Isolators and Contactors PDF

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Isolators and Contactors notes...

Description

Isolator Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works.Isolator is a manually operated mechanical switch which separates a part of the electrical power system normally at off load condition. Types of Isolators Double Break Isolators Single Break Isolators Construction of Double Break Isolators These have three stacks of post insulators as shown in the figure. The central post insulator carries a tubular or flat contact which can be rotated horizontally with rotation of central post insulator. This rod type contact is also called moving contact. The fixed contacts are fixed on the top of the other post insulators which are located at both side of central post insulator. The fixed contacts are generally in the form of spring loaded figure contacts. The rotational movement of moving contact causes to come itself into fixed contacts and isolators becomes closed. The rotation of moving contact in opposite direction make to it out from fixed contacts and isolators becomes open. Rotation of the central post insulator is done by a driving lever mechanism at the base of the post insulator and it connected to operating handle (in case of hand operation) or motor (in case of motorized operation) of the isolator through a mechanical tie rod.

Fig: Double break isolator

Fig: Double break isolator in use

Construction of Single Break Isolators The contact arm is divided into two parts; one carries rod or tubular shape contact and other carries hollow type contact. The contact arm moves due to rotation of the post insulator upon which the contact arms are fitted. Rotation of both post insulators stacks in opposite to each other causes to close the isolator by closing the contact arm. Counter rotation of both post insulators stacks open the contact arm

and isolator becomes in off condition. This motorized form of this type of isolators is generally used but emergency hand driven mechanism is also provided

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Operation of Electrical Isolator As no arc quenching technique is provided in isolator, it must be operated when there is no current flowing through the circuit. No live circuit should be closed or open by isolator operation. A complete live closed circuit must not be opened by isolator operation and also a live circuit must not be closed by isolator operation to avoid huge arcing in between isolator contacts. That is why isolators must be open after circuit breaker is open and these must be closed before circuit breaker is closed. Isolator can be operated by hand locally as well as by motorized mechanism from remote position. Motorized operation arrangement costs more compared to hand operation; hence decision must be taken before choosing an isolator for system whether hand operated or motor operated economically optimum for the system. For voltages up to 145KV system hand operated isolators are used whereas for higher voltage systems like 245 KV or 420 KV and above motorized isolators are used. Application Isolators and circuit brekers

CB Bus-bar

Isolators

Fig: Application of isolator

Fig: Application of isolator

Isolators are used at the both side of circuit breaker in case of supply from both side. In case of supply from one side only the isolators are used at the downstream of circuit breaker this means at the receiving end (load side). Contactor A contactor is an electrically controlled switch used for switching a power circuit. It is 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 are used to control electric motors, lighting, heating, capacitor banks and other electrical loads.

Construction 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 for housing the contact and the electromagnet. Enclosures are made of insulating materials like Bakelite 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. A basic contactor has 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 voltage.

Fig: Inner view of contactor

Fig: Magnetic blowout coil

Fig: Contactor coil and contacts

Operation When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of the contactor. 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. Because arcing and consequent damage occurs just as the contacts are opening or closing, contactors are designed to open and close very rapidly.

Fig: Contactor coil supply diagram

“NO” and “NC” auxiliary contacts NO - Normally open contact NC – Normally close contact

Fig: Contactor supplying a motor

“NO” contact remain open when the contactor coil is not supplied or is de-energized. Just after the contactor coil gets supply, it energizes and pulls the “NO” contacts and the contacts get closed. The “NC” contacts remain closed when the contactor coil is not supplied. Just after the contactor coil gets supplied the coil energizes and makes the “NC” contacts open.

Fig: NO and NC contacts

Fig: NO and NC contacts

Application 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. Motor control Contactors are intensively used to supply motors.

Fig: Use of contactor

Fig: Use of contactor...