--- title: "Large starting current; 3-point starter" subject: "Electrical Machine" chapter: 4 key: "semester_4/electrical_machine:dc-motor-large-starting-current-3point" source: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-motor-large-starting-current-3point site: BCT Engineering --- # Large starting current; 3-point starter Electrical Machine · Chapter 4: DC Motor - Asked in: 2068 Magh, 3+5 marks - 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_machine/dc-motor-large-starting-current-3point - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4 ## Question Why does the dc motor draw large current at starting? Justify it clearly and also describe the working of 3-point dc motor starter. ## Short answer At starting, back emf is zero so armature current is V/R_a, limited only by small armature resistance, causing dangerously high current. The 3-point starter inserts external resistance in series with the armature and gradually cuts it out as speed rises; it also provides no-volt and overload protection via NVC and OLR. ## Answer #### Why the starting current is large The armature current is I_a = (V − E_b) / R_a. At the instant of starting the armature is at rest, so the back emf E_b = KΦN = 0. Hence I_a(start) = V / R_a The armature resistance R_a is very small (about 0.1 to 1 Ω). For example, with V = 220 V and R_a = 0.5 Ω, I_a = 440 A, while the rated current may be only 20 A, i.e. more than 20 times. As the motor speeds up, E_b grows and the current falls to the normal value. This large current would damage the commutator and brushes by sparking, overheat the winding, produce a heavy torque shock on the shaft and cause a voltage dip in the supply. So a starter is used to limit the current to about 1.5 to 2 times full-load current. #### Working of the 3-point starter It has three terminals L (line), A (armature) and F (field). ```text L o--[OLR]--+--handle--> o--o--o--o (studs with R1..R4) | | +--[NVC]--- F (field) A (armature) ``` 1. **Start**: the handle is moved from OFF to the first stud. The supply is connected to the armature through the full starting resistance, and to the shunt field through the no-volt coil. The current is limited. 2. **Acceleration**: as the speed rises, the back emf increases. The handle is moved to the next studs slowly, cutting the resistance out step by step. 3. **Run**: at the last stud the whole resistance is out and the soft-iron piece on the handle is held by the electromagnet of the no-volt coil (NVC). The motor runs with full voltage. 4. **No-volt protection**: if the supply is interrupted, the NVC loses its magnetism and the spring returns the handle to OFF, which avoids a dangerous restart. 5. **Overload protection**: the overload release (OLR) electromagnet short-circuits the NVC on excess current, releasing the handle. The shunt field is always in circuit from the first stud, so that the full field is available at starting and high starting torque is obtained. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a DC motor draw a large current at the instant of starting? A. Back emf E_b = 0 at rest, so I_a = V/R_a with a very small R_a B. The field winding short-circuits C. The flux is maximum at start D. The load torque is infinite Answer: A. E_b = KΦN is zero at standstill, so only the small armature resistance limits the current. (Key point: Cause of large starting current) 2. With V = 220 V and R_a = 0.5 Ω, what is the starting armature current? A. 44 A B. 440 A C. 110 A D. 220 A Answer: B. I_a = V/R_a = 220/0.5 = 440 A, far above a rated 20 A. (Key point: Numerical example) 3. Which is a harmful effect of the large starting current? A. Increase in back emf B. Reduced torque on the shaft C. Sparking at commutator and brushes, overheating and a supply voltage dip D. Lower field flux permanently Answer: C. The heavy current damages commutator and brushes, overheats the winding, gives a torque shock and dips the supply. (Key point: Effects of high current) 4. As the motor speeds up, what is done with the starter handle? A. It is moved back to OFF B. It is kept on the first stud C. It is moved straight to the last stud at once D. It is moved slowly to the next studs, cutting out resistance step by step as back emf rises Answer: D. Rising back emf lets the resistance be removed in steps; at the last stud the no-volt coil holds the handle. 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