Motor Control Methods and Differences

V/F control is the most simple control method of permanent magnet synchronous motor, easy to achieve, low price, it is by changing the frequency while controlling the inverter output voltage, so that the motor flux to maintain a certain, in a wide range of speed regulation operation, the motor power factor and efficiency do not decline, that is, in the control process always keep V/F constant. To keep the stator flux constant. The characteristic of this control method is that the speed of the motor can be open-loop control, and there is no need to introduce any speed, position, voltage or current feedback signals from the motor end, and the control circuit structure is simple, so it is essentially open-loop control.

If the motor voltage is certain and only reduces the frequency, then the magnetic flux is too large, the magnetic circuit is saturated, and the motor will be burned when it is serious. Therefore, the frequency and voltage should be changed proportionally, that is, the output voltage of the inverter is controlled while the frequency is changed, so that the magnetic flux of the motor is maintained to avoid the generation of weak magnetic and magnetic saturation. V/f control is based on this idea, to ensure that the output voltage is proportional to the frequency of the control, so that the motor flux to maintain a certain, to avoid weak magnetic and magnetic saturation phenomenon. V/F control is generally used for fan and pump motor loads.

 

The method of vector control variable frequency speed regulation is to convert the stator currents Ia, Ib and Ic of the asynchronous motor in the three-phase coordinate system to the AC currents Ia1 and Ib1 in the two-phase stationary coordinate system through the three-phase two-phase conversion, and then to the DC current Id and Iq in the synchronous rotating coordinate system through the directional rotation transformation according to the rotor magnetic field. Then, imitating the control method of DC motor, the control quantity of DC motor is obtained, and the control of asynchronous motor is realized by corresponding inverse transformation of coordinates.

The essence of vector control is to equate the AC motor to the DC motor, and control the two components of the speed and the magnetic field independently. By controlling the rotor flux linkage, and then decomposing the stator current, the torque and magnetic field components are obtained, and orthogonal or decoupled control is realized by coordinate transformation. However, in practical application, because the rotor flux is difficult to be accurately observed, the system characteristics are greatly affected by the motor parameters, and the vector rotation transformation used in the control process of the equivalent DC motor is complicated, the actual control effect is difficult to reach the ideal analysis result.

Vector control is divided into closed-loop vector control and open-loop vector control, and then the two control methods are introduced respectively.

 

Open loop vector control, also known as sensorless vector control (SVC), in the late 1980s, Joetten.R successfully combined sensorless technology with vector control and carried out practical applications on permanent magnet synchronous motors. The starting point of this technology to solve the problem is to use the voltage, current and other physical quantities in the stator winding of the motor that can be measured directly as input variables, and estimate the position and speed information of the motor rotor through a certain control algorithm, and then replace the traditional mechanical sensor to realize the closed-loop control of the permanent magnet synchronous motor. This sensorless vector control technology eliminates the magnetic pole position and velocity sensor, simplifies the system structure, reduces the cost, improves the control and calculation accuracy, and has strong reliability, which has become an important research direction in the field of permanent magnet synchronous motor control. However, sensorless vector control needs to accurately estimate the position and speed information of the motor rotor. Once the estimation is inaccurate, the software phase locking failure will be caused, which is easy to cause the permanent magnet synchronous motor to lose step or stop running, and the motor will be damaged in serious cases. Especially in high-speed applications, the sensorless vector control of permanent magnet synchronous motor is complicated, the speed and position signal estimation is unstable, the magnetic flux observation is difficult, and it is difficult to achieve weak magnetic control, and a high-speed position estimation is required to limit the design of current regulator and governor, which reduces the stability of the system.

SVC control is not actually an open loop in the true sense, because under this control mode, the speed outer ring still exists, but at this time, the speed feedback value is not the real speed feedback value of the motor, but the speed value calculated by the inverter according to the motor model as a feedback signal. The motor does not have a speed feedback device, and the inverter relies on the speed observer in its own internal software to calculate the motor speed, so as to achieve the control of the motor speed, which is essentially a "closed-loop control without speed feedback".

 

Closed-loop vector control, that is, FVC control mode, the motor is equipped with an encoder, the speed feedback value is the actual speed measured by the encoder, is the real speed of the motor, the actual measured speed into the speed PI for dynamic adjustment, so that the motor speed is faster and better close to the given speed. Therefore, closed-loop vector control is mostly used in situations with high demand precision and high dynamic response.

Closed-loop vector control, permanent magnet synchronous motor has very good low-speed characteristics, at zero speed can provide a large static moment to balance the load torque, so closed-loop vector control is widely used in permanent magnet synchronous servo motor, injection molding machine, elevator traction machine, wire drawing machine, electric spindle and so on.

 

Direct torque control (DTC) method uses the space vector analysis method to analyze the mathematical model of AC motor directly in the stator stationary coordinate system, build the algorithm model of torque and flux, calculate and control the torque of AC motor, and generate PWM signal with the help of hysteresis controller (Bang-Bang control). The switch state of the inverter is controlled by the switch table directly to obtain the high dynamic performance of the torque.

The basic principle is to make full use of the switching characteristics of the voltage inverter, by constantly switching the voltage state, the stator flux trajectory is approximated to the circle, and the slip frequency is changed by the interspersing of the zero voltage vector to control the motor torque and its rate of change, so that the flux and torque of the AC motor can change rapidly according to the requirements.

 

Asynchronous motor direct torque control (DTC) system consists of inverter, three-phase asynchronous motor, flux estimation, torque estimation, rotor position estimation, switch meter, PI regulator, hysteresis comparator, etc. The error between the given speed and the actual speed of the motor is output by PI regulator as the given signal of torque in the control system. At the same time, according to the three-phase current and voltage values of the motor, the system uses the flux model and the torque model to calculate the flux and torque of the motor respectively, and calculates the rotor position of the motor, the error between the given flux and torque of the motor and the actual value. Finally, the switching voltage vector of the inverter is selected according to their state, so that the motor can adjust the output torque according to the control requirements, and finally achieve the purpose of speed regulation.

 

Control Mode	Advantages	Disadvantages	Application Scenarios
V/F Control	Simple structure, reliable operation, low cost.	Control characteristics The control precision is not high; At low speed, the torque is obviously small.	It is mostly used for energy-saving inverter of fan and pump, and is often used for frequency converter.
Open Loop Vector Control (SVC)	The disadvantages of installing speed sensor in closed-loop vector control are overcome, and good control accuracy is obtained.	Low speed performance is relatively poor, more can not provide zero speed torque output performance, so it can not be used for lifting class load applications, or need low speed large torque heavy load starting occasions.	For the speed range, speed accuracy, dynamic response, low speed torque does not have too many requirements of the field, such as the load for fans, pumps, screw air compressors, etc.
Closed Loop Vector Control(FVC)	High control precision, high real-time response of the motor.	The installation and maintenance cost of mechanical sensors is high. The complexity of the system is increased. Reduced robustness and reliability.	Widely used in permanent magnet synchronous service motor, injection molding machine, elevator traction machine, wire drawing machine, electric main shaft and so on.
Direct Torque Control(DTC)	No need to decouple the stator current, excellent static and dynamic performance; With a stator flux for magnetic field orientation, the stator resistance can be measured as long as the stator resistance is known, so that the system performance is robust to the rotor parameters.	The distortion of stator current and flux linkage caused by the change of stator resistance at low speed; The limit to the inverter switching frequency increase is larger; No current ring, cannot do current protection, need to add current limiting measures.	Suitable for large inertia motion control systems requiring fast torque response, such as electric locomotives.