Field Orientated Control of a Multi-Level PWM Inverter Fed Induction Motor
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17-02-2011, 02:21 PM
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Field Orientated Control of a Multi-Level PWM Inverter Fed Induction Motorr
This thesis presents the simulation of a Field Orientated Control of a Multi level PWM Inverter fed induction motor system and its implementation in terms of programming and code in a real time operating system. Field Orientated Control allows precise controllability and excellent transient behaviour when used to control an induction motor by manipulating the angle and amplitude of the torque and speed producing current vectors. A closed loop feedback control system is employed to give the system stability and rapid response. The implementation of software code for the vector algorithms through a rapid phototyping interface will also be investigated in this project and implimentation
The control of a DC or AC induction motor involves a manipulation of the vector relationship in space of the air gap magnetic flux to the rotor current. From fundamental mathematics, a vector represents both the magnitude and direction of a variable, such as voltage or current. This type of AC variable speed drive gets it name from the fact that the system attempts to separately measure and control the two vector components that make up the overall stator current of the motor. Specifically, it attempts to measure and control the torque-producing current in an AC motor. In a DC motor the switching action of the commutator determines the position of the armature current in relation to the flux giving control over the torque of the motor. It is this aspect of the DC motor that makes precise control a relatively simple and effective procedure.
In an induction motor the rotating flux is responsible for setting up the rotor current and the relationship between them is a function of the slip and certain other variables. This makes controllability a more delicate process that requires some form of closed loop control system to modify the field vectors of the motor. This closed loop system that modifies vector components of the motor is what is known as Vector Control. Due to the vast popularity of AC induction motors in industry this type of delicate control is used frequently to control the speed and the torque of the induction motor. In applications with high dynamic requirements, where speed or load change rapidly, a better form of control is necessary. The dynamic response has to be at least 10 times better than that provided by standard variable voltage variable frequency drives. In the past, DC drives have been effectively used for these difficult applications because of their inherent ability to separately and directly control speed and torque. However, the high maintenance requirements of DC drives has encouraged the development of alternate solutions. Vector Control has evolved to provide a level of dynamic performance for AC drives which is equivalent to or better than DC drives. Thanks to ever advancing technological developments particularly in the area of semiconductor science and microprocessor and DSP refinement the capabilities and knowledge are now accessible to allow greater control over AC motor drives.
1.2 Project objective
The primary objective of this thesis was to simulate and test a Vector Control system with the intent to produce some physical implementation. This thesis is based on the continuation of past Field Orientated Control project and implimentations and provides corrections and advancements made in the simulation and implementation.
Implementation initially consists of the hardware component specifications and information of all sensors and equipment along with a complete blueprint of the Control System. This blueprint was constructed in such a way that it allows for the progression of this project and implimentation in years to come. A large component of the implementation is the code associated with the algorithms and the control system. A detailed prototype design of a Field Orientated Control system was produced in the C programming language but has not been tested. As a secondary objective the project and implimentation will also cover areas of Vector Control code for real time operating using a PC in a Linux environment.
1.3 Past Investigations
This section discusses the problems encountered with precious project and implimentation and the limitations of the current project and implimentation.
Initially the project and implimentation required a great deal of refinement in terms of motor parameter values and general system simulation setup. The values of inductance and resistance’s were determined based on comparison to other similar models as the values obtained from previous work were inconclusive. This led to the induction motor model and all transformations being reconstructed to allow for the desired change to generate the expected phase voltage and current waveforms.
Problems were also encountered in the Beta estimation schematic from the previous project and implimentation. A solution to this was the construction of a new rotor flux estimation to conform with the new design and the respective changes. The value of rotor position angle theta was discovered to be incorrect also due to the induction motor setup. This problem generated incorrect values of current and voltage as the value Beta is used in many of the torque and current estimation algorithms.
