--- title: "1-phase IM not self-starting; two methods" subject: "Electrical Machine" chapter: 7 key: "semester_4/electrical_machine:single-phase-im-not-self-starting-methods" source: https://www.bctengineer.com/q/semester_4/electrical_machine/single-phase-im-not-self-starting-methods site: BCT Engineering --- # 1-phase IM not self-starting; two methods Electrical Machine · Chapter 7: Fractional Kilowatt Motors - Asked in: 2073 Magh, 8 marks; 2068 Bhadra, 8 marks (asked 2 times) - Video: [Single Phase Induction Motor (Construction & Working) Electrical Machines(BEE) 1st year engineering](https://www.youtube.com/watch?v=SdWM_jSu0PY) — Gautam Varde - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/single-phase-im-not-self-starting-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-7 ## Question Why single phase induction motors are not self-starting? Explain any two starting methods for single phase induction motor. ## Short answer Single winding produces pulsating field; at rest forward and backward torques cancel, no starting torque. Methods: split-phase uses high-resistance auxiliary winding for phase split; capacitor-start inserts series capacitor with auxiliary for ~90° shift and high starting torque. ## Answer #### Why single-phase induction motors are not self-starting A single-phase supply on a single stator winding produces a pulsating field along a fixed axis, not a rotating one. By the double revolving field theory, this is the sum of two equal fields, forward and backward, each of half the magnitude, turning at synchronous speed in opposite directions. At standstill both fields have slip 1 and induce equal rotor currents. They produce equal and opposite torques, T_f = T_b, so the net starting torque is zero. The rotor does not start by itself, but if it is pushed in either direction, it continues to rotate in that direction. To start the motor, a rotating field must be created at starting. This is done with a second winding (the auxiliary or starting winding), displaced 90° in space, whose current is out of phase with the main winding current. #### Method 1: Split-phase (resistance-start) motor ```text +--- Main winding (low R, high L) ---+ Supply --| |-- Supply +--- Aux winding (high R) -- Centrifugal switch --+ ``` - The stator has two windings displaced 90° in space, connected in parallel across the supply. - The main winding has low resistance and high inductance, so its current I_m lags V by a large angle (about 60° to 80°). - The auxiliary winding is of thin wire, with a high R/X ratio, so its current I_a lags V by a smaller angle (about 20° to 30°). - The two currents are about 30° to 40° apart in time and 90° apart in space, producing a weak rotating field and a starting torque (1.5 to 2 times full-load torque). - At about 75 to 80 % of synchronous speed, a centrifugal switch cuts out the auxiliary winding, and the motor runs on the main winding. - Uses: fans, blowers, washing machines, grinders (up to about 0.5 kW). #### Method 2: Capacitor-start motor ```text +--- Main winding ---------------+ Supply --| |-- Supply +--- Aux winding -- C -- Switch -+ ``` - A capacitor (electrolytic, 50 to 400 μF) is placed in series with the auxiliary winding and the centrifugal switch. - The capacitor makes the auxiliary current I_a lead the voltage, while I_m lags V. The phase angle between them is close to 90°, so the rotating field is nearly circular. - This gives a high starting torque (3 to 4.5 times full-load torque) with a lower starting current. When the speed reaches about 75 %, the switch opens and the auxiliary winding and capacitor are cut out. - Uses: compressors, refrigerators, pumps, conveyors. Other methods are the shaded-pole motor, the permanent-split (capacitor-run) motor and the capacitor-start capacitor-run motor. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a single-phase induction motor have no starting torque? A. The rotor has too high a resistance B. At standstill the forward and backward fields give equal and opposite torques C. The stator has too many poles D. The supply frequency is too low Answer: B. Both fields have slip 1 at standstill, so T_f = T_b and the net torque is zero. (Key point: Why not self-starting) 2. In a split-phase motor, how does the auxiliary winding differ from the main winding? A. Thicker wire, more turns, low R/X B. Thin wire, fewer turns, high R/X, so its current lags V by a smaller angle C. Same wire as the main winding but connected in series D. Wound with a capacitor inside the slots Answer: B. A high R/X ratio makes I_a lag V by only about 20° to 30°, while I_m lags by 60° to 80°, giving a phase difference. (Key point: Split-phase windings) 3. What does the capacitor do in a capacitor-start motor? A. Makes I_a lead V so that I_a and I_m are nearly 90° apart B. Increases the main winding current C. Stores energy to run the motor at full load D. Reduces the speed of the rotating field Answer: A. The capacitor makes the auxiliary current lead V while I_m lags, so the angle approaches 90° and the field is nearly circular. (Key point: Role of the capacitor) 4. When is the auxiliary winding cut out, and by what? A. At standstill by a thermal relay B. At about 75 to 80 % of synchronous speed by a centrifugal switch C. At synchronous speed by a contactor D. At 20 % of synchronous speed by a fuse Answer: B. The centrifugal switch opens at about 75 to 80 % of N_s, and the motor then runs on the main winding. 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