Cooling tower motor characteristics and basic knowledge(4)

Inverter motor: The frequency conversion technology is actually based on the principle of motor control, and the motor is controlled by the inverter.The motor used for this type of control is called a variable frequency motor.

Common variable frequency motors include: three-phase asynchronous motor, DC brushless motor, AC brushless motor and switched reluctance motor.

Control principle of variable frequency motor

Generally, the control strategy of the variable frequency motor is: constant torque control at the base speed, constant power control above the base speed, and weak magnetic field control in the ultra high speed range.

Base speed: The back EMF is generated when the motor is running, and the back EMF is usually proportional to the speed.Therefore, when the motor runs to a certain speed, the magnitude of the back electromotive force is the same as the magnitude of the applied voltage, and the speed at this time is called the base speed.

Constant torque control:The motor performs constant torque control at the base speed.At this time, the back electromotive force E of the motor is proportional to the rotational speed of the motor.The output power of the motor is proportional to the product of the torque and speed of the motor, so the motor power is proportional to the speed.

Constant power control:When the motor exceeds the base speed, the motor's back electromotive force is kept constant by adjusting the motor excitation current, thereby increasing the motor speed.At this point, the output power of the motor remains basically constant, but the motor torque decreases inversely proportional to the speed.

Weak magnetic control:When the motor speed exceeds a certain value, the excitation current is already quite small, and it can hardly be adjusted. At this time, the field of weak magnetic control is entered.

The speed control and control of the motor is one of the basic technologies of various types of machinery and office and industrial and electrical equipment in industry and agriculture.With the amazing development of power electronics technology and microelectronic technology, the AC speed regulation mode of “special frequency induction induction motor + inverter” is adopted.With its excellent performance and economy, it is guiding a revolutionary change in the field of speed regulation.It greatly improves the degree of mechanical automation and production efficiency, saves energy, improves product qualification rate and product quality, increases power system capacity, miniaturizes equipment, and increases comfort.It will soon replace the traditional mechanical speed regulation and DC speed regulation.