We often say inverter control motor equipment, but specifically how how to control it? In fact, the inverter control of the motor, mainly based on the motor characteristics of the parameters and motor operation requirements, the motor voltage, current, frequency control to achieve the load requirements. Frequency inverter motor control common four main are: U / f constant control, slip frequency control, vector control, direct torque control.
1, U / f constant control
U / f control is to change the motor power supply frequency at the same time change the motor power supply voltage, the motor flux to maintain a certain, in a wide speed range, the motor efficiency, power factor does not decline. Because it is the ratio of control voltage (V) to frequency (F), it is called U / f control. However, the main problem of constant U / f control is the poor performance at low speed. When the speed is very low, the electromagnetic torque can not overcome the large static friction and the torque compensation of the motor can not be properly adjusted to adapt to the change of load torque. Is unable to accurately control the actual motor speed. As the constant U / F inverter is open-loop speed control, the mechanical characteristics of the asynchronous motor can be seen, the set value for the stator frequency is the ideal no-load speed, and the actual motor speed by slip determined, so U / f constant control of the existence of the stability of the error can not be controlled, it can not accurately control the actual motor speed.
2, slip frequency control
The slip frequency is the difference frequency between the AC power frequency applied to the motor and the motor speed. According to the steady mathematical model of induction motor, we can see that when the frequency is constant, the electromagnetic torque of induction motor is proportional to the slip, the mechanical characteristics of a straight line.
Slip frequency control is essentially by controlling the slip frequency to control the torque and current. Slip frequency control needs to detect the motor speed, constitute the speed closed loop, the output of the speed regulator slip frequency, and then the sum of the motor speed and slip frequency as the given frequency of the inverter. Compared with the U / f control, its acceleration and deceleration characteristics and the ability to limit the over-current is improved. In addition, it has a speed regulator, the use of speed feedback form closed-loop control, the speed of static error small. However, to achieve automatic control system steady-state control, but also fail to achieve good dynamic performance.
3, vector control
Vector control, also known as magnetic field orientation control. It was first proposed by West F.Blasschke et al in the early 1970s to illustrate this principle by comparing DC and AC motors. This created a precedent for AC motors and equivalent DC motors. Vector control of frequency control is the induction motor stator in the three-phase coordinate system AC current Ia, Ib, Ic. Through the three-phase – two phase transformation, equivalent to the two-phase stationary current coordinate system Ia1, Ib1, and then according to the rotor magnetic field directional rotation transform, equivalent to the synchronous rotation coordinate system under the direct current Im1, It1 The excitation current of the DC motor; It1 is equivalent to the DC motor armature current), and then imitate the control method of the DC motor to obtain the control of the DC motor, through the corresponding coordinate transformation to achieve asynchronous motor control. The emergence of vector control methods, the induction motor speed control in the field of motor speed in all directions in a dominant position. However, the vector control technology needs to correctly estimate the motor parameters, and how to improve the accuracy of the parameters has been a topic of study.
4, direct torque control
In 1985, Professor DePenbrock of Ruhr University in Germany proposed the theory of direct torque control for the first time. This technique solves the problem of vector control to a large extent. Instead of indirectly controlling the torque by controlling the current and flux linkage, The torque directly as the controlled amount to control. The superiority of torque control lies in that: Torque control is to control the stator flux linkage, it does not need the rotational speed information in essence, and the control has good robustness to all the motor parameters except the stator resistance. The introduced stator flux observing It can easily estimate the synchronous speed information, which can be easily realized without speed sensor, this control is called speed sensorless direct torque control.