I. Vector Control
1. Introduction to Vector Control
Vector control is a field-oriented control method for electric motors. Taking the vector control of an asynchronous motor as an example, it first derives flux linkage equations through the equivalent circuit of the motor, including stator flux linkage, air gap flux linkage, and rotor flux linkage. The air gap flux linkage connects the stator and rotor. Since the rotor current of a typical induction motor is difficult to measure, air gap flux linkage is used as an intermediary to convert it into stator current. Then, some coordinate transformations are performed. First, a 3/2 transformation is used to convert it into stationary dq coordinates. Then, the unit vector generated by the previous flux linkage equations is used to obtain the torque current component and magnetic field current component in rotating coordinates, similar to a DC motor. This achieves decoupling control and accelerates the system's response speed. Finally, a 2/3 transformation is used to generate three-phase AC power to control the motor, thus achieving good performance.
In summary, vector control essentially boils down to four concepts: equivalent circuit, flux linkage equation, torque equation, and coordinate transformation (including static and rotating). Vector control allows for fine-tuning of the motor according to customer needs and can be used for servo motors. It doesn't prioritize motor efficiency above all else, but rather focuses on engineering requirements and continuous feedback control.
2. Detailed Explanation of Vector Control
Vector control concept: The purpose of vector control is to transform an AC motor into an equivalent DC motor, thereby achieving higher speed regulation performance. The vector control method decomposes the stator current vector of an AC three-phase asynchronous motor into a current component that generates the magnetic field (excitation current) and a current component that generates torque (torque current), controlling them separately while simultaneously controlling the amplitude and phase of the two components. This achieves the equivalent of a DC motor. Vector control methods include slip frequency-based vector control, sensorless vector control, and vector control with a speed sensor.
Vector control characteristics: Inverter vector control, depending on whether a speed feedback loop is required, is generally divided into non-feedback vector control and feedback vector control.
1) Feedbackless vector control.
The advantages of feedback-free vector control are:
a) Easy to use, no additional components required by the user.
b) The mechanical characteristics are relatively stiff. Due to the V/F control method, the mechanical characteristics are easy to adjust and will not cause motor magnetic circuit saturation (personal opinion, please feel free to criticize and correct). The disadvantages are: the speed range and dynamic response capability are not as good as those of the feedback control method.
2) It has a feedback vector control mode.
The main advantage of feedback vector control is that its speed regulation performance is superior to that of non-feedback vector control and V/F control. The disadvantages are that a speed measuring device (mostly a rotary encoder) needs to be installed on the motor, and motor frequency conversion is relatively complicated and costly.
Therefore, feedback vector control is generally used in the following situations:
a) Applications requiring a wide speed range (e.g., gantry planers with milling and grinding functions);
b) Situations requiring high dynamic response performance; c) Situations requiring high safety operation.
Scope of application for vector control:
a) Vector control can only be used when one frequency converter controls one motor. Vector control is ineffective when one frequency converter controls multiple motors.
(b) The motor capacity and the motor capacity required by the frequency converter can differ by a maximum of one level. (For example, if the frequency converter requires a motor capacity of 7.5KW, then the minimum actual motor capacity is 5.5KW; a 3.7KW motor would not be suitable.)
c) The number of magnetic poles of the motor is generally best at 2, 4 or 6. If there are more poles, it is recommended to consult the inverter manual.
d) Special motors such as torque motors, deep slot motors, and double squirrel-cage motors cannot use vector control functions. // (Personal opinion, please feel free to criticize and correct.)
Vector control can be considered an advanced technique for AC motors. Voltage-frequency control aims to maintain a constant magnetic flux in the motor, allowing it to maintain high efficiency.
II. V/F Control
1. Overview of V/F Control
V/F control is a method of controlling magnetic flux. This voltage-frequency ratio may be preset in the system to maintain the magnetic flux at a certain level. It is mainly used in frequency converters to save energy consumption of motors.
2. Detailed Explanation of V/F Control
V/F control: If the motor voltage is constant and only the frequency is reduced, the magnetic flux will be too large, causing magnetic circuit saturation, which can burn out the motor in severe cases. Therefore, the frequency and voltage must be changed proportionally. That is, while changing the frequency, the inverter output voltage is controlled to keep the motor's magnetic flux constant and avoid the occurrence of weak magnetic field and magnetic saturation. V/F control is based on this idea, ensuring that the output voltage is proportional to the frequency, thus keeping the motor's magnetic flux constant and avoiding the occurrence of weak magnetic field and magnetic saturation. V/F control is generally used for motor loads such as fans and pumps.
Comparison of vector control and V/F control: Simply put, vector control has a larger torque and is suitable for heavy-load applications and low-frequency applications where torque must be guaranteed. The disadvantage is that few domestic frequency converter manufacturers can truly master this function. Although vector control is mentioned in the manual, it is not reflected in actual applications, or it is too rigid and cannot automatically adjust to the load (for example, when used in industrial washing machines, although vector torque is greater, motor magnetic saturation and other faults may occur when the load changes).
V/F control is currently more widely used in the market. In general applications, the manufacturer's default V/F parameters can be used. Some manufacturers provide parameter setting methods for heavy load, light load, and different loads. Furthermore, most frequency converters allow users to customize V/F curves to suit different applications, although the setup is relatively complicated, but anyone with some experience using frequency converters can do it.
