Single-phase electricity cannot produce a rotating magnetic field. To make the single-phase motor rotate automatically, we can add a starting winding in the stator, which is 90 degrees apart from the main winding in space. The starting winding should be connected in series with a suitable capacitor, so that the phase difference between the current and the main winding is about 90 degrees, which is the so-called phase separation principle.
In this way, two currents with a time difference of 90 degrees flow into two windings with a space difference of 90 degrees, which will generate a (two-phase) rotating magnetic field in space, and the rotor can start automatically under the action of this rotating magnetic field.
Extended data:
Single-phase asynchronous motor consists of stator, rotor, bearing, casing and end cover. Single-phase asynchronous motor is often made into small motor equipment. Its motor capacity is very small, and it only needs single-phase AC power supply. As a driving motor, single-phase asynchronous motor only needs a few watts, dozens of watts or hundreds of watts.
Single-phase asynchronous motor is a rotating motor powered by single-phase AC power supply, and its stator winding is single-phase. When the single-phase alternating current is switched on, an alternating pulsating magnetic field will be generated in the air gap between the stator and the rotor, so the single-phase asynchronous motor cannot start itself.
In AC motor, when the stator winding is energized with AC current, the armature magnetomotive force is established, which has great influence on the energy conversion and running performance of the motor. Therefore, the pulsating magnetomotive force generated by energizing the single-phase alternating current winding with single-phase alternating current can be decomposed into the sum of two rotating magnetomotive forces with equal amplitude and opposite rotation speed, thus establishing the sum of rotating and reverse magnetic fields in the air gap. These two rotating magnetic fields cut the rotor conductor, respectively generating induced electromotive force and induced current in the rotor conductor.
The current interacts with the magnetic field to produce positive and negative electromagnetic torques. The forward electromagnetic torque tries to make the rotor rotate forward; The reverse electromagnetic torque tries to reverse the rotor. The superposition of these two torques is the resultant torque that drives the motor to rotate.
Whether it is a forward rotating magnetic field or a reverse rotating magnetic field, the relationship between their magnitude and slip is the same as that of a three-phase asynchronous motor. If the rotational speed of the motor is n,
For the forward magnetic field, the slip ratio is: s+= (n1-n)/n1= s.
For the reverse magnetic field, the slip ratio is: s-= (-n1-n)/-n1= s.
The main features of single-phase asynchronous motor are:
(1) n = 0, s = 1, t = t++t-= 0, indicating that the single-phase asynchronous motor has no starting torque, and the motor cannot be started unless other measures are taken.
(2) when s≠ 1, T≠0, t has no fixed direction, which depends on the sign of S.
(3) Due to the existence of reverse torque, the resultant torque is also reduced, so the overload capacity of single-phase asynchronous motor is low.
Working principle of capacitor split-phase starting
When starting, the switch K is closed, so that the phase difference between the two winding currents I 1 and I2 is about 90, thus generating a rotating magnetic field and making the motor rotate. The rotation is positive
Usually, the centrifugal switch is turned off and the starting winding is cut off.
Working principle of shield pole single-phase motor
After the stator is energized, part of the magnetic flux passes through the short-circuit ring, in which induced current is generated. The current in the short-circuit ring hinders the change of magnetic flux, which leads to the phase difference between the magnetic flux generated by the short-circuit ring part and the part without the short-circuit ring, thus forming a rotating magnetic field and making the rotor rotate.
References:
Baidu encyclopedia-single-phase electric machinery