Considerable consideration into choice of Digital Signal Processors concluded that the use of the TMS320C40 DSP was not financially viable. The actual DSP chip was but the problem occurred in the creation of an evaluation board to allow the computer to program the DSP with the appropriate algorithms. To purchase or built a specific board was not within the university budget, therefore a financially viable alternative was found that allowed greater flexibility and reduced costs. The solution was to simulate the Vector Control system using a real time operating system RTAI in a LINUX environment with the I/O components connected to a parallel bus communicating with the computer.
Therefore on this basis a chapter was added to the project and implimentation with deals with the real time control of an induction motor based on a PC rather than a DSP. The Texas instruments TMS320 DSP will still be an important focus on the project and implimentation and if budget constraints do not apply can be implemented in future project and implimentations. Therefore data sheets and information is provided for reference.
2 Theory Review
2.1 Three phase induction motor
For industrial and mining applications the 3-phase AC induction motor is the prime mover for the vast majority of machines and processes. The beauty of such a device is that it can be operated directly from the mains or controlled by adjustable frequency drives such as PWM inverters. The importance of the AC induction motor to the economy is paramount as they are used in more than 90% of all motor applications for example driving pumps, fans, compressors, mixers, mills, conveyors and crushers. The popularity of such a motor stems to its simplicity, reliability and low cost making it a very economically viable choice. To clearly understand how a Vector Control system works it is essential to firstly understand the principal operation of the squirrel cage induction motor.
The AC induction Motor is comprised of two electromagnetic parts:
1) Stator which is stationary
2) Rotor which rotates about the ends supported by bearings
The stator and rotor are each comprised of an electrical circuit made of insulated copper or aluminium to carry current and a magnetic circuit usually made of laminated steel to carry magnetic flux. The stator the outer stationary part of the motor consists of an outer cylindrical frame, a magnetic path and insulated electrical windings. The outer cylindrical frame is made of some metal alloy which incorporates and mountings or support brackets. The magnetic path is comprised of a set of slotted steel laminations pressed into the cylindrical space inside the outer frame which is laminated to reduce eddy currents and hence reduce losses. The insulated electrical windings are placed inside the slots of the laminated magnetic path and in the case of a 3-phase motor 3 sets of windings are required.
The rotor or rotating part of the motor consists of a set of slotted steel laminations pressed together in the form of a cylindrical magnetic path and the electrical circuit. In the case of this project and implimentation specifically the squirrel cage induction motor is used as opposed to the wound rotor type. This type of AC induction motor is comprised of a set of copper or aluminium bars installed into the slots which are connected to an end ring at each end of the rotor. Thus the construction of this type of motor resembles a cage hence the name “squirrel cage” motor. The aluminium rotor bars are in direct contact with the steel laminations but the rotor current tends to flow through the aluminium bars not the laminations.
The connection of the stator terminals of an AC induction motor to a 3-Phase AC power supply induces a 3-Phase alternating current to flow in the stator windings. The presence of these currents establishes a fluctuating magnetic flux that rotates around inside the stator. This speed of rotation in synchronization with the frequency is named the synchronous speed. In its simplest form the induction motor consists of 3 fixed stator windings spaced 120 degrees apart. The flux completes one rotation for every cycle of the supply voltage and so on a 50Hz power supply the stator flux rotates at a speed of 50 revolutions per second or equivalently 3000 RPM. Therefore the number of poles of the motor is inversely proportional to its operating speed. The synchronous speed is a function of the number of poles of the motor and the supply frequency as shown in the relationship below, pfn1200×=
Where n0 Synchronous rotating speed in rev/min
f Power supply frequency in hertz
p Number of poles
Initially the voltage supplied from the magnetic field created by the stator current induces a current flow in the rotor bars. The rotating stator magnetic flux passes from the stator iron path, across the air-gap between the stator and rotor and penetrates the rotor iron path. Therefore as the magnetic field rotates the lines of flux cut the rotor conductors and consistent with Faraday’s law induces a voltage in the rotor windings which is relative to the rate of change of flux. A magnetic field is set up by the current flow through the rotor bars which is attributed to the short circuiting of the rotor bars by the end rings. It is this magnetic field that interacts with the rotating stator flux to produce the rotational force and in accordance with Lenz’s law the rotor will accelerate to flow in the direction of rotating flux.