An electrical contactor is a switching device, widely used for the switching of motors, capacitors (for power factor correction) and lights. As the name indicates it is used to make or break contacts as like an ordinary on-off switch. The only difference is that the contactors have an electromagnet that holds the contacts when energized whereas switches do not have it.
The basic operation of a contactors is similar to that of a relay but their contacts can carry much more current than relays. Relays cannot be directly used in circuits where the current exceeds 20 amperes. In such conditions, contactors can be used. They are available in a wide range of ratings and forms. Also, they are available up to the ampere rating of 12500A. They cannot provide short circuit protection but can only make or break contacts when excited.
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Constructional features of Contactor
A contactor consists of an electromagnet, contacts and spring enclosed inside an enclosure. In some them, have built-in economizers, that can reduce power consumption. Certain arrangements for arc extinction is also made inside for making and breaking the operation.
An electromagnet is a key component in contactors without which it cannot function. It requires an additional supply for excitation. It drains negligible current from the supply during excitation. These electromagnets will be hollow cylindrical in shape. A rod (armature) with spring return arrangement will be placed in the hollow cylindrical electromagnet.
In some contactors, this electromagnet is split into two halves. One of the halves is fixed and the other is movable. Movable power contacts are fixed to the movable electromagnet. Under normal condition, these two halves of electromagnets are held apart using a spring in between.
In contactor with AC coil, the electromagnetic core is made up of laminated soft iron to reduce eddy current losses and in contactor with DC coil, the electromagnetic core is made up of solid steel/ soft ironic core since there is no risk of eddy current loss in DC.
A typical contactor consists of two sets of contacts, of which one is stationary and the other is moveable. Silver tin oxide (AgSnO2), silver nickel (AgNi) and silver cadmium oxide (AgCdO)are the normally used contact materials. These materials have high welding resistance and stable arc resistance. Silver cadmium oxide and silver nickel are used in contactors of less ampere rating whereas Silver tin oxide is used in those of high ampere rating and in DC contactors.
The movable set of contacts is attached to the armature or movable electromagnet. Contact material must withstand mechanical stresses, arcs, erosion and must have very low resistance.
Electromagnet and contacts are packed inside an enclosure made of plastic, ceramic or Bakelite, which protects it from dust and external environment and ensures safe opening and closing of contacts.
Arc extinction is one of the key functionalities of a contactor. AC arcs can be easily extinguished since it passes through zero twice for every cycle. Hence arc suppressors can do the job. But on the case of DC arcs, magnetic blowouts or specially designed arc chutes for arc extinction are necessary. depending upon the application various arc suppression arrangements are made in contactors out of which arc chutes are one of them.
An economizer circuit is the one used to reduce the power consumed by the coil. An economizer circuit supplies a high current during pickup and later it supplies adequate power to keep the contacts closed. It is not mandatory that all contactors must have an economizer circuit.
Operating principle of contactors
The operating principle of a contactor is very simple. Whenever the electromagnetic coil is energized, an electromagnetic field is produced. This electromagnetic field attracts the metallic rod (armature) towards the gap in the hollow cylindrical magnet.
In contactors with split electromagnets, the movable half of the electromagnet is attracted towards the fixed electromagnet. This action closes the contacts. The contacts remain closed as long as the electromagnet remains excited. When the coil is de-energized, moving contact is pushed back to its normal position by the spring. Contactors are designed to open and close contacts rapidly. Moving contacts may bounce as it rapidly makes contacts with the fixed contacts. Bifurcated contacts are used in some contactors to avoid bouncing.
The input to the contactor coil can be AC or DC (available in various voltage ranges starting from 12Vac/ 12Vdc to 690Vac) or even universal. The universal coils are the ones that can operate on AC as well as DC voltages. A small amount of power is drained by the contactor coil during its operation. Economiser circuits are used to reduce the power consumed by the contactor during its operation.
Contactors with AC coils have shading coils. Otherwise, the contactor may chatter every time the alternating current crosses zero. Shading coils delay demagnetization of the magnetic core and avoids chattering. Shading is not required in DC coils as the flux produced is constant.
Arc Suppression in contactors
Arc occurs between the contacts every time when contacts are closed or opened under load. Arc formed during the breaking of a load is more destructive and may damage the contacts, hence reducing the life of the contactor. In addition to that high temperature of arc degrades the gases surrounding the contacts and forms harmful gases such as carbon mono-oxide, ozone etc. This may affect the mechanical durability of the contactors. Several methods are adopted for control and extinction of arcs.
As mentioned earlier, DC arcs are more severe compared to AC arcs. In DC contactors magnetic blowouts are used to propagate the arcs towards specially designed arc chutes and extinguishing it by splitting it. Contactors used in low voltage AC applications (690Volt or less), atmospheric air surrounding the contacts extinguishes the arc. For medium voltage and high voltage applications vacuum contactors are used to avoid the risk of arc.
Categorization of contactors
Few important IEC utilisation categories are below:
Contactors are categorised based on the type of load (IEC utilisation categories – 60947) and current and power rating (NEMA size).
- AC-1: Non-inductive or slightly inductive and resistive heating type of loads
- AC-2: Starting of slip ring induction motor
- AC-3: Starting and switching off Squirrel-cage motors during the running time
- AC-15: Control of AC electromagnets.
- AC-56b:- Switching of capacitor banks
- DC–1: Non-inductive or slightly inductive and resistive heating type of loads
- DC-2: Starting, inching and dynamic breaking of DC shunt motors
- DC-3: Starting, inching and dynamic breaking of DC series motors
- DC-13: Control of DC electromagnets
NEMA size is based on the maximum continuous current and horsepower rating of the induction motor controlled by the contactor. In NEMA standard contactors are designated as size 00,0,1,2,3,4,5,6,7,8,9.
Application of contactors
Contactors are used in motor starter either Direct-on-line or Star Delta along with thermal overload relays or motor protection circuit breakers. Even in our homes, one can find it inside the pump starters. Normally in a motor stater circuit, a contactor is used for switching, along with overload relay and short circuit protection devices.
Switching of capacitor Banks
In capacitor banks, capacitor switching contactors are used to switch capacitors based on the reactive power requirements. Capacitor switching contactors are specially designed to control high transient currents during switching of contactors.
Contactors are used in the switching of street, commercial and residential lights. Especially timer controlled lighting systems uses contactors for switching. Latch type contactors are also available. In this type, two coils are available, one for opening and the other for closing. Closing coil closes the contacts, when excited and cuts off the supply to the coil. Contact is then held closed mechanically. The second coil is used for opening the contacts.
Selection of contactors
Contactors are selected based on the following:
- Application-based on IEC utilisation category.
- Load current and voltage.
- Control voltage available – For selecting coil voltage.
How to check a contactor?
A contactor can be checked whether it is “open” or “closed” using an ohmmeter. If the resistance between the input and output terminals is infinite then the contactor is opened and if the ohmmeter reading is zero then it denoted that the contacts are closed.