Simulink model of a controlled AC drive

Abstract

A model of the ABB ACS800 industrial drive using Matlab Simulink is presented in this report. The ACS800 (ABB, 2011) is a drive designed for industrial applications and contains a rectifier, DC link , inverter and an AC drive. A choice can be made between speed control or torque control of the drive shaft. The feedback for the control loop is supplied by a motor model instead of encoders on the motor shaft. Chapter 1 shows the derivation of a full motor model that is used to approximate the behavior of the AC drive. The motor input is a three phase, AC voltage that is generated in an AC-DC-AC converter, discussed in chapter 2. Switching the gates of the inverter allows control over the generated three phase output voltage. The switching commands are generated by the controller. Speed control can be realized as scalar control, or by using vector control. The control of the torque can only be done with the latter scheme. The used scalar control uses a direct relationship between the motor input peak voltage amplitude and the rotational frequency. It is named Volts per Hertz control (chapter 3). The desired motor input voltage wave is realized as a PWM signal by choosing appropriate switch commands for the inverter. Vector control (chapter 4) uses the motor input voltage to control the magnetic flux amplitude generated in the stator of the motor and the output torque. The switch commands for the inverter are chosen through the direct torque control (DTC) algorithm where the estimated stator flux and the estimated output torque, acquired from the motor model, are compared to reference values. The torque reference is obtained from the speed controller in chapter 5