--- title: "Why synchronous motor is not self starting; damper starting" subject: "Electric Machines II" chapter: 2 key: "semester_5/electric_machines_ii:motor-not-self-starting-damper" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-not-self-starting-damper site: BCT Engineering --- # Why synchronous motor is not self starting; damper starting Electric Machines II · Chapter 2: Three Phase Synchronous Motor - Asked in: 2071 Chaitra, 6 marks; 2071 Shrawan, 6 marks (asked 2 times) - Video: [SM39 Why Synchronous Motor is Not Self Starting ?](https://www.youtube.com/watch?v=k5CEMtQzAsY) — Lectures in Electrical Engineering - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-not-self-starting-damper - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2 ## Question Explain why the synchronous motor is not self starting and explain damper winding method of starting of synchronous motor. ## Short answer The synchronous motor has zero average starting torque because the rotating stator field reverses torque every half-cycle, and rotor inertia prevents acceleration. Damper winding, a squirrel cage on pole faces, enables induction-motor starting: the field is short-circuited, stator energised, then DC excitation applied near synchronous speed to lock rotor into synchronism. ## Answer #### Why a synchronous motor is not self-starting 1. When balanced 3-phase supply is given to the stator, a magnetic field rotating at synchronous speed $N_s = 120f/P$ is set up. 2. The rotor poles are excited by DC and are stationary at start. 3. At one instant, a stator N pole is in front of a rotor S pole: attraction tends to turn the rotor in the direction of the field (say clockwise). 4. Half a cycle later (1/100 s at 50 Hz), the stator poles have moved by one pole pitch; now a stator N is in front of the rotor N: repulsion tends to turn the rotor anticlockwise. 5. Because of its inertia, the rotor cannot move in such a short time; the torque reverses every half cycle, so the **average starting torque is zero**. The rotor only vibrates. ```text t = 0: stator N -> attracts rotor S -> torque CW t = T/2: stator S -> repels rotor S -> torque CCW average torque = 0 (rotor stays at rest) ``` Hence the rotor must first be brought close to synchronous speed by some other means. #### Damper winding method (starting as induction motor) **Construction:** copper or brass bars are placed in slots in the rotor pole faces and short-circuited at both ends by end rings, forming a **squirrel-cage** (damper / amortisseur) winding. **Procedure** 1. Do not connect the DC supply to the field winding. Short-circuit it through a discharge resistance (never leave it open, because the rotating field would induce a very high voltage in the many-turn field winding). 2. Switch on the 3-phase supply (often at reduced voltage through an auto-transformer or star–delta starter to limit current). 3. The rotating field induces currents in the damper bars, and the motor **starts as an induction motor**, accelerating to about 95–98% of synchronous speed. 4. Now apply DC excitation to the field. The rotor poles lock with the stator field and are **pulled into synchronism**. 5. At synchronous speed there is no relative motion, so no current flows in the damper bars; they carry current only during speed changes, when they also **damp hunting**. **Advantages:** simple, no separate starting motor, also prevents hunting. **Limitation:** low starting torque, so it suits motors that start at light load. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a synchronous motor produce no average starting torque? A. The stator field does not rotate at start B. Torque on the stationary rotor reverses every half cycle, so the average is zero C. The DC field is too weak D. The rotor has no poles Answer: B. The stator field rotates at Ns but the rotor is at rest; attraction and repulsion alternate each half cycle and inertia prevents the rotor following, so average torque is zero. (Key point: Why no starting torque) 2. During damper-winding starting, why is the DC field winding kept open or shorted through a discharge resistance? A. To increase the speed B. To avoid very high induced voltage in the many-turn field winding C. To save DC power D. To lock the rotor Answer: B. The rotating field would induce a dangerously high emf in the many-turn field winding if it were left open-circuited. (Key point: Field winding during start) 3. After the motor reaches about 95-98% of synchronous speed as an induction motor, what is done? A. The supply is removed B. DC excitation is applied and the rotor is pulled into synchronism C. The damper bars are removed D. The speed is raised by a pony motor Answer: B. With DC excitation the rotor poles lock with the rotating stator field and the motor runs at Ns. (Key point: Pull-in) 4. At synchronous speed, the damper bars: A. Carry maximum current B. Carry no current, but damp hunting during speed changes C. Produce the main running torque D. Must be disconnected Answer: B. No relative motion means no induced current; they act only when speed varies, damping hunting. 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