areas.

iii) Due to inadequate heat ventilation, coil gets heated up and leads to insulation failure.

Provide suitable cooling of the coil.

iv) -Peak voltage surges while coil operation. Induced voltage in the coil is proportional to rate of change of current and the inductance of the coil. While switching off the contactor the di/dt is very high and induces high voltages, which is much greater than the coil operating voltage. This may result burning of coils.

Provide suitable resistance across the coil. The added resistance discharges the stored energy in the coil when supply cuts keeping di/dt to small.

Provide a series resistance to the coil to reduce the coil across voltage as well as coil holding current which in term reduces the self induced EMF.

v) -Natural aging of coil insulation.

Check coil resistances periodically. Use varnish to increase the coil insulation.

-Inter turn short circuit of coil turns due to coil insulation failure, Thermal aging of the coil insulation.

Replace the Coil.

vi) -High control voltages and low frequencies are also causes of coil burning. Use Suitable fuses for protecting the coil.

vii) -Under voltage failure of magnet to seal-in. In this case coil draws more and more current and resulting coil burning.

Excessive pitting and welding of contacts

Pitting means worn out of the contactor tips due to the mechanical impact and due to arching in every contactor operation. The excessive pitting causes welding of the contactor tips. IN this case contactor fails to open even though supply cuts.

Main Reasons for this type of problems are

i) -In adequate rating of contactor according to drive duty and rapid inching. In this case the current drawn by the contactor is more than its rating.

Use suitable contactor.

ii) -A drive starting current is 7 to 8 times more than the rated current. So prolonged starting current may leads to contactor tips welding.

iii) -Insufficient contact pressure leads to sparking at tips due to improper contact and gets weld.

Do regular checkups and adjust the contact tips and springs.

iv) -While Short circuit conditions if fuse is fails to isolate the fault, leads to tips welding.

v) -Due to more number of operations and due to bounce excessive pitting will takes place. Bounce is minimized by, reducing the speed of armature movements and improving the material property of contacts.

More hum noisy magnets

Some contactors while operation produce a low continuous sound, which we known as humming. The main reasons for the humming noise are due to -Minor gap in magnetic faces due to dirt or rust accumulation on magnet faces or due to cracked shaded ring. To prevent these clean the cores faces regularly and protect the faces from rust.

-Loose stocking of core laminations vibrate while contactor operation and produce high hum noise. Check contactor core regularly.

Failure to pick up and seal Sometimes contactors may not respond for the close command. In those cases check the following conditions

-Contactor unable to prick up due to low control voltage at coil end.

Check the control supply voltage, check for loose connections in the circuit and tightened it if any.

-Coil open or shorted. Due to dry soldering, thermal insulation failure or aging coil may open or shorted. In this case replace the coil.

-Control circuit problems. Check the control circuit, interlocks etc.,

-Due to mechanical obstruction of contact assembly. Check for the foreign material, arc chute positions and mechanical interlocks etc.,

Failure to drop out

In some cases Contactors may not respond for the open command. This problem we call as contactor fails to drop out. The main reasons are -Control supply may extend continuously due to earth faults or control circuit disorders. Check and normalize the control circuit. In case of earth faults in the circuit isolate them immediately.

-Worn out or rusted parts will cause binding. Hence contactor tips get welded and oppose to drop in absence of control supply. Replace those parts with new parts.

-Residual magnetism due to lack of air gap in magnet path. Core may saturate and behaves as a permanent magnet. Replace the core. Over heating of the contactor Owing to the following reasons, Contactor temperature rises abnormally and gives poor performance

-Improper terminations of contactors terminals and loose Connections are the main reason for over heating.

Always use proper size of lugs and ensure sufficient contact area at terminals.

Avoid loose connections by tightening the terminals regularly.

- High contact resistance at tips due to carbonization of tips. Insufficient contact pressure also offers high resistance and causes temperature rise at rated power output.

Clean the tips frequently. Check the contact area and pressure.

-Improper selection of contactor according to drive duty and rating. So contactor has to conduct more than their rated currents, which in turn produce high temperature.

Always select the contactor accordingly the drive.

