Single-Phase Induction Motor
AC Motor
The motor in fans, pumps, washing machines and compressors. A single-phase winding alone produces no starting torque, so every type adds a trick — an auxiliary winding, capacitor or shaded pole — to get the rotor turning.
Schematic symbol
Motor
How single-phase induction motor appears in a circuit diagram.
Types & variants
Key specs
Power rating
W / HPFractional horsepower is typical: ceiling fans ~70 W, domestic pumps 0.5–1 HP.
Voltage
V (AC)230 V 50 Hz single phase for Indian domestic supply.
Capacitor value
µFStart capacitors: 60–300 µF electrolytic (seconds only); run capacitors: 1.5–50 µF film, continuously rated.
Speed
RPMSame synchronous-speed rules as three-phase: 2-pole ~2800, 4-pole ~1400 at 50 Hz.
Starting torque
% of full loadShaded pole ~50%, PSC ~50–100%, capacitor start 200–300% — pick by how hard the load is to get moving.
Markings
Nameplate with W/HP, V, A, RPM, capacitor µF and voltage. The centrifugal switch (if any) is inside the end shield. Fan motors often mark only the capacitor value.
Standard values
Ceiling fan capacitors: 2.25–3.15 µF 440 V. Pump motors: 0.5/1 HP with 36–50 µF run caps. Compressors ship with matched start + run capacitor pairs and sometimes a starting relay.
How to choose
1) Match HP and speed to the load. 2) High starting torque (compressor, loaded pump) → capacitor start (or start+run). 3) Continuous quiet duty (fan, blower) → PSC. 4) Tiny loads where efficiency doesn't matter → shaded pole. 5) Always replace capacitors with the same µF and equal-or-higher voltage rating.
Pinout & package
Main winding (thick wire, low resistance) and auxiliary/start winding (thin wire, higher resistance) share one terminal; capacitor in series with the auxiliary winding. Frame styles match small three-phase motors; fan motors are skeleton frames built into the appliance.
Example circuits
- Ceiling fan: PSC motor with speed regulated by series capacitor or TRIAC regulator
- Domestic water pump: capacitor-start motor with centrifugal switch
- Refrigerator compressor: current relay + start capacitor kicking the auxiliary winding
- Direction reversal by swapping auxiliary winding connections
Common failures
Dead run/start capacitor (motor hums but won't start — spin it by hand and it runs), stuck or burnt centrifugal switch, dry bearings/bushes (slow start, hot motor), and burnt auxiliary winding when the switch fails to disconnect it.
How to test
Identify windings by resistance: main is lower, auxiliary higher, and end-to-end (in series through the common tap) is their sum. Test the capacitor on a meter's µF range against its marked value. If it hums but a hand-spin starts it, the start circuit (cap/switch/relay) is at fault, not the winding.
Substitutes
Match HP, speed, frame and capacitor arrangement. A PSC motor cannot simply replace a capacitor-start motor on a hard-starting load. Where three-phase supply exists, a small three-phase motor + VFD is smoother and more efficient.