What Is Synchronous Rotation?
The synchronous speed, also called the speed of the rotating magnetic field, is represented by n 1 and its unit is "r / min". Its size is determined by the frequency of the AC power source and the number of magnetic pole pairs of the magnetic field.
When the rotating magnetic field has For magnetic poles, the magnetic field turns in space every time the alternating current changes turn. So synchronous speed for
In the formula:
-The frequency of the current;
Number of magnetic pole pairs generated by the stator winding.
The frequency (i.e. power frequency) of China's power grid supply current is (That is, to complete the change of 50 cycles per second), then the speed of a pair of rotating magnetic fields is 50r / min × 60 r / min = 3000 r / min.
If the arrangement of the stator windings is different, the number of pairs of magnetic poles generated is also different, as shown in the figure of the motor. If the two windings that are 180 ° apart from each other are connected in series as a phase winding (for example, the U-phase winding is versus (Composed in series), and finally the three-phase winding is connected to the three-phase power according to the star connection (or delta connection), and a rotating magnetic field with two pairs of magnetic poles can be generated.
Whenever the alternating current changes for one cycle, the rotating magnetic field of the two poles rotates through a 360 ° (ie 1 revolution) mechanical angle in space. The four-pole rotating magnetic field only rotates 180 ° (that is, 1/2 revolution) in the space during a week of current change. [1]
The speed of the motor is represented by n. The rotor is driven by a rotating magnetic field. When the asynchronous motor is in the electric state, its speed is always less than the synchronous speed n 1. This is because the rotor rotates in the same direction as the magnetic field. The rotor rotates slower than the magnetic field. It is only possible for the winding to cut the magnetic lines of force and generate an induced current, and the rotor can also be affected by the magnetic torque.
If any This means that there is no relative movement between the rotor and the magnetic field, and the rotor does not cut the magnetic lines of force, no induced current will be generated in the rotor, and it will not be affected by the magnetic torque. If this happens, the rotor will gradually decelerate under the resistance torque (from friction or load), making . When the electromagnetic torque and resistance torque (the sum of friction torque and load torque) of the rotor are balanced The rotor keeps rotating at a constant speed. Therefore, when an asynchronous motor is running normally, this is why this type of motor is called an "asynchronous" motor. Because the current in the rotor is not supplied by the power supply, but is generated by electromagnetic induction, this type of motor is also called an induction motor. [1]
