--- title: "High starting current; 3-point starter" subject: "Electrical Machines" chapter: 4 key: "semester_4/electrical_machines:dc-motor-starter-high-starting-current" source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-starter-high-starting-current site: BCT Engineering --- # High starting current; 3-point starter Electrical Machines · Chapter 4: DC Motor - Asked in: 2075 Baisakh (old course), 2+6 marks; 2073 Bhadra (old course), 8 marks; 2068 Magh, 3+5 marks; 2066 Magh (old course), 8 marks (asked 4 times) - Video: [3 point starter | 3 point starter dc motor | three point starter of dc motor | starter | dc machine](https://www.youtube.com/watch?v=jgJ0pwGkMsg) — 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/dc-motor-starter-high-starting-current - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4 ## Question Why does a dc motor draw a large current at starting? Explain the necessity of a starter and describe the working of a 3-point dc motor starter with neat diagram. ## Short answer At start, back emf is zero, so I_a = V/R_a, drawing 15–20 times full-load current, causing damage, voltage dip, and excessive torque. A starter inserts external resistance in series with the armature to limit current, cutting it out as speed rises. A 3-point starter provides no-volt and overload release protection. ## Answer #### Why the starting current is large **Reason:** the armature current of a DC motor is $$ I_a = \frac{V - E_b}{R_a} $$ At the instant of starting the armature is at rest, so the back emf $E_b = \dfrac{\Phi ZNP}{60A} = 0$. The current is then limited only by the very small armature resistance: $$ I_{a(start)} = \frac{V}{R_a} $$ For example, a 220 V motor with $R_a = 0.5\ \Omega$ would draw $220/0.5 = 440$ A, about 15–20 times its full-load current (say 25 A). #### Starter **Necessity of a starter:** this large current would - burn the armature winding and damage the commutator and brushes (heavy sparking), - cause a large voltage dip on the supply and affect other consumers, - produce a very high starting torque that can shock and damage the shaft, gears and load. A starter inserts an external resistance in series with the armature at starting and removes it step by step as the motor gains speed and $E_b$ builds up. It limits the starting current to about 1.25–2 times full-load current: $$ R_{start} = \frac{V}{I_{start}} - R_a $$ It also gives protection (no-volt and overload release). #### 3-point starter **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. Why is the starting current of a dc motor large? A. Back emf is zero at standstill, so current is V/Ra B. Field flux is maximum C. Supply voltage is higher at start D. Armature resistance is very large Answer: A. Ia = (V - Eb)/Ra and Eb = 0 at start. (Key point: Large starting current) 2. What does a starter do? A. Increases the supply voltage B. Adds series resistance at start and cuts it out step by step as Eb builds up C. Increases field resistance D. Short-circuits the field Answer: B. This limits current to about 1.25-2 times full load. (Key point: Necessity of starter) 3. The no-volt release coil in a 3-point starter: A. Limits the starting current B. Protects against overload only C. Holds the handle at ON and releases it to OFF on supply failure D. Reduces the field current Answer: C. On supply failure NVC loses magnetism and the spring returns the handle. (Key point: NVC function) 4. What is the drawback of the 3-point starter? A. It has no overload protection B. It cannot start any shunt motor C. It needs four resistances D. NVC is in series with the field; weak field current may fail to hold the handle Answer: D. The 4-point starter puts the NVC across the supply. (Key point: Drawback of 3-point) ## Related in Electrical Machines - [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× - [Need of DC motor starter and three-point starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-starter-necessity-three-point) — Electrical Machines I, chapter 4, asked 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 - [Starting current; speed and % drop of shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-high-starting-current-speed-drop) — Electrical Machines, chapter 4 - [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 - [Starting current and starting resistance of shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-starting-current-80pc) — Electrical Machines I, chapter 4 ## Related in other subjects - [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 - [Three terminal DC motor starter](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Athree-terminal-dc-starter) — Electrical Machine, chapter 4 - [DC shunt motor speed at reduced voltage and starting resistance](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reduced-voltage-starting-resistance) — Utilization of Electrical Energy, chapter 3 - [Armature-controlled DC motor transfer function and block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-armature-dc-motor) — Control Systems, chapter 2