Introduction to Microprocessors and Control of Electric Drives:
Introduction to Microprocessors and Control of Electric Drives – Conventionally, control of thyristor converter fed electrical drives (both dc and ac) is performed using analog discrete components. In the drive control, conventional speed control systems have been improved continuously and gradually in terms of their control performance, as the control devices have changed from magnetic amplifiers to transistors and IC amplifiers. This hardware oriented control of drives becomes more and more complex with the increase in sophistication required, i.e. the requirements of high precision, performance and reliability. The various controllers, the control circuits for the power converters are realised using hardware oriented circuits. The control suffers from the drift and parameter variations of the components due to temperature and the improvement is rather limited. However, the control has the advantage that it is capable of processing the signals almost simultaneously.
Because of the involved complexity and the limitations of improvement in performance due to drift problems, several sophisticated methods of controlling ac motors, such as vector control of induction motors, did not become popular until the developments in the area of drive technology, which have been very rapid in the recent past. With these the control is possible with fewer components with the advantages of easy maintenance, economy, serviceability and general applicability. Also this rapid and fast development of digital technology has been the opening for new concepts of control using microprocessors and associated digital components, A/D converters, etc. These have resulted in compact control systems for drives which effectively replace the complex hardware oriented control using analog discrete components.
The digital processing of speed and angular position is more exact and less expensive than analog processing, which makes use of tachogenerators and position transducers. The control is free from drift and parameter variations.
Features of Microprocessors and Control of Electric Drives
The main features of digital control with Microprocessors and Control of Electric Drives that it is mainly software oriented. When once the necessary algorithms are developed, the software can be used for different plants with only a few modifications. This is not possible with dedicated hardware control.
The drive technology using thyristor converter fed dc or ac motors, as expected, has become very popular owing to the developments in the area of thyristor technology, high speed digital systems, microprocessors and microcomputers. The future has already begun for the use of microprocessors and microcomputers in the area of drives. Sophisticated controls of these drives with high performance and reliability, complicated hardware oriented controls, such as four quadrant operation of dc motors using dual converters, field orientation or vector control both in constant flux and flux weakening modes of induction motors and margin angle control of synchronous motors can be realised with software programs on the microprocessor and microcomputers with least possible hardware. This hardware includes the interfacing of the microprocessor to the system. Sometimes to reduce the burden on the microprocessor and to make it available to perform other functions effectively, the firing circuits with necessary pulse transformers are realised in terms of hardware. The sensor measuring the terminal quantities must be interfaced through proper AID converters. The controls become economically feasible. Various controllers in the closed loop controls can be realised on the microprocessor by means of software programs. Optimised PWM strategies for the ac motor can be realised. Advances of new digital techniques make it possible to manage different operating conditions. Microprocessors and Control of Electric Drives can be very easily adopted in the parameter identification and adaptation problems using machine models. A slip energy recovery scheme for both super and subsynchronous operation using vector control can be realised using high performance processors.
Various sophistications in the control of synchronous motors have also become feasible. One such sophistication is on-line determination or estimation of the rotor position without any rotor position sensor on the shaft. The identification and state estimation processes involved are very easily implemented using Microprocessors and Control of Electric Drives.
An attempt is made in this chapter to describe the role of microprocessor and its function in the control of electrical drive. The architecture and the types of microprocessors are not discussed here. A few applications of the microprocessor in the dc and ac drives are discussed to bring out clearly the stages involved in the design of the control system.