--- title: "Need of DC motor starter and three-point starter" subject: "Electrical Machines I" chapter: 4 key: "semester_4/electrical_machines_i:dc-starter-necessity-three-point" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/dc-starter-necessity-three-point site: BCT Engineering --- # Need of DC motor starter and three-point starter Electrical Machines I · Chapter 4: DC Motor - Asked in: 2080 Chaitra, 2+6 marks; 2075 Baisakh, 2+6 marks; 2074 Bhadra, 8 marks; 2073 Bhadra, 8 marks (asked 4 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_i/dc-starter-necessity-three-point - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4 ## Question What is the necessity of a starter in a d.c. motor? Describe the working of 3-point starter. ## Short answer Without a starter, starting current V/Ra would be excessive. The 3-point starter inserts resistance in series with the armature, gradually cut out as back emf builds; a no-volt coil holds the handle at ON, and an overload release trips it on overcurrent. ## Answer A **starter** is a device that puts a resistance in series with the armature of a DC motor while starting, and cuts it out step by step as the motor speeds up. It also provides protection against no-voltage and overload. #### Necessity of a starter In a DC motor, $I_a = \dfrac{V - E_b}{R_a}$. At the instant of starting, speed is zero, so back emf $E_b = 0$: $$ I_{a,start} = \frac{V}{R_a} $$ $R_a$ is very small, so this current is huge. For example, a 220 V motor with $R_a = 0.5\ \Omega$ would draw $220/0.5 = 440$ A, perhaps 15-20 times full-load current. This would: - burn the armature winding and damage insulation, - cause heavy sparking at the commutator and brushes, - give a sudden large torque that can damage the shaft and load, - cause a big voltage dip on the supply. So an external resistance $R$ is added at start: $I_{start} = V/(R_a + R)$, and it is cut out as $E_b$ builds up. Small motors (below about 1 hp) can be started directly because their $R_a$ is higher and inertia low. #### Three-point starter It has three terminals: **L** (line), **F** (field) and **A** (armature). It is used for shunt and compound motors. ```text L (+) o----[OLR coil]----o Handle pivot \ studs: 1 2 3 4 5 ... ON o--R--o--R--o--R--o--R--o | (starting resistance) | | to A (armature) [NVC]--- to F (field) (holding magnet) Supply -ve ------ motor armature and field return ``` **Parts** - Starting resistance divided into sections between studs. - Handle with a soft-iron piece, held in OFF position by a spring. - **No-volt coil (NVC)** / hold-on coil: in series with the shunt field. - **Overload release (OLR)** coil: in series with the line, with an armature that shorts the NVC when it is lifted. **Working** 1. The handle is moved from OFF to stud 1. The field gets full supply voltage through the NVC, so full flux is produced. All starting resistance is in series with the armature, so starting current is limited. 2. As the motor speeds up, $E_b$ grows and current falls; the handle is moved stud by stud, cutting out resistance. 3. At the last (ON) stud all resistance is cut out of the armature circuit. The NVC, energised by field current, holds the handle against the spring. **Protections** - **No-voltage protection:** if supply fails or the field circuit opens, the NVC loses its magnetism and the spring returns the handle to OFF. So the motor cannot restart at full voltage without the starter when supply returns. It also prevents dangerous over-speeding if the field opens. - **Overload protection:** if current exceeds a set value, the OLR lifts its armature and short-circuits the NVC. The handle is released to OFF and the motor is disconnected. **Drawback:** for speed control by field resistance, a large field resistance weakens the NVC current, and the handle may release unwantedly. This is overcome by the **four-point starter**, where the NVC is connected directly across the supply. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the starting current of a DC motor huge without a starter? A. Speed is zero so back emf is zero and only the small Ra limits the current B. The field flux is at its maximum so the armature draws more current C. The supply voltage is higher at the instant of switching D. The brushes short-circuit the armature winding Answer: A. At start Eb = 0, so Ia = V/Ra, and Ra is very small. (Key point: Zero back emf at start) 2. What does a starter do during starting? A. Increases the field current to raise the back emf instantly B. Inserts resistance in series with the armature and cuts it out step by step as speed rises C. Reduces the supply frequency until the motor speeds up D. Connects extra resistance in series with the field permanently Answer: B. The series armature resistance limits the current, and is removed as Eb builds up. (Key point: Role of starter) 3. In a three-point starter, what happens when the supply fails? A. The handle stays at ON so the motor restarts automatically B. The overload coil holds the handle at the last stud C. The no-volt coil loses magnetism and the spring returns the handle to OFF D. The starting resistance is cut in permanently Answer: C. The NVC holds the handle only while energised; on loss of supply the spring releases it, preventing restart at full voltage. (Key point: No-volt coil action) 4. What is the drawback of the three-point starter that the four-point starter overcomes? A. It cannot limit the starting current B. It cannot be used with shunt motors C. It gives no overload protection at all D. With a large field regulator resistance the NVC current becomes weak and may release the handle Answer: D. The NVC is in series with the field in a three-point starter, so weak field current weakens it; the four-point starter places it across the supply. (Key point: Three-point drawback) ## Related in Electrical Machines I - [Need of starter and four-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-four-point-starter) — Electrical Machines I, chapter 4 - [Need of starter and induction motor starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-starter-methods) — Electrical Machines I, chapter 5 - [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× - [Back emf and its role in DC motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aback-emf-dc-motor-role) — Electrical Machines I, chapter 4, asked 6× - [DC shunt motor with open field circuit](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-field-open) — Electrical Machines I, chapter 4 - [DC motor principle and role of commutator and brushes](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-commutator-brushes) — Electrical Machines I, chapter 4 ## Related in other subjects - [Working of 3-point DC motor starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Athree-point-starter-working) — Electrical Machines, chapter 4, asked 2× - [Three terminal DC motor starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Athree-terminal-dc-starter) — Electrical Machine, chapter 4 - [High starting current; 3-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-starter-high-starting-current) — Electrical Machines, chapter 4, asked 4× - [Large starting current; 3-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-motor-large-starting-current-3point) — Electrical Machine, chapter 4