Why does a single-phase induction motor not start by itself?

A single winding produces a pulsating field made of two equal rotating fields in opposite directions. At standstill their torques cancel, so the starting torque is zero. A second, phase-shifted field is needed to start.

What does the capacitor do in a single-phase motor?

It shifts the current in the auxiliary winding towards 90° ahead of the main winding current. The two windings then make a rotating field, which gives high starting torque, or smoother running if the capacitor stays in circuit.

What is the centrifugal switch for?

It disconnects the start winding, and start capacitor, when the rotor reaches about 75% of synchronous speed. Without it, the start winding would overheat and add losses during running.

How do you reverse a single-phase induction motor?

Swap the connections of the auxiliary winding relative to the main winding. Swapping the two supply leads has no effect. A shaded-pole motor cannot be reversed electrically because the ring sets the direction.

Which is better, capacitor-start or PSC?

Capacitor-start gives much higher starting torque for hard-to-start loads like compressors. PSC has no switch and runs more smoothly with a better power factor, but its starting torque is low, so it suits fans and light loads.

Why is a shaded-pole motor so inefficient?

The shading ring carries current continuously and the field it produces is weak and far from circular, so the motor has low torque, high slip and high losses. It is chosen for price and simplicity in very small fans.

How single-phase induction motors start: split-phase, capacitor and shaded-pole

A single winding on a single-phase supply makes a pulsating field, not a rotating one, so a stationary rotor feels no net torque. Every single-phase induction motor therefore adds a second, phase-shifted field at start-up, using a higher-resistance winding, a capacitor or a copper shading ring.

Updated November 8, 2026

A pulsating field is two rotating fields

The main winding of a single-phase motor creates a magnetic field that stays on one axis and varies sinusoidally with time. This pulsating field can be split mathematically into two rotating fields of half the amplitude, one turning forward and one backward, both at synchronous speed N_s = 120 × f / p.

A stationary rotor sees both fields with a slip of 1. They produce equal and opposite torques, so the sum is zero: there is no starting torque, and the rotor can sit humming in either direction. If the rotor is turned by hand, the forward field has slip s and the backward field has slip 2 − s. The forward torque then wins, and the motor accelerates and carries its load.

This double-field picture also explains why a running single-phase motor is less capable than a three-phase one: the backward field always wastes some torque and adds losses. The torque also pulsates at twice the supply frequency, 100 Hz on a 50 Hz supply, which is the source of the hum and vibration.

The cure: a second field out of phase

If a second winding, displaced 90 electrical degrees in space, carries a current that is shifted in time, the two fields combine into a field that rotates in one direction. The closer the time shift gets to 90°, and the closer the two currents are in magnitude, the better the rotation and the starting torque.

All methods are ways of obtaining that time shift from a single supply. Except for shaded-pole motors, the extra winding is called the auxiliary or start winding, and the main winding is called the run winding.

The motor types

Split-phase: the auxiliary winding is wound with thinner wire and has more resistance and less reactance than the main winding, which gives a small phase shift of perhaps 30°. Starting torque is moderate, roughly one and a half times rated torque, with high starting current. Used for fans, blowers and small pumps.
Capacitor-start: a capacitor in series with the auxiliary winding brings its current close to 90° ahead of the main current. Starting torque is high, around three times rated, with lower current. Used for compressors, pumps and workshop tools.
Permanent-split capacitor (PSC): the capacitor and auxiliary winding stay in circuit at all times. Starting torque is low, a fraction of rated torque, but the motor runs smoothly with a better power factor and no switch. Used for fans and some pumps.
Capacitor-start capacitor-run (CSCR): a large start capacitor is switched out at speed while a smaller run capacitor stays in. It combines the high starting torque of capacitor-start with the smooth running of PSC, and suits heavy-duty compressors.
Shaded-pole: a copper ring around part of each pole delays the flux in that part and creates a weak sweep of the field across the pole. Starting torque is low, efficiency is poor, but the motor is very cheap and robust. Used for small fans, in the 5 to 100 W range, and for appliance blowers.

The centrifugal switch

A start winding with a large capacitor is only meant for a few seconds. At about 75% of synchronous speed a centrifugal switch on the shaft, with spring-loaded weights that move outwards with speed, opens and disconnects the start winding (and start capacitor). The motor then runs on the main winding, and on the run capacitor if it has one. Electronic relays or PTC thermistors do the same job in many modern motors.

If the motor cannot reach the cut-out speed, because it is overloaded or the voltage is low, the start winding remains connected and overheats quickly. A hum and no rotation, or a motor that starts and then runs very hot, often points to a failed switch or capacitor.

