--- title: "Contactor construction and remote control circuit" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:contactor-remote-control" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-remote-control site: BCT Engineering --- # Contactor construction and remote control circuit Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2078 Chaitra, 6 marks - Video: [The Difference Between Contactors And Relays - ELECTROMAGNETIC SWITCHES electricians use](https://www.youtube.com/watch?v=YwAm2D-mm_g) — Electrician U - 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-remote-control - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question Explain the construction and operating principle of contactor. Draw the circuit diagram for remote control of a contactor. ## Short answer A contactor uses an electromagnetic coil to close main contacts and a spring to open them. For remote control, Start buttons are wired in parallel and Stop buttons in series; a hold-in auxiliary contact maintains the coil after start is released, providing no-volt protection. ## Answer A **contactor** is an electrically operated switch in which a magnetic coil closes the main contacts and a spring opens them when the coil is de-energised. It is used for frequent on-load switching and remote control of motors and other loads. #### Construction - **Coil and laminated core** (electromagnet), AC cores with a shading ring. - **Armature** carrying the moving contacts through an insulated carrier. - **Main contacts** (normally 3 NO, silver alloy) for the power circuit. - **Auxiliary contacts** (NO/NC) for latching, interlocking and indication. - **Return spring**, **arc chutes/splitters**, moulded **housing** and terminals. #### Operating principle 1. Control voltage on the coil produces flux; the armature is attracted against the spring. 2. Main contacts close and the load is energised; auxiliary contacts change state. 3. Removing coil voltage lets the spring open the contacts; the arc is extinguished in the arc chute at current zero. 4. Since the coil drops out at low voltage, the contactor also gives no-volt protection. #### Circuit for remote control of a contactor START buttons are connected **in parallel** and STOP buttons **in series**, so the motor can be started or stopped from the local panel or from a distant point. The hold-in auxiliary contact K is connected in parallel with the START buttons. Only thin control wires go to the remote station. ```text L --[Fuse]--[STOP local]--[STOP remote]--+--------------+--------------+ (NC) (NC) | | | [START local] [START remote] [K aux NO] (NO) (NO) (hold) | | | +--------------+--------------+ | [OL relay NC] | ( K coil ) | N ------------------------------------------------------+ Power: L1,L2,L3 -> fuses -> K main -> OL -> Motor ``` #### Working of the remote scheme 1. Pressing any START button energises coil K; the main contacts close and the motor runs. 2. The auxiliary contact K (in parallel with the START buttons) closes and **holds** the coil after the button is released. 3. Pressing any STOP button opens the series path; the coil drops out and the motor stops. 4. The overload relay NC contact also opens the coil path on overload. 5. Indicating lamps through auxiliary contacts show ON/OFF status at the remote point. This arrangement allows control of large motors (pumps, fans, cranes) from a control room using low-power control wiring, and safe automatic disconnection on overload or supply failure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In remote control of a contactor, how are multiple START buttons connected? A. In series B. In parallel C. In series with the coil neutral D. Across the main contacts Answer: B. Any START button in parallel can energise the coil. (Key point: START wiring) 2. How are multiple STOP buttons connected? A. In parallel B. Across the coil C. In series, so pressing any one opens the coil path D. Through the K auxiliary contact Answer: C. Series NC STOP buttons mean any one breaks the circuit. (Key point: STOP wiring) 3. What holds the coil energised after a START button is released? A. Remote STOP button B. K auxiliary NO contact in parallel with the START buttons C. The overload relay D. Indicating lamp Answer: B. The auxiliary contact closes with the main contacts and bridges the START buttons. (Key point: Holding) 4. What is a main advantage of this arrangement? A. Heavy power cables run to the remote station B. Only thin low-power control wires go to the remote station C. Fuses are not needed D. The motor needs no overload relay Answer: B. Control circuit carries only coil current, so remote stations need light wiring. (Key point: Advantage of remote scheme) ## Related in Switchgear and Protection - [Construction and principle of contactor](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acontactor-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× - [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 - [Feeder switching guideline; contactor in place of CB](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afeeder-isolator-cb-guideline) — Switchgear and Protection, chapter 4 ## Related in other subjects - [Data logger and remote control block diagrams](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adata-logger-remote-control) — Basic Electronics Engineering, chapter 7 - [Report on road construction in remote Nepal](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Areport-road-construction-remote-nepal) — Communication English, chapter 9 - [Remote control or digital multimeter](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aremote-control-or-dmm) — Basic Electronics Engineering, chapter 7 - [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×