--- title: "Contactor for motor control and protection" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:contactor-motor-control-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-motor-control-protection site: BCT Engineering --- # Contactor for motor control and protection Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2070 Bhadra, 6 marks; 2073 Magh, 6 marks; 2077 Chaitra, 5 marks (asked 3 times) - 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-motor-control-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question How contactors are used to control and protect the motors? Explain the working principle of the scheme with necessary circuits. ## Short answer In a DOL starter, pressing start energises contactor coil, closing main contacts; an auxiliary NO contact seals it. Stop button or thermal overload relay (set 100–110% FLC) opens the coil circuit to stop motor. HRC fuses handle short circuits; undervoltage drops out the contactor, preventing automatic restart. ## Answer A **contactor** is an electrically operated switch whose contacts are closed by an electromagnet (coil) and held closed as long as the coil is energised. It is designed for frequent making and breaking of normal load currents, so it is the standard device for **starting, stopping and protecting motors** when combined with an overload relay and fuses (a DOL starter). #### Scheme: DOL starter using a contactor ```text Power circuit Control circuit L1 L2 L3 L ---[F]---+ | | | | [F][F][F] fuses (SC) [STOP NC] | | | | [K][K][K] main contacts +--[START NO]--+ | | | | | [OL OL OL] thermal relay +----[K aux]---+ | | | | ( M ) 3-ph motor [OL NC contact] | (K coil)---N ``` #### Working 1. **Starting:** pressing the START button energises the contactor coil K. The armature is pulled in and the three main contacts close, so the motor gets full supply. 2. **Holding (latching):** an auxiliary NO contact of K, connected across START, closes at the same time. When the START button is released, the coil stays energised through this contact. 3. **Stopping:** pressing STOP (NC) breaks the coil circuit; the coil de-energises, the spring opens the main contacts and the motor stops. 4. **Overload protection:** the thermal overload relay (bimetal strips in each line) heats up if the motor current stays above its setting. Its NC contact in the coil circuit opens, the contactor drops out and the motor is disconnected. It is set at about 100–110% of motor full-load current with a time delay so that starting current does not trip it. 5. **Short-circuit protection:** a contactor cannot break large fault currents, so back-up **HRC fuses** or an MCCB ahead of it clear short circuits. 6. **Under-voltage (no-volt) protection:** if the supply fails or falls to about 70–80% of rated value, the coil cannot hold the armature and the contactor opens. When the supply returns, the motor does **not** restart by itself because the hold-in contact is open; the operator must press START again. This prevents accidents and a large simultaneous starting load. 7. **Single phasing:** a differential type overload relay or phase-failure relay opens the coil circuit if one phase is lost. #### Other control functions - Remote control: extra START/STOP buttons can be wired in parallel (START) and in series (STOP) from any location. - Interlocking: in a reversing starter, two contactors are electrically interlocked by NC auxiliary contacts so that both cannot close together. - Star-delta and auto-transformer starters use two or three contactors with a timer. Thus the contactor gives frequent switching, remote and automatic control, and with its overload relay and fuses, complete protection of the motor. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. After releasing the START button, why does the contactor stay on? A. The START button is mechanically locked B. An auxiliary NO contact of K across START holds the coil energised C. The overload relay holds it D. The main contacts supply the coil Answer: B. The hold-in auxiliary contact bypasses the released START button. (Key point: Latching) 2. How does the thermal overload relay protect the motor in the scheme? A. Its NC contact in the coil circuit opens so the contactor drops out B. It opens the main fuses C. It increases coil voltage D. It short-circuits the coil Answer: A. Bimetal heating opens the NC contact in series with the coil, de-energising the contactor. (Key point: Overload protection) 3. Why are back-up HRC fuses used with the contactor? A. The contactor cannot break large fault currents B. The coil needs fuses to pick up C. Fuses start the motor D. Fuses give no-volt protection Answer: A. Contactors are limited to overload-level breaking, so fuses or MCCB clear short circuits. (Key point: Short-circuit protection) 4. After supply failure and return, a contactor-controlled motor: A. Restarts automatically B. Does not restart until START is pressed again C. Trips the fuses D. Runs in reverse Answer: B. The hold-in contact has opened, so the operator must press START, preventing sudden restart. (Key point: No-volt protection) ## Related in Switchgear and Protection - [Fuse and contactor ratings for induction motor](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amotor-fuse-contactor-selection) — Switchgear and Protection, chapter 3 - [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 - [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 - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 - [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3× - [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1 ## Related in other subjects - [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4× - [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-working-torque-equation) — Electrical Machines, chapter 4, asked 2× - [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3 - [Boost regulator working principle](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aboost-regulator-principle) — Advanced Electronics, chapter 8, asked 2×