--- title: "Not self starting and permanent split capacitor motor" subject: "Electric Machines II" chapter: 3 key: "semester_5/electric_machines_ii:not-self-starting-psc-motor" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/not-self-starting-psc-motor site: BCT Engineering --- # Not self starting and permanent split capacitor motor Electric Machines II · Chapter 3: Fractional Kilowatt Motors - Asked in: 2073 Chaitra, 3+5 marks - Video: [PSC Motor Breakdown (Permanent Split Capacitor)](https://www.youtube.com/watch?v=5_nGB0qtJrY) — Jersey Mike HVAC - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/not-self-starting-psc-motor - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3 ## Question Explain why single phase induction motor is not self starting? Also explain working principle and application of permanently split phase capacitor motor. ## Short answer A single-phase winding produces a pulsating field; by double revolving field theory it splits into two equal counter-rotating fields whose torques cancel at start, yielding zero net torque. A permanently split capacitor motor adds a capacitor in the auxiliary winding to create a phase split, producing a rotating field for self-starting; used in fans, blowers, etc. ## Answer #### Why a single-phase induction motor is not self-starting A single-phase stator winding produces a **pulsating** (alternating) field along one fixed axis, not a rotating field. By **double field revolving theory**, this field equals two fields of half amplitude, $\phi_m/2$, rotating at synchronous speed in opposite directions. - At standstill the rotor slip with respect to both fields is $s = 1$. - Both fields induce equal rotor currents and produce **equal and opposite torques**, $T_f = T_b$. - Net starting torque $T = T_f - T_b = 0$, so the rotor only hums and does not start. (Cross-field view: rotor currents at standstill produce a field in line with the stator field, so there is no torque-producing displacement.) If the rotor is pushed in either direction, $T_f \ne T_b$ and it accelerates in that direction. So an auxiliary means is needed to create a rotating field at start. #### Permanent split capacitor (PSC) motor **Construction:** - Main winding and auxiliary winding displaced $90^\circ$ electrical in space. - One **oil-filled paper capacitor** of small value (e.g. 2–20 μF) is permanently connected in series with the auxiliary winding. There is **no centrifugal switch**. - Both windings are usually identical (same copper), so the motor works like a two-phase motor. ```text o-------+---------------+ | | Main Auxiliary winding winding 1-ph | | supply | [C] (permanent) | | o-------+---------------+ ``` **Working principle:** 1. The capacitor makes $I_a$ lead the supply voltage, while $I_m$ lags it. The angle between them is large (about $90^\circ$ at the design load). 2. Two space-displaced windings carrying time-displaced currents produce a **rotating magnetic field** (nearly uniform at rated load), and the cage rotor starts and runs like a two-phase induction motor. 3. Because the capacitor is chosen for good running, the starting torque is only moderate (about 50–100% of full-load torque). 4. Direction can be reversed easily by switching the capacitor from one winding to the other. ```text I_a ^ alpha ~ 90 deg | -------+--------> V \ v I_m ``` **Features:** no switch to fail, high power factor, good efficiency, quiet and smooth running, speed control by tapped windings or voltage. **Applications:** - Ceiling fans, table fans, exhaust fans and blowers - Air-conditioner and refrigerator fan motors, room coolers - Oil burners, office machines, motor-operated valves and reversible drives ## 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 hum but not start? A. Its stator has no winding B. At standstill T_f = T_b, so net torque is zero C. The rotor current is zero D. Supply frequency is too low Answer: B. Equal and opposite torques from the forward and backward fields cancel at slip 1. (Key point: Zero starting torque) 2. A permanent split capacitor motor has: A. A starting capacitor with centrifugal switch B. One small oil-filled capacitor permanently in series with the auxiliary winding and no switch C. No auxiliary winding D. A shading ring Answer: B. The capacitor stays in circuit, so no centrifugal switch is needed. (Key point: PSC construction) 3. The starting torque of a PSC motor is: A. Very high, 300% of full load B. Zero C. Moderate, since the capacitor is chosen for running D. Same as a three-phase motor Answer: C. The capacitor is sized for good running performance, so starting torque is only about 50-100% of full load. (Key point: PSC starting torque) 4. How is the PSC motor direction reversed, and where is it used? A. Swap supply to DC; compressors only B. Move the capacitor from one winding to the other; fans and blowers C. Cannot be reversed; clocks D. Reverse the rotor; cranes Answer: B. Switching the capacitor to the other winding reverses the field; PSC motors drive fans, blowers and similar loads. 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