VF Control of 3-Phase Induction Motor using Space Vector Modulation
Created: May 18, 2016
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The Variable Frequency or VF control is generally used in AC induction motor control. It uses Sine PWM as its traditional algorithm but it has implementation drawbacks. Some of its drawbacks include: the Sine PWM is not able to utilize available DC bus supply voltage (VDC) to the Voltage Source Inverter (VSI), and this algorithm gives more Total Harmonic Distortion (THD). In Space Vector Modulation (SVM), these drawbacks of Sine PWM algorithm are overcome. It increases the VDC utilization and minimizes THD as well as switching loss. Provided in this design, the capability to evaluate the VF control using SVM with its advantages.
This PICDEM MC development board design has several parts. The PIC18F4431 MCU is the main component as it implements the VF control using the SVM algorithm. The implementation of VF control is done by reading the analog potentiometer (R44) connected to RA1/AN1. Using the CONVERT_MANUAL_COUNT_TO_HZ routine, the potentiometer setting is converted to the required motor speed in Hz. The potentiometer setting is also interpreted as the modulation index, m. To get the required motor speed, the vector update step size is calculated by calling the CALCULATE_UPDATE_STEP_SIZE routine. All protection routines (overcurrent, overvoltage and overtemperature) are checked at a fixed time interval (presently, 5ms set by overflow rate of the Timer1). The acceleration and deceleration routine are called at one-second intervals. The power circuit is optically isolated to the control circuit that allows the programming and debugging tools to be plugged into the development board, when the main power is connected to the board. An on-board flyback power supply generates +5VD, with respect to the digital ground used for powering up the control circuit, including the PICmicro device. The +5VA and +15VA are generated with respect to the power ground (negative of DC bus). The feedback interface circuit is powered by the +5VA, while the +15VA supplies power to the IGBT drivers located inside the Integrated Power Module (IPM). The board communicates with a host PC over a serial port configured with an on- chip Enhanced USART (EUSART). The on-board user interface has two toggle switches, a potentiometer and four LEDs for indication. The switch SW1 is used to toggle between the motor Run and Stop. The SW2 is used to toggle the motor rotation direction. A potentiometer is used to set the speed reference as well as the modulation index. The LEDs are used for indication of different states of control.
The design is suitable for industrial control system design development since it improves overall system efficiency. The 3-phase line-to-neutral sine waves required for driving the 3-phase AC induction motor can be represented as 120° phase-shifted vectors (/VRN, /VYN, and /VBN) in space. For a balanced 3-phase system, these vectors sum to zero. Therefore, they can be expressed as a single space reference vector (/VS). By controlling the amplitude and the frequency of /VS, the motor voltage and the motor frequency can be controlled. Hence, this algorithm is known as the SVM. Aside of industrial application, it is also applicable to operations that require both voltage and frequency control.