Asynchronous motor working principle
When the stator of the asynchronous motor is connected to the three-phase power supply, three-phase current flows through the stator, and the stator current generates a series of air-gap rotational magnetic density. The main function of this is to rotate the magnetic density of the fundamental wave air gap rotating at the synchronous speed and along the phase sequence of the winding. The size of the synchronous speed determines the frequency of the sub-grid and the number of winding pole pairs, ie
Figure 5-1(a) is a schematic diagram of a two-pole asynchronous motor. The n1 arrow indicates the direction of rotation of the air gap magnetic density. The largest circle on the innermost side represents the rotor. Two small circles represent the conductors of the rotor winding. The rotor has not yet turned up. The air gap is rotated and magnetically densely represented by N and S poles. At the instant shown in the figure, the N pole is above and the S pole is below.
Thus, the rotor conductor cuts the air gap to rotate the magnetic density to induce the potential, and its direction is as shown in [Picture] and ⊙ in Figure 5-1. Since the rotor winding is short-circuited, there is current in the rotor winding. At the instant shown in Figure 5-1, the direction of the current in the conductor is assumed to be in phase with the induced potential.
According to the polarity and current direction of the air gap rotating magnetic density, it can be seen by using the left-hand rule that an electromagnetic torque in the same direction as the air gap rotating magnetic density acts on the rotor. If this electromagnetic torque can overcome the load torque applied to the rotor, the rotor can rotate and accelerate the rotation. As long as the rotational speed of the rotor is lower than the synchronous rotational speed, the induced potential and the current direction in the rotor conductor are constant, and the direction of the electromagnetic torque is also constant, which is the torque of the driving property.
Thus, the stator aspect of the asynchronous machine is changed from the absorption of electrical power from the grid to the transmission of electrical power to the grid, ie in the operating state of the generator.
If other mechanical drive motor rotor is rotated in the opposite direction of the air gap rotation magnetic density, that is, s>1, as shown in Figure 5-1(c). At this time, the direction of the potential and current in the rotor is still the same as that in the working state of the motor, and the direction of the electromagnetic torque acting on the rotor is still consistent with the direction in which the air gap is rotated and magnetically dense, however. The opposite of the actual steering of the rotor. It can be seen that the electromagnetic torque at this time is opposite to the direction of the torque that the dragging machine adds to the rotor of the motor, and the electromagnetic torque is the braking torque. We call this situation the motor is in the electromagnetic brake operating state.
In addition to absorbing the mechanical power of the dragging machine, the motor also absorbs electrical power from the grid. These two parts of the power are finally converted into heat energy by the loss inside the motor.