--- title: "Working of 3-point DC motor starter" subject: "Electrical Machines" chapter: 4 key: "semester_4/electrical_machines:three-point-starter-working" source: https://www.bctengineer.com/q/semester_4/electrical_machines/three-point-starter-working site: BCT Engineering --- # Working of 3-point DC motor starter Electrical Machines · Chapter 4: DC Motor - Asked in: 2074 Bhadra (old course); 2070 Bhadra, 5 marks (asked 2 times) - Video: [Three Point Starter: Explanation & Working Principle](https://www.youtube.com/watch?v=MPmgx_0V_04) — Electrical4U - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/three-point-starter-working - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4 ## Question Explain the operation of 3-point dc motor starter with neat diagram. ## Short answer A 3-point starter connects a resistance in series with the armature at start and cuts it out in steps as speed rises. It uses a no-volt release coil to hold the handle at ON and return it to OFF on supply failure, and an overload release coil that short-circuits the no-volt coil on excessive armature current, releasing the handle. ## Answer A **3-point starter** is a device that limits the starting current of a DC shunt (or compound) motor by connecting a resistance in series with the armature at starting, and cutting it out in steps as the motor speeds up. It also gives no-volt and overload protection. Reason it is needed: at standstill $E_b = 0$, so $I_a = V/R_a$ is very large. **Parts of a 3-point starter:** three terminals **L** (line), **A** (armature) and **F** (field); a starting resistance divided into sections with studs; a spring-loaded handle (starting arm) with a brass arc; a **no-volt release coil (NVC)**, an **overload release coil (OLR)** and a soft-iron piece on the handle. ```text L (+) | OLR (overload coil) +------[~~~]---+ | handle (spring loaded) OFF 1 2 3 4 ON o----- soft iron | | | | | piece, held by NVC +[R]+[R]+[R]+---+---------> A (armature) | +--- brass arc ---[NVC]----[ Field ]--> F L (-) ---------------------------- supply - ``` **Working:** 1. With the handle at OFF, the circuit is open. The handle is moved slowly to stud 1. The full starting resistance is now in series with the armature, and the shunt field gets full supply voltage through the brass arc and NVC. This gives full flux, so the motor starts with high torque and low current. 2. As the motor speeds up, $E_b$ rises. The handle is moved stud by stud, cutting out resistance sections so the armature current stays within limits. 3. At the last stud (ON), the whole resistance is cut out and the motor is directly across the supply. The soft-iron piece on the handle touches the **NVC electromagnet**, which holds the handle there against the spring. 4. **No-volt protection:** if the supply fails, the NVC loses its magnetism, the spring returns the handle to OFF, and the motor does not restart with the resistance out when the supply returns. 5. **Overload protection:** if the armature current exceeds a set value, the OLR electromagnet lifts its armature and short-circuits the NVC. The NVC releases the handle, and the spring returns it to OFF. **Drawback:** the NVC is in series with the field. If the field current is reduced for speed control, the NVC may be too weak to hold the handle. A **4-point starter** places the NVC in a separate branch across the supply to avoid this. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The three terminals of a 3-point starter are: A. L, A and N B. L, A and F C. A, F and B D. L, M and F Answer: B. Line, armature and field. (Key point: Terminals) 2. At starting, the handle is moved to the first stud: A. No resistance is in the circuit B. The field is open C. Full resistance is in series with the armature and the field gets full voltage D. The armature gets full voltage Answer: C. This gives full flux, high torque and limited current. (Key point: Starting) 3. On supply failure the handle returns to OFF because: A. The OLR trips B. The resistance melts C. The brass arc opens D. The NVC loses magnetism and the spring pulls it back Answer: D. This prevents restarting with the resistance out. (Key point: No-volt protection) 4. The overload release coil: A. Short-circuits the NVC so that the handle is released B. Adds resistance C. Opens the field D. Increases the supply Answer: A. The NVC then releases the handle. 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