What are the main methods of AC speed regulation?

Permanent magnet synchronous motor AC speed regulation system, switched reluctance motor AC speed regulation system and asynchronous motor AC frequency conversion speed regulation system. The rare earth used to manufacture permanent magnet synchronous motor is relatively expensive and has no price advantage; At present, switched reluctance motor has not been productized or serialized, so it needs to be specially molded and manufactured, and the initial investment is large; However, the AC variable frequency speed regulation technology is quite mature, and with the rapid development of various switching power devices, the carrier frequency of PWM modulation can be quite high, so the system performance is high, especially the direct torque control (DTC) technology developed in the last 20 years, which does not need to convert the AC motor into an equivalent DC motor, thus eliminating many complex calculations in the vector rotation transformation; It is not necessary to imitate the control of DC motor, nor to simplify the mathematical model of AC motor to solve problems. Direct torque control (DTC) has become a hot topic of variable frequency speed regulation because of its fast torque response, high speed accuracy, novel control idea, simple control structure, and direct control means (direct torque control).