- Poor heat ventilation also causes for temperature rise. Contactor flash over

Generally most of the times contactor flash over occur due to -While starting the bigger drives if contactor opens due any reason, inducing high voltage (di/dt) causes a flash over.

- If control gear in conductive dust environment flash over may occur.

- Due to condensation of moisture (wherever relative humidity more than 90%) flash over may occur. Provide space heaters.

- If separator between phases (phase barrier) gets broken while open/close time, flash over may occur due to phase to phase short circuits.

Sluggish in operations: Contactor responds slowly and moves lazily. It is also a most common problem facing with contactors over period of a time and the increased operations. This may be due to

- Low control voltage at the coil end. Check the control circuit.

-Due to less no of magnetizing turns, or may be due to inter-turn shorting of coil turns. To overcome this problem, replace the coil.

-Due to influence of dust in the path of core traveling area, contactor may be sluggish in operations. Clean the core path area.

-Due to improper strength of restoring springs. Adjust the spring tension periodically and replace the rusted, broken springs. Good maintenance steps to follow

-Execute routine inspections. Inspect terminals, contacts, arc chambers periodically.

Connections have tendency to loose with time and no. of operations, Hence tighten terminals regularly. Use always-suitable torque of terminals tightening.

-Do not alter contact arrangement and never remove any unutilized contact from the contactor. This will effects the balance / performance of the contactor.

- Observe always nameplate details of contactor and select correct size of contactor, suitable to the motor rating, duty cycle and control voltage.

-Never operate contactor with out back up fuse, because contactor is not designed for short circuit currents. Select proper fuse rating suitable to fault level.

- In dust prone areas keep contactor in enclosure or clean contactor periodically.

- Clean the contactor tips periodically with suitable cleaners. Don’t apply grease to contactor parts. Grease reduces the contact area and affects the performance.

- In case if tips gets roughened, clean dirt and carbon with solvents and keep as its original location. Do not grind the contractor tips. Do not use emery paper to clean the tips. Do not use abrasives to clean the contactors as they will remove the silver from contact tips and drastically reduce the contact life.

-In high humidity and chemical zone areas, Occasional spray with CRC 2-26 to prevent reaction of the silver parts with the atmosphere and minimizes damage.

-Do not operate the contactor with out the Arc chamber. Do not use broken / cracked damaged arc chambers. Such arc chambers can cause to flashovers hence replace them.

-In case of rust / dust accumulated on the magnet faces, clean them with CRC or Chamois leather. Do not use degreasing agent as petrol. Do not clean the magnets with sharp / pointed objects.

-Never replace parts other than the standard spares. Parts other than the standard spares are designed to last for the lifetime of the contactor. -Do not replace slightly pitted contact. Replace contacts when the contact tip has become less than 40% of its original volume.

-The contact condition should be inspected after a fault, i.e. short circuits. If the contacts are slightly welded separate them with a screw diver. If the contacts are not easily separable, i.e. permanently welded, replace the contacts and use the contactor, provided its other parts are in proper condition.

-Replace contacts in pairs, fixed and moving contacts of one pole together. This ensures proper matching of the contact tips.

-Check the pole face regularly. If pole face is not matching properly due to deformations replace the magnet cores.

-Check the contact tips pressure and adjust the springs accordingly. Don’t elongate or compress the restraining springs is contactor.

-Ensure the bounce free operation of contactor.

-Select always suitable control fuses in control circuits for the protection of the coil in case of faults.

-Provide suitable surge protectors across the coil to protect the coil from high voltage surges.

-Strictly maintain anti condensation of heaters in side the panel where relative humidity is more than 90%. i.e. at sea shore areas.

REFERENCES 1. Switch gear and Controls -Robert W.Smeaton 2. Industrial power system analysis- Beeman 3. Siemens Contactor service manual 4. Crompton Contactor service manual 5. Power station & Sub station maintenance – S.Leznov A.Taits

WRITTEN BY

CH.RAGHAVARAO Dy.General Manager (E&I) ESSAR STEEL LTD, VISAKHAPATNAM. chcrr@yahoo.com, powerch@gmail.com