--- title: "Construction and principle of contactor" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:contactor-construction-principle" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-construction-principle site: BCT Engineering --- # Construction and principle of contactor Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2072 Asoj, 6 marks - Video: [What is a Contactor? | Working Principles](https://www.youtube.com/watch?v=08ozhRb7HEU) — RealPars - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-construction-principle - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question Explain the construction and operating principle of contactor. ## Short answer A contactor contains an electromagnet (coil, laminated core with shading ring), an armature carrying silver‑alloy main and auxiliary contacts, a return spring, and arc chutes. Energising the coil attracts the armature, closing the contacts; de‑energising releases the spring to open them and extinguish the arc. ## Answer A **contactor** is an electromagnetically operated switch used to make and break a power circuit many times, under normal load conditions, by a small control signal. It is used for motors, heaters, lighting and capacitor banks. #### Construction ```text +------------------------+ | fixed contacts (3) | | == == == | | moving contacts | | -- -- -- <- bridge | | [ armature ] | | ^ spring | | [ coil on core ] | +------------------------+ A1 A2 = coil terminals ``` 1. **Electromagnet (coil and core):** a laminated E- or U-shaped iron core with a coil. The coil works on AC or DC (24 V, 110 V, 230 V, 415 V). AC cores carry a **shading ring** to stop chatter. 2. **Armature:** the movable iron part attracted by the magnet; the moving contacts are fixed to it through an insulating carrier. 3. **Main contacts (power contacts):** usually three NO poles, made of silver alloy (silver–cadmium oxide or silver–tin oxide) for low resistance and resistance to welding. Double-break bridge contacts are common. 4. **Auxiliary contacts:** small NO and NC contacts for holding, interlocking and signalling. 5. **Return spring:** opens the contacts when the coil is de-energised. 6. **Arc chutes / arc splitters:** splitter plates or blow-out coils in larger contactors cool and lengthen the arc. 7. **Enclosure and frame:** moulded insulating housing with terminals. #### Operating principle 1. When the control voltage is applied to the coil, current in the coil sets up flux in the core. 2. The magnetic pull exceeds the spring force; the armature moves and **closes the main contacts** (and changes over the auxiliary contacts). The load is connected. 3. The coil current is small compared to the load current, so a small switch or PLC output can control a large load. 4. When the coil is de-energised (STOP button, overload relay or supply failure), flux collapses and the **spring opens the contacts**. The arc formed is quenched in the arc chute at the next current zero. 5. Because pick-up needs about 85% and drop-out happens below about 60–70% of coil voltage, a contactor gives built-in **no-volt protection**. #### Main features - Designed for very high mechanical life (millions of operations) and frequent switching. - Breaking capacity is limited to a few times rated current (overload/motor stall current), not short-circuit current; fuses or MCCBs give short-circuit back-up. - Utilisation categories (IEC 60947-4-1) such as AC-1 (resistive), AC-3 (squirrel-cage motor start and run), AC-4 (inching, plugging) define the duty. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the purpose of the shading ring on an AC contactor core? A. To increase coil current B. To stop chatter (hum) of the armature C. To quench the arc D. To act as a return spring Answer: B. It creates a phase-shifted flux so pull never falls to zero, preventing chatter. (Key point: Shading ring) 2. What opens the main contacts when the coil is de-energised? A. The return spring B. The arc chute C. Gravity on the bridge D. A second coil Answer: A. Flux collapses and the return spring pulls the armature and contacts open. (Key point: Operation) 3. Why can a small switch or PLC output control a large load through a contactor? A. Coil current is small compared with load current B. The load flows through the coil C. The core carries the load current D. Contacts are in series with the PLC Answer: A. Only the small coil current is switched by the control device; the main contacts carry the load. (Key point: Control principle) 4. Which statement about contactor breaking capacity is correct? A. It is rated for full short-circuit currents B. It is limited to a few times rated current, so fuses or MCCB give short-circuit back-up C. It cannot break any current D. It is unlimited because of arc chutes Answer: B. Contactors handle overload/stall currents, not short circuits. (Key point: Limits and duty) ## Related in Switchgear and Protection - [Contactor construction and remote control circuit](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acontactor-remote-control) — Switchgear and Protection, chapter 4 - [Isolator construction, principle and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-construction-principle) — Switchgear and Protection, chapter 4 - [Contactor for motor control and protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acontactor-motor-control-protection) — Switchgear and Protection, chapter 4, asked 3× ## Related in other subjects - [Transformer operating principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-principle-emf-equation) — Electrical Machines I, chapter 2, asked 6× - [Servo motor operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — Electrical Machine, chapter 7, asked 2× - [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2× - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5