--- title: "Justify not self starting; two starting methods" subject: "Electric Machines II" chapter: 2 key: "semester_5/electric_machines_ii:motor-starting-two-methods" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-starting-two-methods site: BCT Engineering --- # Justify not self starting; two starting methods Electric Machines II · Chapter 2: Three Phase Synchronous Motor - Asked in: 2070 Chaitra, 2+4 marks - 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-starting-two-methods - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2 ## Question Justify, synchronous motor is not self starting. Explain any two starting methods of synchronous motor. ## Short answer Synchronous motor has zero average starting torque because torque reverses every half-cycle. Two starting methods: (1) Damper winding – starts as induction motor, then DC excitation pulls rotor into synchronism. (2) Pony motor – auxiliary motor brings it to near synchronous speed, then it is synchronised to supply. ## Answer A synchronous motor with excited field has **zero average starting torque**, so it is not self-starting. #### Justification - On switching on, the stator field rotates at $N_s = 120f/P$ immediately, while the excited rotor poles are stationary and the rotor has high inertia. - At one instant a stator N pole faces a rotor S pole and the rotor is pulled, say, clockwise. Half a cycle later (0.01 s at 50 Hz) a stator S pole faces the same rotor pole and pushes it anticlockwise. - The torque reverses every half cycle; the rotor cannot follow, so the **average torque is zero** and it only vibrates. ```text t = 0 : N over S -> attract -> CW t = T/2 : S over S -> repel -> CCW average torque = 0 ``` #### Method 1: Damper winding (induction motor starting) - Bars in pole-face slots, shorted by end rings, form a squirrel cage. - Field winding is shorted through a discharge resistor; reduced voltage is applied to the stator. - The motor starts as an induction motor and reaches about 95-98 % of $N_s$. - DC excitation is then switched on; rotor poles lock with stator poles and the motor runs at $N_s$. The damper then carries no current, except during hunting, which it damps. #### Method 2: Pony (auxiliary) motor ```text [Pony motor] ==shaft== [Sync motor] ---> load (small IM or DC motor) | synchroscope/lamps ``` - A small induction motor (with fewer poles) or DC motor coupled to the shaft runs the unloaded synchronous motor up to about synchronous speed. - The DC field is excited, and the stator is connected to the supply when the voltage, frequency and phase sequence match (checked with a synchroscope or lamps), as in synchronising an alternator. - The pony motor is then disconnected (or left running idle on the shaft). - Suitable only for starting without load. Other methods: slip-ring induction motor starting, variable frequency (inverter) starting, and using the DC exciter as a motor. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does the excited synchronous motor not self-start? A. The stator field is stationary B. Torque on the stationary rotor reverses every half cycle, giving zero average torque C. The rotor is demagnetised D. The torque is constant but small Answer: B. A stator N pole attracts the rotor S pole, then half a cycle later a stator S pole repels it; the rotor only vibrates. (Key point: Zero average torque) 2. In damper-winding starting, what is done before DC excitation is applied? A. The motor runs as an induction motor to about 95-98 percent of Ns, with the field shorted through a discharge resistor B. The rotor is held at standstill C. The stator is supplied with DC D. The field is excited at full strength from the start Answer: A. Reduced voltage starts it as an induction motor; then DC excitation locks the rotor to the stator field. (Key point: Damper winding start) 3. How is a pony motor used? A. It runs the unloaded synchronous motor to near synchronous speed; the stator is connected when voltage, frequency and phase sequence match B. It supplies the stator C. It provides DC to the field D. It is used with full load on the shaft Answer: A. The pony motor brings it near Ns, then synchronising is done as for an alternator, checked by synchroscope or lamps. (Key point: Pony motor method) 4. What is a limitation of the pony motor method? A. It cannot start at all B. It is suitable only for starting without load C. It needs a variable frequency supply D. It increases starting torque to many times rated Answer: B. A small auxiliary motor can only run the synchronous motor up to speed unloaded. 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