What is a single-phase induction motor?
Working principle
A single-phase induction motor runs from an ordinary two-wire supply, so its main winding produces a pulsating magnetic field instead of a rotating one. Double-revolving-field theory splits that pulsating field into two equal fields turning in opposite directions. At standstill their torques cancel exactly, so the motor cannot start by itself. Once the rotor is turning in either direction, the forward field wins and the motor keeps accelerating on its own.
Starting methods
To get a starting torque, a second auxiliary (start) winding is placed 90 electrical degrees from the main winding and fed with a current that is out of phase with it. In a split-phase motor this comes from a higher-resistance winding, in a capacitor-start motor from a series capacitor, and in a permanent split capacitor (PSC) motor from a capacitor left in circuit. A CSCR motor combines a large start capacitor with a small run capacitor, while a shaded-pole motor needs no winding at all, only a copper ring on each pole. A centrifugal switch typically drops the start winding out at about 75 % of synchronous speed.
Torque, slip and efficiency
Synchronous speed is still ns = 120 × f / p, so a 4-pole motor on 50 Hz has a field speed of 1500 rpm. Compared with a three-phase machine, the single-phase motor runs at higher slip, with a lower power factor and efficiency. Its torque also pulsates at twice the supply frequency, 100 Hz on a 50 Hz supply, which shows up as hum and vibration. Swapping the auxiliary winding connections reverses the direction of rotation.
Where it is used
Single-phase induction motors power the everyday machines found wherever only a household supply exists: fans, water pumps, compressors, washing machines and bench grinders. Capacitor-start designs deliver roughly three times rated torque at start and suit compressors and loaded pumps. CSCR motors add the better efficiency of a run capacitor for heavy duty, while split-phase and PSC types suit fans and light loads. Shaded-pole motors, with weak starting torque and low efficiency, stay limited to tiny fans and pumps below about 100 W. Above a few kilowatts, three-phase motors are preferred.
Frequently asked questions
- Why does a single-phase induction motor need a capacitor to start?
- A single-phase winding creates a pulsating field that is the sum of two equal fields rotating in opposite directions, so at standstill the net torque is zero. A capacitor in series with an auxiliary winding shifts its current by up to 90°, producing a rotating field that gives the rotor a push. Once it is turning, the motor can keep running without it.
- How do you reverse the direction of a single-phase induction motor?
- Swap the connections of either the main winding or the auxiliary (start) winding, but not both. This reverses the phase relationship between the two currents and therefore the direction of the rotating field. Reversing the supply wires does nothing, because both windings flip together. In the simulator, use the rotation reversal control and watch the rotor turn the other way.
- What is the difference between a capacitor-start and a capacitor-run (PSC) motor?
- A capacitor-start motor uses a large capacitor only during starting and disconnects it with a centrifugal switch at about 75 % of synchronous speed. It gives high starting torque, around three times rated. A PSC motor keeps a smaller capacitor connected continuously, with no switch. Starting torque is low, around 0.6 times rated, but running is smoother and more efficient.