Asynchronous motor
Asynchronous motors, also known as induction motors, are often referred to as squirrel-cage motors because of the "squirrel-cage" rotor, which is the most widely used rotor structure.
From the structural form of the motor alone, the basic structure of the asynchronous motor appears when the permanent magnet rotor of the brushless DC motor is replaced with a squirrel-cage winding rotor.
Induction motor
Asynchronous motor, the rotor itself is just a coil, no current is flowing in. However, the winding coil is excited by the rotating magnetic field in the space, and an induced current is generated inside. The current is subjected to Lorentz force and motion occurs. The direction and speed of the coil motion is completely dependent on the direction and speed of the rotating magnetic field.
The rotational speed of the rotating magnetic field here is called the synchronous rotational speed of the motor. The rotation speed of the rotor, due to the motion generated by the induced current, always lags behind the synchronous rotation speed by a certain angular difference, and as the load increases, the angular difference also increases.
Here, in an asynchronous motor, it is necessary to figure out how the rotating magnetic field is generated.
Asynchronous motor rotating magnetic field
The curve in the figure is the excitation current waveform of the stator winding of the asynchronous motor, a set of sine waves. The following three circles are three moments of interception interval ωt=π/2, and the direction of the stator magnetic field at this moment is analyzed separately.
The excitation current indicates that the current flows to the inside of the paper surface with "x", and "•" indicates the flow to the outside of the paper surface. From the left to the first circle, take ωt=2π/3 on the sine wave pattern. At this time, iu>0, iv<0, iw<0. According to the right-hand rule, the magnetic line diagram can be drawn, as shown in the figure. Shown with the dotted line of the arrow. The magnetic lines of force are equivalent to a pair of magnets having an upper portion having an S pole and a lower portion having an N pole mounted on the stator;
The second circle in the middle corresponds to the second value point ωt=7π/6 on the sinogram, spaced π/2 from the first value point; the third circle corresponds to the third value on the sine curve Point, spaced from the second value point by π/2. According to the current direction at each moment, the magnetic field diagram at this moment can be drawn.
According to the continuous observation of three circles, it can be found that with the change of the phase angle of the input current, the S pole and the N pole of the equivalent magnetic pole on the stator rotate counterclockwise corresponding angles, that is, a rotating magnetic field is formed.
