VFD stands for Variable Frequency Drive, which is a device used to control the speed and torque of electric motors by varying the frequency and voltage supplied to the motor. Vector and scalar control are two different methods used to achieve this control.
Scalar Control:
Scalar control, also known as V/f (Voltage-to-Frequency) control, is a basic and traditional method used in VFDs. In scalar control, the VFD maintains a fixed ratio between the voltage and frequency supplied to the motor. As the frequency of the voltage supplied to the motor changes, the voltage is also changed proportionally to maintain the V/f ratio. This method is suitable for simple applications where speed control is not very critical, such as in fans and pumps.
However, scalar control has limitations when it comes to providing precise control of motor speed, especially at low speeds or during dynamic changes in load. It does not take into account the motor's actual magnetic characteristics or the effects of varying load conditions.
Vector Control:
Vector control, also known as Field-Oriented Control (FOC) or Vector Drive, is a more advanced method used in modern VFDs. Vector control offers improved performance and accuracy in controlling motor speed and torque across a wide range of operating conditions.
In vector control, the VFD decouples the control of the motor's magnetic flux and torque components. This means that the control system can adjust both the magnitude and angle of these components independently. By doing so, the VFD can accurately control the motor's speed and torque, even at low speeds and under varying load conditions. Vector control essentially provides the ability to mimic the behavior of a DC motor, offering high-performance control of AC motors.
There are two main types of vector control:
Direct Torque Control (DTC): This method directly controls the motor's torque and flux without needing to calculate the motor's rotor position. It provides fast and accurate torque and speed control.
Indirect Field-Oriented Control (IFOC): This method calculates the rotor's position and then uses this information to control the motor's torque and flux. It's a more complex approach but offers precise control similar to DTC.
In summary, while scalar control is a simple method suitable for basic applications, vector control provides much better performance and accuracy, making it the preferred choice for applications that require precise control of motor speed and torque, such as industrial automation and robotics.