--- title: "Synchronous motor not self-starting; starting methods" subject: "Electrical Machines" chapter: 6 key: "semester_4/electrical_machines:sync-motor-not-self-starting-methods" source: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-motor-not-self-starting-methods site: BCT Engineering --- # Synchronous motor not self-starting; starting methods Electrical Machines · Chapter 6: Three Phase Synchronous Machines - Asked in: 2078 Baisakh, 6 marks; 2070 Magh, 8 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 Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-motor-not-self-starting-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6 ## Question Why synchronous motor is not self starting? Explain the methods used to start this motor (including starting using damper winding). ## Short answer The stat or field rot ates at synchronous speed while rot or poles are station ary, causing tor que reversal each half-cycle and zero average tor que. Start ing methods: damper winding (st arts as in duction mot or then syn chron izes), pony mot or, variable-fre quency supply, or slip-ring in duction start with later DC excitation. ## Answer #### Why a synchronous motor is not self-starting When a three-phase supply is applied to the stator, a field rotating at synchronous speed $N_s = \frac{120f}{P}$ is produced. The rotor poles, excited by DC, are stationary. A rotor pole is attracted by a stator pole, but in the next half cycle (1/100 s at 50 Hz) the stator pole has moved on, an opposite pole has taken its place, and the force reverses. Because of its inertia, the rotor cannot accelerate to $N_s$ in that short time. The torque reverses every half cycle, so the average starting torque is **zero**. The rotor must first be brought close to synchronous speed by other means. #### Starting methods 1. **Using a pony (auxiliary) motor:** a small induction or DC motor drives the rotor to near synchronous speed. The field is then excited, the machine is synchronised with the supply like an alternator, and the pony motor is disconnected. It is used for large motors that can be started without load. 2. **Using damper windings (induction motor method):** squirrel-cage type bars are placed in the pole faces and short-circuited by end rings. - The field winding is short-circuited through a resistance (to avoid high induced voltages), and the stator is supplied at reduced voltage (by an autotransformer or star-delta starter). - The damper winding develops torque like a squirrel-cage rotor, and the motor accelerates to about 95 % of $N_s$. - DC excitation is then applied. The rotor pulls into step (synchronises), and the damper winding then carries no current. 3. **Using a variable-frequency supply:** the frequency is raised gradually from a very low value, so the rotor stays locked to the slowly rotating field as it speeds up. 4. **As a slip-ring induction motor (synchronous-induction motor):** the rotor carries a three-phase winding brought out to slip rings. It is started like a slip-ring induction motor with external resistance in the rotor circuit, which is cut out as the speed rises. Near full speed, DC excitation is applied through the slip rings and the rotor pulls into synchronism. This gives a high starting torque and is used where the motor must start against load. ```text Supply | [Autotransformer] -> stator rotor: damper bars shorted field winding --[R]-- (shorted) after ~95% Ns: remove R, apply DC field ``` Except in method 4, the motor is started on no load or light load because the pull-in torque is limited. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a synchronous motor have no self-starting torque? A. The stator has no field B. Rotor poles are stationary and the stator pole passes so fast that the torque reverses each half cycle, averaging zero C. The rotor is too light D. DC excitation is absent Answer: B. Because of rotor inertia, the rotor cannot follow the rotating field, so the alternating attraction and repulsion averages to zero. (Key point: Why no starting torque) 2. In damper-winding starting, what is done with the field winding? A. Left open B. Closed through a resistance to avoid high induced voltage C. Connected to full DC at once D. Connected to the stator Answer: B. The many-turn field would develop dangerous voltage if open, so it is shorted through a resistor. (Key point: Damper winding method) 3. When is DC excitation applied in the damper method? A. Before the stator supply B. After the motor reaches about 95 % of Ns C. At 10 % of Ns D. Only at standstill Answer: B. The motor accelerates as an induction motor to about 95 % of Ns and then the rotor pulls into step. (Key point: When DC is applied) 4. What is the role of a pony motor? A. To provide field excitation B. To bring the rotor near synchronous speed before synchronising C. To reduce stator voltage D. To damp hunting Answer: B. The pony motor runs the rotor to near Ns; the machine is then synchronised like an alternator and the pony motor is disconnected. 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