Reversing a split-phase or capacitor motor is done by swapping the connections of the auxiliary winding relative to the main winding. Reversing the supply leads does nothing, since both windings reverse together.

Worked example

Take a four-pole, 50 Hz capacitor-start motor.

1. Synchronous speed: N_s = 120 × 50 / 4 = 1500 rpm. 2. The centrifugal switch opens at 75% of N_s = 1125 rpm. 3. At 1400 rpm the forward-field slip is s = (1500 − 1400) / 1500 = 0.067, or 6.7%, while the backward field sees a slip of 2 − s = 1.93, so it contributes very little torque at that speed. 4. The torque pulsation frequency is 2 × 50 = 100 Hz.

For a capacitor motor, the start capacitor is on the order of hundreds of microfarads per kilowatt, much larger than the run capacitor. A motor with a start capacitor that is too small has a weak start, and one that is too large draws excessive current and stresses the winding.

How the simulator models single-phase motors

The simulator is a lesson model, labelled as such in the interface. The synchronous speed is 120 × f / p. The operating point is found where the torque–speed curve, the sum of main and auxiliary winding torques, crosses a constant load torque, and the slip follows from that point.

Starting torque comes from fixed per-type ratios at the best capacitor value, and a capacitor far from that value reduces it: about 3 times rated torque for capacitor-start and CSCR, about 1.5 times for split-phase, about 0.6 times for PSC and about 0.4 times for shaded-pole. The centrifugal switch cuts out the start winding at 75% of synchronous speed (not in PSC and shaded-pole motors). The best-match capacitors are about 250 µF per kW for the start capacitor and 35 µF per kW for a run capacitor (PSC and CSCR), so a 0.75 kW CSCR motor would have about 190 µF and 26 µF.

With the auxiliary winding disconnected the starting torque is zero and the rotor stays locked, with locked-rotor current flowing. The model is not a double-field equivalent circuit and is not for sizing capacitors or protection.

Try it in the single-phase induction motor simulator

Open the single-phase induction motor simulator.

Set Auxiliary winding to Disconnected (not shown for shaded-pole). The motor will not turn, and a warning explains that the main winding alone has no starting torque. In the 3D scene the main-winding field only pulsates.
Set it back to Connected: a rotating field appears and the rotor accelerates.
Change Motor type between Split-phase, Capacitor-start, PSC (run capacitor), Capacitor-start capacitor-run (CSCR) and Shaded-pole, and compare the Starting torque readout (in multiples of rated torque) and the Start winding state (Engaged or Disconnected; for shaded-pole the readout is called Shading rings).
Vary the Capacitor value (not shown for split-phase and shaded-pole). The hint shows the best match in µF; far from it, a warning appears and the starting torque drops. For CSCR there are separate Start capacitor and Run capacitor sliders.
Raise the Mechanical load until a motor cannot start. Shaded-pole motors have so little starting torque that a modest load already keeps the rotor stalled, and a warning says so.
Switch Rotation direction to Reversed (hidden for shaded-pole, whose direction is fixed by the ring). Then run the timed experiment to see speed and current over time.

The experiment shows that the second winding or ring is only about starting, and the main winding does the running.

Try it in the simulator

Open the Single-phase induction motor simulator

Frequently asked questions

Why does a single-phase induction motor not start by itself?: A single winding produces a pulsating field made of two equal rotating fields in opposite directions. At standstill their torques cancel, so the starting torque is zero. A second, phase-shifted field is needed to start.
What does the capacitor do in a single-phase motor?: It shifts the current in the auxiliary winding towards 90° ahead of the main winding current. The two windings then make a rotating field, which gives high starting torque, or smoother running if the capacitor stays in circuit.
What is the centrifugal switch for?: It disconnects the start winding, and start capacitor, when the rotor reaches about 75% of synchronous speed. Without it, the start winding would overheat and add losses during running.
How do you reverse a single-phase induction motor?: Swap the connections of the auxiliary winding relative to the main winding. Swapping the two supply leads has no effect. A shaded-pole motor cannot be reversed electrically because the ring sets the direction.
Which is better, capacitor-start or PSC?: Capacitor-start gives much higher starting torque for hard-to-start loads like compressors. PSC has no switch and runs more smoothly with a better power factor, but its starting torque is low, so it suits fans and light loads.
Why is a shaded-pole motor so inefficient?: The shading ring carries current continuously and the field it produces is weak and far from circular, so the motor has low torque, high slip and high losses. It is chosen for price and simplicity in very small fans.