--- title: "Single phase motor not self-starting; DFRT" subject: "Electrical Machines" chapter: 7 key: "semester_4/electrical_machines:spim-not-self-starting-dfrt" source: https://www.bctengineer.com/q/semester_4/electrical_machines/spim-not-self-starting-dfrt site: BCT Engineering --- # Single phase motor not self-starting; DFRT Electrical Machines · Chapter 7: Fractional Kilowatt Motors - Asked in: 2078 Poush, 2+6 marks - Video: [double revolving field theory of single phase induction motor | double revolving theory](https://www.youtube.com/watch?v=pXmPCSc9di0) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/spim-not-self-starting-dfrt - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-7 ## Question Why single phase induction motor is not self-starting? Explain Double Field Revolving theory of single phase motor. ## Short answer The pulsating field from a single-phase supply is equivalent to two counter-rotating fluxes of half magnitude at synchronous speed. At standstill, forward and backward torques are equal and opposite, so net starting torque is zero and the motor is not self-starting. ## Answer #### Why a single-phase induction motor is not self-starting When a single-phase ac supply is connected to the stator winding, it produces a **pulsating** (alternating) magnetic field along a fixed axis, not a rotating field. This field induces currents in the stationary rotor, but the torque produced on one half of the rotor is equal and opposite to that on the other half, so the net starting torque is zero. The rotor stays at rest. However, if it is turned by hand in either direction, it accelerates in that direction. #### Double field revolving theory The single-phase winding carrying alternating current produces a **pulsating** (alternating) flux along a fixed axis: $\phi = \phi_m\cos\omega t$. According to the **double field revolving theory**, this pulsating flux is equivalent to two rotating fluxes, each of constant magnitude $\phi_m/2$, rotating at synchronous speed $N_s = \frac{120f}{P}$ in **opposite directions**. $$ \phi_m\cos\omega t = \frac{\phi_m}{2}e^{j\omega t} + \frac{\phi_m}{2}e^{-j\omega t} $$ - $\phi_f$: forward-rotating component (in the direction of the rotor). - $\phi_b$: backward-rotating component (opposite). ```text phi_f = phi_m/2 --> (anticlockwise) \ / phi_b \ / resultant = phi_m cos(wt) = phi_m/2 X along fixed axis (pulsating) (clockwise) ``` At every instant the vertical components cancel and the horizontal components add up, giving the pulsating flux $\phi_m\cos\omega t$. #### Rotor behaviour - The forward slip is $s_f = s$ and the backward slip is $s_b = 2 - s$, and the net torque is $T = T_f - T_b$. - At standstill $T_f = T_b$, so the net torque is zero (not self-starting). - Once the rotor is started in a direction, $T_f > T_b$ in that direction, and it accelerates to a speed slightly less than $N_s$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a single-phase winding fail to start the rotor? A. It produces a pulsating field with no rotation, so net torque at standstill is zero B. It produces a rotating field too fast to follow C. It produces a DC field D. The rotor is open-circuited Answer: A. The field is pulsating on a fixed axis; the torque on one half of the rotor cancels the other half. (Key point: Why not self-starting) 2. In DFRT, how do the two component fields rotate? A. In the same direction at Ns B. In opposite directions at Ns, each phi_m/2 C. In opposite directions at Ns/2, each phi_m D. One rotates, one is stationary Answer: B. The pulsating flux equals two fields of phi_m/2 rotating at Ns in opposite directions. (Key point: DFRT) 3. What happens if the rotor is turned by hand? A. It stops B. It reverses C. It stays at rest D. It accelerates in that direction since Tf > Tb, reaching slightly below Ns Answer: D. Once rotating, the forward torque exceeds the backward torque and the rotor accelerates. (Key point: Running after a push) ## Close variants set in other exams - [DFRT and making single phase motor self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adfrt-self-starting-explain) — Electrical Machines, chapter 7, asked 3× ## Related in Electrical Machines - [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 - [Double field revolving theory](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adfrt-single-phase-motor) — Electrical Machines, chapter 7, asked 3× - [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× - [Need of starter and induction motor starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-starter-methods) — Electrical Machines I, chapter 5 ## Related in other subjects - [Double field theory; not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adfrt-not-self-starting-proof) — Electrical Machine, chapter 7, asked 2× - [Double field revolving theory and a starting method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adfrt-one-starting-method) — Electric Machines II, chapter 3, asked 5× - [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× - [Double field revolving theory](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adouble-field-revolving-theory) — Electric Machines II, chapter 3, asked 5×