A frequency converter (VDC) is a device used to regulate the speed of an electric motor by changing the power supply frequency and voltage. In VDC technology, the dual-loop control of the current loop and speed loop is crucial for ensuring stable and efficient motor operation. The current loop, the inner loop of the VDC's internal control system, is primarily responsible for rapidly adjusting the motor's current; the speed loop, the outer loop, further controls the motor's speed based on the current loop. This article will detail the adjustment methods for the current and speed loops of VDC parameters.
(I) Current Loop
The input to the current loop is the output of the speed loop's PID control, called the "current loop setpoint." The current loop compares this setpoint with the "current loop feedback" value, and the difference is adjusted by the PID controller within the current loop before being output to the motor. The output of the current loop is the phase current of each phase of the motor. The feedback of the current loop does not come from the encoder, but from Hall effect sensors installed inside the driver in each phase. These sensors convert the current into a current-voltage signal through magnetic field induction and feed it back to the current loop. The main function of the current loop is to quickly respond to current changes, suppress current fluctuations, reduce motor losses, and ensure the stability of the motor current.
(II) Velocity Loop
The speed loop's input consists of the position loop's PID control output and the position setpoint feedforward value, collectively known as the "speed setpoint." The speed loop compares this setpoint with the "speed loop feedback" value, and the difference is output after PID control (primarily proportional gain and integral processing) within the speed loop, becoming the "current loop setpoint." The speed loop's feedback comes from the encoder's feedback value, processed by the "speed calculator." The speed loop's function is to control the motor's speed, enabling it to track the commanded speed. Its control accuracy and response speed directly affect the motor's dynamic performance and positioning accuracy.
The current loop and speed loop work together in a frequency converter to achieve precise control of motor speed and current. The fast response of the current loop ensures the stability of the motor current, while the speed loop, based on this, enables precise adjustment of the motor speed. Output current determines torque, torque determines acceleration, and acceleration determines speed; therefore, the current loop indirectly affects the speed loop. In practical applications, there are differences between open-loop and closed-loop current loops. The current response of an open-loop loop may be faster than that of a closed-loop loop because the closed loop requires more time to integrate speed feedback for comprehensive control. Therefore, when adjusting parameters, it is necessary to consider the interaction between the two loops and set the parameters appropriately to achieve the best control effect.
The dual-loop control of the current and speed loops in a frequency converter plays a crucial role in motor control. By deeply understanding the working principles and interactions of these two control loops, we can better optimize the performance of the frequency converter and achieve efficient and stable motor operation. In practical applications, the dual-loop control parameters should be adjusted and optimized according to the specific motor type and load characteristics. Advanced control algorithms and intelligent adjustment technologies should be employed to further improve the performance and stability of the dual-loop control.
