。 It is the torque output on the motor shaft when the motor operates at rated speed and outputs rated power under rated voltage, which can be obtained from the motor nameplate parameters using equation (1-99).

(2) Maximum torque memix. It reflects the ability of the motor to drive the maximum load. The ratio of the maximum torque mamx5 of the motor to the rated torque M, λ = MX / Mn, is called the overload factor. Generally, the overload factor of three-phase asynchronous motor is 1.8 ~ 2.2. When selecting a motor, the possible maximum load torque must be considered. The maximum torque of the motor can be calculated according to the overload coefficient of the selected motor, which must be greater than the maximum load torque. Since the maximum torque mmax of three-phase asynchronous motor is directly proportional to the square of voltage, memx is very sensitive to the fluctuation of voltage. Pay attention to the change of voltage when using.

(3) Starting torque mesro is the starting torque of the motor, which reflects the starting ability of the motor with load. If the starting torque M. > M. (load torque), the motor can start, otherwise it will not start.

The mechanical characteristic curve of the three-phase asynchronous motor also reflects the adaptive load capacity of the motor, that is, the electromagnetic torque of the motor can be adjusted automatically with the change of load. For example, when starting, MX > M1. When the motor starts, the speed n gradually increases, and the electromagnetic torque M. also increases; when reaching point B, M. reaches the maximum; as n continues to increase, me begins to decline, along the Ba line segment; when the speed reaches the rated speed NN, the electromagnetic torque and the load torque are the same, and the speed no longer increases, stable operation. If the external load increases, i.e. the load torque M. increases, the electromagnetic torque M. will be temporarily smaller than M., resulting in the decrease of the speed n, and the slip s will increase, resulting in the increase of the current in the rotor coil, resulting in the increase of the electromagnetic torque, so as to achieve a new torque balance and then the constant operation. Similar analysis can be made for the reduction of external load. The three-phase asynchronous motor works normally on the curve AB section.

In addition, the artificial mechanical characteristics of the motor can be obtained by changing the relevant parameters of the motor, for example, changing the stator voltage U1, the resistance of the stator circuit or putting three-phase symmetrical reactance in series in the stator circuit can speed the three-phase asynchronous motor.

4. Speed regulation of three-phase asynchronous motor

According to formula (1-97) and formula (1-98), the e-xpression of speed n of three-phase asynchronous motor is obtained

n=(1-s)60f1/p

Where s - slip rate;

F1 --- power frequency;

P --- number of poles.

It can be seen from formula (1-100) that there are three ways to change the rotating speed of the three-phase asynchronous motor rotor: changing the pole pairs p; changing the power frequency f. (variable frequency speed regulation); changing the slip s. The following focuses on the frequency control technology.

With the development of AC frequency control technology, high-voltage frequency control technology, as a large capacity drive, has been widely used. At present, high-voltage high-power motors have been widely used in power, petrochemical, water supply, metallurgy, transportation and other industries. Their capacity is generally hundreds to thousands of kilowatts, and the electric power consumed by them has accounted for about 30% of the power generated in China. Therefore, their energy-saving transformation of frequency conversion and speed regulation has great potential. Field experiments show that the high voltage frequency conversion technology can effectively save energy, facilitate the speed regulation of AF foam pump motor and improve the efficiency of pipeline transportation. Therefore, the research and development of high-voltage and high-power inverter has been widely valued.

High voltage and high power inverter can be divided into two types: high low high type inverter and direct high voltage type inverter.