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Microprocessor Controlled Electric Drive (Unit 4) Block Diagrams of Microprocessor based Drives
Advantages of microprocessor / microcontroller controlled electric drives compared to the dedicated hardware controller of analog digital circuits: (a)The use of microprocessors reduces the complexity of the system. (b)The controller becomes cost effective as a microcontroller board can be used for signal generation, feedback control, monitoring, protection and diagnostic functions, data logging, fault event logging, fault retrieval, remote monitoring etc. Functions can be implemented easily unlike analogue / digital circuitry based controller. (d) Reliability of the drive is higher with microprocessors than with dedicated hardware systems, as the number of components in controller is reduced. (e) Ease of modification or upgradations, if required during commissioning or regular operation. The control software can be modified easily and drive performance can be improved. The software module have provision of addition, deletion and upgradation. (f) The controller is free from drift and parameter variations due to temperature while in operation (g) Control accuracy is higher compared to dedicated analogue/digital controller. (f) A generalised hardware supported by a flexible software can take care of any motor (DC, AC, Special Motor) drive applications.
Challenges with Microprocessor based Drives The communication between the microprocessor and the analog circuitry is accomplished by A/D or D/A converters. There are sampling or quantizing errors with ADC and DAC. Errors can be minimised by increasing the bit size. The response is slower than that of a dedicated hardware. The dedicated hardware can handle the signals and process them almost simultaneously, without any time delay. The microprocessors on the other hand process the signals in a sequence or in a serial manner, causing a delay in processing. Microprocessor based controllers are discreet time systems. Sampling time should be small enough (depending on application) to avoid loss of any critical input and output data. The development of necessary software may be costly and time consuming. The cost may be justified depending on the size of the production or application. With the remarkable progress in the area of microelectronics, availability of reliable and computationally efficient microprocessors, microcontrollers, digital signal controller and field programmable array have made the electric drive system fast efficient, reliable and operator friendly for various applications.
Functions performed by Microprocessor in Electric Drive Systems (a) Generation of firing pulses or PWM switching signals for the converters (b) Generation of necessary reference signals for the drive (c) Nonlinear function generation. (d) Estimation of feedback signals and computation of reference quantities which cannot be directly measured, such as torque and flux (e) Implementation of open loop and closed loop control (current controllers, speed controllers, position controllers etc.) (f) On-line implementation of various mathematical computations (g) Processing the measured signals, such as voltage, current, speed etc. (h) Measurement of speed (i) Storing and processing the information of controlled quantities (j) Identification and adaptation of variable parameters (k)Adaptive control and optimisation (l) General sequencing control (m) Display, Monitoring, data logging andAlarms (n) Diagnostics and protections etc.
Control of Electric Drives using Microprocessor / Microcontroller / Digital Signal Controller DC Drives Induction Motor (Squirrel cage, Slipring) Synchronous Motor (Conventional, PM) Switched Reluctance Motor BLDC Motor Stepper Motor
Microprocessor controlled Dual Controlled DC motor Drive
Functions Performed by Microprocessor in a Typical Vector Controlled Drive i. Processing of the signals obtained from the shaft encoder to determine the rotor speed and also the rotor angle. This rotor angle has to be used in the transformations from one frame to another. ii. The flux estimation using the terminal voltages, currents and speed, based on one of the machine models. iii. The computations with respect to phase and coordinate transformations to identify the two components of the current. After the necessary control these components must be transformed to provide the reference values for comparison with actual phase currents. iv. The speed and current loops in the feedback control. The implementation of controllers in these loops.
v. To produce gate signals for both machine side converter and line side converter. The machine side converter decides the frequency whereas line side one decides the current/voltage. The firing signals to the line side converter are obtained in the same way as described for a dual converter. The firing delay is decided by the current/voltage required in the dc link. The firing signals to the machine side converter decide the frequency. The speed of the motor is added to the output of the slip controller to decide the frequency of the inverter output. The addition must be precise because a large quantity is added to a small one. The digital addition in a microprocessor is accurate. So the microprocessor must be capable of providing or generating the firing signals to the machine side converter also. vi. Data acquisition The microprocessor must acquire the feedback signals in the digital form. A transfer of the data to CPU is required. A flow of the data both from and to the processor is required. vii. Limiting the control Variables Linearisation of non-linear functions used in the control as well as non-linear behaviour of the converter during discontinuous conduction. The compensation of variable gain during discontinuous conduction and field weakening modes.
Microprocessor based control of a synchronous motor Microprocessor based control of a synchronous motor using terminal voltage sensing I/O Terminal
