A single-phase asynchronous motor is a rotating electrical machine powered by a single-phase AC power source, and its stator winding is single-phase. When connected to single-phase alternating current, it will generate an alternating pulsating magnetic field in the air gap of the stator and rotor, so the single-phase asynchronous motor cannot start automatically. [2] In the AC motor, when the stator winding passes the AC current, the armature magnetomotive force is established, which has a great influence on the energy conversion and running performance of the motor. Therefore, the single-phase AC winding is connected to the single-phase AC to generate a pulsed magnetomotive force, which can be decomposed into two rotating magnetomotive force sums with equal amplitudes and opposite rotational speeds, thereby establishing forward and reverse rotations in the air gap. magnetic field and. These two rotating magnetic fields cut the rotor conductor and generate induced electromotive force and induced current in the rotor conductor, respectively.
This current interacts with the magnetic field to generate positive and negative electromagnetic torques. Forward electromagnetic torque attempts to make the rotor rotate forward; reverse electromagnetic torque attempts to reverse the rotor. The superposition of these two torques is the resultant torque that drives the motor to rotate.
Regardless of 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 speed of the motor is n,
Then for the forward magnetic field, the slip rate is: s+=(n1-n)/n1=s
For the reverse magnetic field, the slip is: s-=(-n1-n)/-n1=s
From the T-s curve of the single-phase asynchronous motor, it can be seen that the main characteristics of the 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. If no other measures are taken, the motor cannot be started.
(2) When s≠1, T≠0, T has no fixed direction, it depends on the positive and negative of s.
(3) Due to the existence of the reverse torque, the combined torque is also reduced, so the overload capacity of the single-phase asynchronous motor is low.
The working principle of capacitor split-phase starting
When starting, the switch K is closed, so that the phase difference between the currents I1 and I2 of the two windings is about 90°, thereby generating a rotating magnetic field, and the motor rotates; the rotation is positive.
Often the centrifugal switch is thrown off and the start winding is cut off.
The working principle of the shaded pole single-phase motor
After the stator is fed with current, part of the magnetic flux passes through the short-circuit ring and induces current in it. The current in the short-circuit ring hinders the change of the magnetic flux, resulting in a phase difference between the magnetic flux generated by the part with the short-circuit ring and the part without the short-circuit ring, thereby forming a rotating magnetic field and making the rotor rotate.
The direction of rotation of the motor in the T-s curve diagram of a single-phase asynchronous motor: instantaneous needle rotation. Because the magnetic flux without the shorted loop portion leads the magnetic flux with the shorted loop portion.
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