--- title: "Starting methods of synchronous motor" subject: "Electrical Machines" chapter: 6 key: "semester_4/electrical_machines:sync-motor-starting-methods" source: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-motor-starting-methods site: BCT Engineering --- # Starting methods of synchronous motor Electrical Machines · Chapter 6: Three Phase Synchronous Machines - Asked in: 2065 Chaitra (old course), 8 marks; 2068 Magh, 4 marks (asked 2 times) - Video: [SM40 Starting Methods of Synchronous Motor](https://www.youtube.com/watch?v=iOvlQ5BhZcI) — 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-starting-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6 ## Question Explain the starting methods of synchronous motor. ## Short answer A synchronous mot or lack s starting tor que because the rot at ing field reverse es tor que each half-cycle. Common start ing: damper (amort isseur) winding start s it as an in duction mot or, then DC excitation pull s it into syn chron ism. Other methods include pony mot or, variable-fre quency drives, and slip-ring in duction mot or start ing. ## Answer A synchronous motor has **no starting torque** because the stator field rotates at $N_s = \frac{120f}{P}$ while the rotor is stationary. The rotor poles are attracted and then repelled in each half cycle, so the average torque is zero. The motor must be brought near synchronous speed before DC field excitation is applied. The common methods are given below. #### 1. Using a pony (auxiliary) motor A small induction or DC motor, mechanically coupled to the shaft, runs the rotor close to $N_s$ without load. The rotor is then excited and synchronised with the supply in the same manner as an alternator (lamp or synchroscope method), the pony motor is disconnected, and the load is applied. It needs an extra motor and is used for large machines. #### 2. Using damper (amortisseur) winding Short-circuited copper bars are placed in slots in the pole faces and joined by end rings, similar to a squirrel-cage rotor. - Reduced voltage is applied to the stator. The field winding is closed through a resistance (about 10 times the field resistance) to avoid dangerous high voltage in the many-turn field. - The motor starts as an induction motor and rises to about 95 % of $N_s$. - DC field is applied, and the rotor locks into synchronism. - Once running at $N_s$ there is no relative motion, so the damper winding carries no current. It also reduces hunting. ```text stator ---(autotransformer / star-delta)--- supply rotor: pole-face damper bars + end rings field winding --[discharge resistor]-- ``` #### 3. Using variable frequency supply An inverter supplies the stator at very low frequency at first, so the rotating field is slow and the rotor locks to it. The frequency is gradually raised to the rated value. This is used in modern drives. #### 4. Using the DC motor method The exciter or a DC machine on the same shaft runs the machine as a DC motor to bring it to speed. Of these, the damper-winding method is the most common. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the average starting torque zero? A. Rotor poles are attracted and repelled each half cycle as the stator field rotates past a stationary rotor B. The field winding is open C. Supply frequency is zero D. Rotor resistance is high Answer: A. The torque reverses every half cycle, so its average over time is zero. (Key point: Zero average torque) 2. About how large is the resistor in the field circuit during damper starting? A. Zero B. About 10 times the field resistance C. Equal to armature resistance D. Infinite Answer: B. A resistance of about 10 times the field resistance avoids dangerous voltage in the field. (Key point: Damper resistor) 3. Once running at Ns, why does the damper winding carry no current? A. It is disconnected B. There is no relative motion between rotor and field C. Voltage is zero D. The field is shorted Answer: B. With no relative motion no emf is induced in the damper bars; they also help reduce hunting. (Key point: Damper winding after starting) 4. How does a variable-frequency supply start a synchronous motor? A. It raises voltage suddenly B. It starts at very low frequency so the rotor locks to the slow field, then raises frequency C. It reverses phase sequence D. It uses a pony motor Answer: B. At low frequency the field rotates slowly so the rotor can lock on; frequency is then raised gradually. 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