I. Vector Control Method of Frequency Converter
Vector control uses vector coordinate circuits to control the magnitude and phase of the motor's stator current, thereby controlling the excitation current and torque current in the d, q, and 0 coordinate systems, and ultimately controlling the motor torque. By controlling the sequence and timing of the vectors' actions, as well as the duration of the zero vector, various PWM waves can be generated to achieve different control objectives. For example, a PWM wave with the fewest switching operations can be generated to reduce switching losses. Currently, the vector control methods practically used in frequency converters are mainly two types: slip frequency-based vector control and sensorless vector control.
Both slip frequency-based vector control and slip frequency control have the same steady-state characteristics. However, slip frequency-based vector control requires coordinate transformation to control the phase of the motor stator current, ensuring it meets certain conditions to eliminate fluctuations during torque current transients. Therefore, slip frequency-based vector control offers significantly improved output characteristics compared to slip frequency control. However, this control method is a closed-loop control, requiring a speed sensor on the motor, thus limiting its application.
Sensor-based vector control controls the excitation current and torque current separately through coordinate transformation. It then identifies the speed by controlling the voltage and current on the motor stator windings to control both the excitation and torque currents. This control method offers a wide speed range, high starting torque, reliable operation, and ease of use. However, the calculations are complex and generally require a dedicated processor. Therefore, its real-time performance is not ideal, and the control accuracy is affected by the accuracy of the calculations.
Variable frequency control is generally divided into
1: V/F control (scalar) controls the voltage.
2: V/F+PG control is achieved by controlling the voltage and the encoder.
3: No power steering (PF), vector control is achieved by controlling torque.
4: Vector control with PG is controlled by encoder and torque, also known as true current vector.
Vector refers to a quantity that has both direction and magnitude. PG: Encoder
In V/F, V refers to the output voltage and F refers to the frequency.
II. Differences between Vector Control and Scalar Control
Vector control refers to a control system that can perform precise control of a motor, tailoring the control to the motor's actual conditions to ensure stable and efficient operation. Scalar control, on the other hand, controls the motor as a whole, without performing precise control over its internal components.
(I) Differences
1. Different control precision
Vector control offers higher control precision and can achieve more refined control effects. Scalar control, on the other hand, has relatively lower control precision.
2. Differences in control difficulty
Vector control is relatively more difficult to implement, requiring a thorough understanding and mastery of the motor's details to achieve effective control. Scalar control, on the other hand, is relatively simpler, requiring only a grasp of some basic control techniques.
3. Different scope of application
Vector control has a wider range of applications and is suitable for various complex control situations. Scalar control, on the other hand, is suitable for situations where control requirements are not high.
4. Different gear impedances
Vector control has relatively low gear impedance, allowing for more precise motor control. Scalar control, on the other hand, has relatively high gear impedance, making fine adjustments during control impossible.
(II) Application Scenarios of Vector Control and Scalar Control
1. Applications of Vector Control
Vector control is suitable for applications with high control requirements, such as precision machining, high-speed motion control, petrochemicals, and pharmaceuticals. In these industries, precise control of motor operation is essential, and vector control can better meet these needs.
2. Applications of scalar control
Scalar control is suitable for applications with less stringent control requirements, such as ventilation, pumping, and fan manufacturing. In these industries, the control requirements for motors are not high, and scalar control can better meet these needs.
