In AC motor, when the stator winding passes through AC current, the armature magnetomotive force is established, which has a great impact on the energy conversion and operation performance of the motor. Therefore, when the single-phase AC winding is connected with the single-phase AC, the pulsating magnetomotive force is generated. The magnetomotive force can be decomposed into the sum of two rotating magnetomotive forces with equal amplitude and opposite speed, so as to establish the sum of forward and reverse magnetic fields in the air gap. These two rotating magnetic fields cut the rotor conductor and generate induced electromotive force and induced current in the rotor conductor respectively.
The current interacts with the magnetic field to produce positive and negative electromagnetic torque. Forward electromagnetic torque attempts to make the rotor rotate forward; The reverse electromagnetic torque attempts to reverse the rotor. The superposition of these two torques is the synthetic torque that drives the motor to rotate.
Whether it is forward rotating magnetic field or reverse rotating magnetic field, the relationship between their size and slip is the same as that of three-phase induction motor. If the speed of the motor is n.
For the forward rotating magnetic field, the slip rate 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 induction motor are:
(1) N = 0, s = 1, t = t + + T - = 0, indicating that the single-phase induction 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, which depends on the positive and negative of S.
(3) Due to the existence of reverse torque, the synthetic torque is also reduced, so the overload capacity of single-phase induction 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 I1 and I2 is about 90 °, resulting in a rotating magnetic field and the motor rotates; After the rotation is normal, the centrifugal switch is thrown off and the starting winding is cut off.
Working principle of shaded pole single-phase motor:
After the stator is energized with current, part of the magnetic flux passes through the short-circuit ring and generates induced current in it. The current in the short-circuit ring hinders the change of magnetic flux, resulting in a phase difference between the magnetic flux generated by the part with short-circuit ring and the part without short-circuit ring, so as to form a rotating magnetic field and make the rotor rotate.
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