For applications of small power (from few watts until some hundreds) the single-phase asynchronous motor is often used. 

With standstill rotor we get no rotating magnetic field, but an alternative field that can be considered as the sum of two fields both having half an amplitude as the one of the alternating filed and rotating with opposite speeds, n₀'=n₀ and n₀"=-n₀. 

In first approximation, we can study the behaviour of the motor by superposing the effects that each one of the two fields produces on the rotor by acting independently from the other one. If we suppose that the rotor is moving with a speed n_S, it presents some relative speeds different as to the two rotating fields and therefore we can define two different slidings, s' and s".

They assume the same value s'=s"=1 when the rotor is standstill (n = 0), while they are respectively s'=0 and s"=2, when the rotor turns at the synchronism speed n₀' or viceversa s'=2 and s"=0 when the rotor turns at the synchronism speed n₀". At each one of the rotating fields an electromechanic torque corresponds with a run as a function of the speed similar to the one of Fig. 2.

The two torques C' and C", shown in Figure 2  as a function of n_r, have a symmetric run as to the origin; the total torque C is given by their sum and it is null when the rotor is standstill: Ca=C(O)=O.

Fig. 2 - Torque run as a function of the speed

Starting of the single-phase motor

Being the static torque null, the motor can't start naturally. 
However, when it is standstill it is in an instable balance condition: if it is set to rotation, even slow, in a direction or in the other one, a not null electromechanic torque develops that tends to make it accelerate until reaching a stable balance condition.. 
So that the motor can start on its own it is therefore enough to equip it with an auxiliary static torque. 
Generally this is obtained by producing an auxiliary magnetic field with a sinusoidal run, shifted in space and out of phase as to the main field, that, by superposing to this one, produces an even small rotating field, but enough to create the static torque.

Capacitor motor
	The auxiliary field can be produced with an auxiliary winding spatially shifted as to the main single-phase winding, to which a timely out of phase current is applied as to the one of the other; at this purpose the auxiliary winding is series connected to a capacitor and therefore the series is supplied at the same voltage applied to the main winding (side Figures).  Once the starting has occurred the auxiliary circuit is cut out by switching S off manually or automatically (by means of a centrifugal switch).  To carry out this type of operation we can use a common three-phase motor, whose third phase is used as an auxiliary winding.	Fig. 3 - Ap=Main winding, Aa=Auxiliary winding Fig. 4
Screened pole motor
	For powers until some tenths of watt we use screened pole motors too.  The stator is a salient pole one, each one equipped with a longitudinal slot, where a big conductor turn is set surrounding a portion of the pole(side Figure 5). Each turn can be considered as the short circuited secondary of a transformer whose primary is made up of the main winding. The alternating flux chained with the turn induces in it a current out of phase as to the main winding one, so to generate a flux φa out of phase as to the main flux φp that goes through the left part of the pole.  The spatial shift and the time phase difference between the two fluxes are modest, but generally enough to guarantee the required static torque.	Fig. 5 - S=Turn, R=Rotor, As=Stator windings