--- title: "Why single-phase IM is not self starting; methods" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:single-phase-not-self-starting-methods" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-phase-not-self-starting-methods site: BCT Engineering --- # Why single-phase IM is not self starting; methods Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2078 Bhadra, 2+1+3 marks; 2076 Asoj, 6 marks (asked 2 times) - Video: [Why 1ph Induction Motor is not Self Starting | why connect Capacitor in Motor| Hindi](https://www.youtube.com/watch?v=Xr0RTKxICoA) — Electrical With Abhishek Joshi - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-phase-not-self-starting-methods - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Why is single-phase induction motor not self-starting? What are the various starting methods of the single-phase induction motors? Explain any two methods in detail. ## Short answer A single-phase induction motor produces a pulsating field that gives zero net starting torque. Starting methods use phase splitting: resistance split-phase adds a high-resistance auxiliary winding displaced 90°, giving ~25–30° phase shift and moderate torque; capacitor-start places a capacitor in the auxiliary, achieving ~90° shift for high torque, both disconnected by a centrifugal switch. ## Answer A single-phase induction motor is **not self-starting** because its single winding produces a **pulsating** field, not a rotating one, so the net starting torque is zero. #### Why it is not self-starting By double field revolving theory, the pulsating flux $\phi_m\cos\omega t$ equals two fields of $\phi_m/2$ rotating in opposite directions at $N_s$. At standstill both have slip 1 and produce equal and opposite torques: $T_f = T_b$, so $T_{net} = 0$. If the rotor is pushed in either direction, the torque in that direction becomes larger and the motor keeps running. #### Starting methods To start, a rotating field must be produced, usually by **phase splitting**: 1. Resistance split-phase motor 2. Capacitor-start motor 3. Capacitor-start capacitor-run (two-value capacitor) motor 4. Permanent split capacitor (PSC) motor 5. Shaded-pole motor 6. Repulsion-start induction-run motor #### 1. Resistance split-phase method ```text AC o---+--------------+ | | Main wdg (low R, Aux wdg (high R, high X) low X) | | | centrifugal S o------+--------------+ ``` - An **auxiliary winding** with high resistance (thin wire) and low reactance is placed $90^\circ$ electrical from the main winding. - Main current $I_m$ lags $V$ by a large angle; auxiliary current $I_a$ lags by a smaller angle, giving a phase difference of about $25-30^\circ$. - These two currents produce a rotating field and starting torque $T_{st} \propto I_mI_a\sin\alpha$ (moderate, about 1.5-2 times full load). - A centrifugal switch disconnects the auxiliary winding at about 75 % of $N_s$. - Used for fans, blowers, washing machines. #### 2. Capacitor-start method ```text AC o---+--------------+ | | Main wdg Aux wdg | | | C (electrolytic) | | | centrifugal S o------+--------------+ ``` - A capacitor in series with the auxiliary winding makes $I_a$ **lead** $V$, so the angle between $I_m$ and $I_a$ is close to $90^\circ$. - Starting torque is high (3-4.5 times full-load torque) with lower starting current. - The auxiliary winding and capacitor are cut out by the centrifugal switch at 70-80 % of $N_s$. - Used for compressors, pumps, refrigerators and air-conditioners. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A single-phase induction motor is not self-starting because: A. Its winding produces a pulsating field, giving zero net starting torque B. The rotor has no cage C. The supply frequency is too low D. The stator has no winding Answer: A. The pulsating field equals two opposite rotating fields giving equal and opposite torques at standstill. (Key point: Why not self-starting) 2. How are single-phase motors usually made to start? A. By phase splitting to produce a rotating field B. By increasing supply voltage C. By removing the rotor D. By reducing the load to zero only Answer: A. An auxiliary winding with a phase difference to the main winding produces a rotating field and starting torque. (Key point: Principle of starting) 3. In the resistance split-phase motor the phase difference between Im and Ia comes from: A. A capacitor in the auxiliary winding B. Auxiliary winding of high R and low X, with main winding of low R and high X C. A shading ring D. A DC supply Answer: B. The high-R auxiliary current lags V less than the main current, giving about 25-30 degrees. (Key point: Resistance split-phase) 4. Compared with resistance split-phase, the capacitor-start method gives: A. Lower starting torque B. A phase angle near 90 degrees and higher starting torque (3-4.5 times full load) C. No need for a centrifugal switch D. Higher starting current for the same torque Answer: B. The capacitor makes Ia lead V so alpha is near 90 degrees, giving a high starting torque. (Key point: Capacitor-start advantage) ## Related in Electric Machines II - [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 - [Single phase IM principle, torque-slip and starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-principle-torque-slip) — Electric Machines II, chapter 3 - [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× - [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 - [Split-phase motor winding currents at starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asplit-phase-starting-currents) — Electric Machines II, chapter 3 ## Related in other subjects - [1-phase IM not self-starting; two methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asingle-phase-im-not-self-starting-methods) — Electrical Machine, chapter 7, asked 2× - [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 - [Reason: single phase motor not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-reason) — Electrical Machines, chapter 7 - [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×