Controlling 3-Phase AC Induction Motors Using the PIC18F4431
Author: Jon Burroughs Microchip Technology Inc. The three peripherals and their features are: 1. Power Control PWM (PCPWM) module: • Up to 8 output channels • Complimentary PWM outputs • Two hardware Fault protection inputs • PWM resolution up to 14 bits • Edge-aligned or center-aligned operation • Flexible dead time •Simultaneous update of duty cycle and period Motion Feedback Module (MFM), comprised of a Quadrature Encoder Interface (QEI) and an Input Capture module (IC): • Standard quadrature encoder inputs (QEA, QEB and Index) (QEI) • High and Low Resolution Position Measurement modes (QEI) • Velocity Measurement mode using Timer5 (QEI) • Interrupt with configurable priority on event detection (QEI) • Pulse WidthMeasurement and Period Measurement modes (IC) • Edge and state change capture (IC) High-Speed Analog-to-Digital Converter (HSADC): • Two independent sample-and-hold circuits • Single or multi-channel selection • Sequential or Simultaneous Conversion modes • Four-word FIFO result buffer with flexible interrupts
Previous Microchip authors (Parekh and Yedamale) have described theimplementation of a 3-phase AC induction motor (ACIM) control with PICmicro® devices. The first application note (AN843) detailed ACIM control with the PIC18F452. More recently, ACIM control has been implemented with the PIC16F7X7 family of devices (AN889). This application note describes how the PIC18F4431 may be used to control an ACIM using open and closed-loop V/f control strategies. Theapplication code is built incrementally and demonstrates the following control methods: 1. 2. 3. Voltage-frequency (V/f) control Voltage-frequency control with current feedback Voltage-frequency control with velocity feedback and PID control
The PIC18F4431 incorporates a set of innovative peripherals, designed especially for motor control applications. The utility of these peripherals isdemonstrated in both open and closed-loop three-phase ACIM motor applications. It is assumed that the reader is already familiar with the theory and nomenclature of AC induction motors. For an excellent introduction to the basic concepts of induction motors control, please refer to Microchip’s application note AN887, “AC Induction Motor Fundamentals” (DS00887).
USING THE PIC18F4431 FOR MOTORCONTROL
Before getting into actual control applications, we should understand what distinguishes the PIC18F4431 from other Microchip devices used for motor control. The core is a set of unique peripherals that simplify external hardware requirements and also enable higher levels of motor control capability than the PIC18F452 or PIC16F7X7.
As we shall see, each of these features provides a distinctadvantage in implementing more sophisticated motor control applications.
2004 Microchip Technology Inc.
Motor Drive Requirements
Practically speaking, control of a 3-phase AC induction motor requires pulse-width modulated control of the six switches of a 3-phase inverter bridge connected to the motor’s stator windings (Figure 1). The six switchesform 3 pairs of “half-bridges”, which can be used to connect the leg of a winding to the positive or the negative high-voltage DC bus. As shown in the figure, two switches on the same “half-bridge” must never be on simultaneously, otherwise the positive and negative buses will be shorted together. This condition would result in a destructive event known as “shoot-through”. If one switch is on,then the other must be off; thus, they are driven as complementary pairs. It should also be noted that the switching devices used in the half-bridge (in this case, IGBTs) often require more time to turn off than to turn on. For this reason, a minimum dead time must be inserted between the off and on time of complimentary channels. In AN843 and AN889, three PWM outputs were used to drive a 3-phase...