--- title: "Three terminal DC motor starter" subject: "Electrical Machine" chapter: 4 key: "semester_4/electrical_machine:three-terminal-dc-starter" source: https://www.bctengineer.com/q/semester_4/electrical_machine/three-terminal-dc-starter site: BCT Engineering --- # Three terminal DC motor starter Electrical Machine · Chapter 4: DC Motor - Asked in: 2070 Bhadra, 5 marks - Video: [Three Point Starter Explained | DC Motor Starter (Step-by-Step)](https://www.youtube.com/watch?v=z8ZTC7tF8g8) — Faraday Jr - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/three-terminal-dc-starter - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4 ## Question With the help of a neat sketch, explain the working principle of three terminal DC motor starter. ## Short answer A 3-point starter inserts resistance in series with the armature at start to limit current, then cuts it out gradually as back emf builds. The no-volt coil holds the handle ON under normal voltage; overload release shorts the coil on excess current, allowing a spring to return handle to OFF, providing protection. ## Answer A DC shunt motor has a very small armature resistance, so at standstill (E_b = 0) the current V/R_a would be many times the rated current. A starter adds resistance in series with the armature during starting and removes it step by step as the motor gains speed. The 3-point starter has three terminals: **L (line), A (armature), F (field)**. #### Construction ```text Supply (+) | L o---[OLR coil]---o pivot \ handle (soft-iron piece; spring pulls it back to OFF) \ OFF (1) (2) (3) (4) (5) <- studs | | | | | +-R1-+-R2-+-R3-+-R4-+----------o A ---(Armature)----+ | | +--brass arc--[NVC]------------o F ---(Shunt field)-+ | Supply (-) -----------------------------------------------------+ ``` Main parts: 1. **Starting resistance** in sections between the studs. 2. **Handle** (starter arm) moving over the studs; a soft-iron piece is attached to it. 3. **No-volt coil (hold-on coil)** in series with the shunt field. Acting as an electromagnet, it holds the handle in the ON position. 4. **Overload release (OLR)** electromagnet with a trip lever, which short-circuits the no-volt coil when the current is excessive. 5. **Spring**, which returns the handle to the OFF position. #### Working 1. With the supply switched on, the handle is moved slowly to stud 1. The full resistance is in the armature circuit, while the shunt field gets full voltage through the no-volt coil. A small starting current flows. 2. As the motor accelerates, back emf builds up, and the handle is moved to the next studs, cutting out resistance step by step. 3. At the last stud, the handle is held by the electromagnet of the no-volt coil, and all the resistance is cut out. The motor runs on full voltage. 4. **No-volt protection**: if the supply fails, the no-volt coil is de-energised and the spring pulls the handle back to OFF, so the motor does not restart with all the resistance cut out. 5. **Overload protection**: on overload, the OLR magnet lifts its lever and shorts the no-volt coil, so the handle is released and returns to OFF. **Disadvantage:** the no-volt coil is in series with the field, so when the field current is reduced for speed control it may release the handle. The 4-point starter removes this problem. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is extra resistance put in series with the armature of a DC shunt motor at starting? A. Because the back emf is zero at standstill and the small R_a would allow a very large current B. To increase the speed quickly C. Because the supply voltage is too low at starting D. Because the field flux is zero at standstill Answer: A. At standstill E_b = 0, so the current V/R_a would be many times rated. The starter resistance limits it. (Key point: Why a starter is needed) 2. What does the no-volt (hold-on) coil do in a 3-point starter? A. It increases the armature resistance B. It holds the handle at the ON position by an electromagnet and releases it on supply failure C. It short-circuits the field winding D. It limits the starting current Answer: B. The coil, in series with the shunt field, magnetically holds the soft-iron piece on the handle; if the supply fails the spring returns the handle to OFF. (Key point: No-volt coil role) 3. How does the over-load release (OLR) protect the motor? A. It reverses the motor B. It adds more resistance to the armature C. It short-circuits the no-volt coil so the handle is released and returns to OFF D. It disconnects the field winding only Answer: C. On excess current the OLR magnet lifts its lever and shorts the no-volt coil, releasing the handle. (Key point: Overload protection) 4. What is the main disadvantage of the 3-point starter? A. It cannot limit starting current B. It needs four supply wires C. It gives no overload protection D. Reducing the field current for speed control may weaken the no-volt coil and release the handle Answer: D. The no-volt coil is in series with the field, so a weak field current may release the handle; the 4-point starter removes this problem. 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