--- title: "Need of starter and four-point starter" subject: "Electrical Machines I" chapter: 4 key: "semester_4/electrical_machines_i:dc-motor-four-point-starter" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-motor-four-point-starter site: BCT Engineering --- # Need of starter and four-point starter Electrical Machines I · Chapter 4: DC Motor - Asked in: 2082 Kartik (new course), 6 marks - Video: [4 point starter | 4 point starter dc motor | four point starter | four point starter of dc motor](https://www.youtube.com/watch?v=VyiLsyrzKB0) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-motor-four-point-starter - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4 ## Question Explain the need of starter in a DC motor and describe the working principle of 4 point starter with a neat sketch. ## Short answer A starter limits the huge armature current at standstill, when Eb=0 makes Ia=V/Ra. The four-point starter connects the no-volt coil directly across the supply through a protective resistance, independent of field current, so field control does not cause false tripping. It provides no-voltage and overload protection. ## Answer #### Need for a starter In a DC motor $I_a = \dfrac{V - E_b}{R_a}$. At start the speed is zero, so $E_b = 0$ and $I_a = V/R_a$. Since $R_a$ is very small (e.g. 0.5 Ω), a 220 V motor would draw about 440 A, many times rated current. This would burn the winding, cause heavy sparking at the commutator, give a damaging torque jerk and a dip in supply voltage. A **starter** puts a resistance in series with the armature at start and cuts it out step by step as $E_b$ builds up. It also gives no-voltage and overload protection. #### Four-point starter It has four terminals: **L** (line), **A** (armature), **F** (field) and **N** (no-volt coil return to supply). The key change from a three-point starter is that the **no-volt coil (NVC) is connected directly across the supply** through a protective resistance, not in series with the field. ```text L o--[OLR]--o pivot/handle \ studs 1 2 3 4 5 ... ON o-R-o-R-o-R-o-R-o | F o---- shunt field A o---- armature (brass arc from stud 1) [NVC]--[R protective]--o N ---- supply (-) ``` **Working** 1. The handle is moved from OFF to stud 1. Current divides into three parallel paths: (a) armature through the full starting resistance, (b) shunt field through the brass arc, getting full voltage, and (c) the NVC with its protective resistance, across the supply. 2. As speed rises, $E_b$ increases; the handle is moved stud by stud, cutting out the starting resistance. 3. At the ON position the armature is connected straight to the supply and the NVC holds the handle against its spring. **Protections** - **No-voltage release:** if supply fails, the NVC is de-energised and the spring returns the handle to OFF. - **Overload release (OLR):** on excess current the OLR attracts its armature and shorts the NVC, releasing the handle. #### Why four-point In a three-point starter the NVC carries field current. When a large field regulator resistance is used to raise speed, field current falls, NVC becomes weak and the handle may release while the motor is running. In a four-point starter the NVC current does not depend on field current, so **field control over a wide range is possible** without false tripping. Drawback: if the field circuit opens, the NVC still holds the handle, so there is no protection against over-speeding due to field failure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a starter needed for a DC motor? A. At start Eb = 0 so Ia = V/Ra is very large B. The field current is too small at start C. The armature winding has high resistance D. The brushes cannot carry rated current Answer: A. A series starting resistance limits this current until back emf builds up. (Key point: Why starter) 2. How is the no-volt coil connected in a four-point starter? A. In series with the shunt field B. Directly across the supply through a protective resistance C. In series with the armature D. In parallel with the starting resistance Answer: B. The NVC current is then independent of the field current. (Key point: NVC connection) 3. What is the main advantage of the four-point over the three-point starter? A. It gives a higher starting torque B. It needs no starting resistance C. Wide field speed control is possible without the handle releasing D. It protects against field failure over-speeding Answer: C. A weak field no longer weakens the NVC current. (Key point: Advantage) 4. What is a drawback of the four-point starter? A. It cannot give overload protection B. It trips whenever field resistance is raised C. It cannot be used for shunt motors D. If the field circuit opens, the NVC still holds the handle so no over-speed protection Answer: D. The NVC is not in the field circuit, so field failure does not release the handle. 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