--- title: "Synchronous motor starting methods and damper winding" subject: "Electric Machines II" chapter: 2 key: "semester_5/electric_machines_ii:motor-starting-methods-damper" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-starting-methods-damper site: BCT Engineering --- # Synchronous motor starting methods and damper winding Electric Machines II · Chapter 2: Three Phase Synchronous Motor - Asked in: 2078 Kartik, 7 marks - Video: [starting methods of synchronous motor | starting of synchronous motor | method of starting | machine](https://www.youtube.com/watch?v=TRN9fFM2rCQ) — Mind of Engineer - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-starting-methods-damper - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2 ## Question Explain why synchronous motor does not have self-starting torque? Write starting methods and explain the damper winding starting method. ## Answer A synchronous motor has **no net starting torque** when its field is excited and the stator is switched on, so it must be brought near synchronous speed by some other means. #### Why it is not self-starting - The stator 3-phase supply produces a field rotating at $N_s = 120f/P$ (1500 rpm for 4-pole, 50 Hz) instantly. - The rotor poles are fixed by DC excitation, and the heavy rotor is at rest. - Suppose a stator N pole is just ahead of a rotor S pole: attraction pulls the rotor one way. Half a cycle (0.01 s) later a stator S pole sits there and pushes the rotor the opposite way. - Due to rotor inertia it cannot follow these rapid reversals, so the **average torque is zero**; the rotor only vibrates. ```text t = 0 : stator N over rotor S -> torque CW t = T/2 : stator S over rotor S -> torque CCW average over one cycle -> 0 ``` #### Starting methods 1. **Damper (amortisseur) winding / induction motor starting** - most common. 2. **Pony (auxiliary) motor** - a small induction motor or DC machine on the same shaft brings the rotor near $N_s$, then the field is excited and the machine is synchronised. 3. **Slip-ring induction motor method** - rotor has a 3-phase winding connected through slip rings and resistances; starts as a slip-ring induction motor, then DC is fed to the rotor. 4. **Variable frequency supply** (inverter) - frequency is raised slowly from zero so the rotor follows the field from rest. 5. **DC exciter on shaft** used as a DC motor to run the machine up. #### Damper winding starting method - Copper or brass bars are placed in slots in the pole faces and shorted at both ends by end rings, forming a partial **squirrel cage**. - **Step 1:** Field winding is not connected to DC; it is shorted through a discharge resistance (to avoid very high induced voltage in it). Reduced voltage (autotransformer or star-delta) is applied to the stator. - **Step 2:** The rotating field induces currents in the damper bars, producing induction-motor torque. The rotor accelerates to about 95-97 % of $N_s$. - **Step 3:** DC excitation is switched on. The rotor poles lock with the stator poles (synchronising or "pull-in" torque) and the rotor runs at $N_s$. - **Step 4:** At synchronous speed there is no relative motion, so no current flows in the damper bars; they carry current only when speed changes, which also **damps hunting**. ```text 3-ph AC --> [Autotransformer] --> Stator | Rotor: pole faces with damper bars | Field --[discharge R]-- (start) | Field --[DC exciter ]-- (near Ns) | ``` The motor starts with low torque because the cage is small; hence this method suits pumps, fans and compressors started at light load. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the average torque on a stationary synchronous-motor rotor zero? A. The rotor is too light B. Torque direction reverses every half cycle (0.01 s at 50 Hz) and inertia prevents the rotor following C. The stator field is stationary D. The supply is single phase Answer: B. Attraction and repulsion alternate each half cycle, and the heavy rotor cannot respond to such fast reversals. (Key point: Average torque zero) 2. Which is NOT a listed starting method for a synchronous motor? A. Damper winding (induction start) B. Pony motor C. Variable frequency supply D. Adding a series resistor in the DC field to run at higher speed Answer: D. Listed methods are damper winding, pony motor, slip-ring induction method, variable frequency, and DC exciter as motor. (Key point: Starting methods) 3. How does variable-frequency starting work? A. Frequency is raised slowly from zero so the rotor follows the field from rest B. Frequency is held at 50 Hz C. The rotor is spun by hand D. The field is reversed repeatedly Answer: A. At very low frequency the field rotates slowly so the rotor can lock and follow it as frequency increases. (Key point: Variable frequency) 4. Why is damper-winding starting suited to pumps, fans and compressors started at light load? A. It gives very high starting torque B. The cage is small, so starting torque is low C. It needs no supply D. It works only at no voltage Answer: B. The partial cage on pole faces gives low starting torque, so only light-load starts are suitable. (Key point: Damper start limitation) ## Related in Electric Machines II - [Why synchronous motor is not self starting; damper starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-not-self-starting-damper) — Electric Machines II, chapter 2, asked 2× - [Damper winding to make motor self starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adamper-winding-self-starting) — Electric Machines II, chapter 2 - [Justify synchronous motor is not self starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-not-self-starting-justify) — Electric Machines II, chapter 2 - [Not self starting and permanent split capacitor motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Anot-self-starting-psc-motor) — Electric Machines II, chapter 3 - [Justify not self starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-starting-two-methods) — Electric Machines II, chapter 2 - [Why single-phase IM is not self starting; methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-not-self-starting-methods) — Electric Machines II, chapter 3, asked 2× ## Related in other subjects - [Synchronous motor not self-starting; starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-not-self-starting-methods) — Electrical Machines, chapter 6, asked 2× - [Sync motor not self-starting; starting method](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-motor-not-self-starting-method) — Electrical Machine, chapter 6, asked 2× - [Single phase motor not self-starting; DFRT](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-dfrt) — Electrical Machines, chapter 7, asked 3× - [Single phase motor not self-starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-starting-methods) — Electrical Machines, chapter 7