24-12-2012, 02:56 PM
Digital Control of a Three Phase Induction Motor
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Abstract
Over the past two decades technological advances in power electronics and an increasing demand for high performance industrial machinery has contributed to rapid developments in digital motor control. This field of study has numerous applications in the areas of manufacturing, mining and transportation. Such is the diversity of digital motor control that it is sometimes difficult to determine which techniques are best suited to a particular application. The purpose of this thesis was to research, design, simulate and implement the most feasible induction motor control algorithm for use in an Electric Vehicle Application.
For those readers who are unfamiliar with the practical aspects of induction motor control a thorough review of electric machinery, power electronics and Pulse Width Modulation strategies was presented. A number of mathematical techniques that are particularly relevant to this thesis were also revised. The theory behind Volts per Hertz control, Field Oriented Control and Direct Torque Control was presented with a particular emphasis on each algorithms practical implementation on a DSP system. A thorough evaluation of these control strategies was also performed using criteria that included their computational complexity, steady state and dynamic performance, resilience to parameter variation and whether or not they could be operated on existing hardware.
Introduction
Major improvements in modern industrial processes over the past 50 years can be largely attributed to advances in variable speed motor drives. Prior to the 1950’s most factories used DC motors because three phase induction motors could only be operated at one frequency [1]. Now thanks to advances in power electronic devices and the advent of DSP technology fast, reliable and cost effective control of induction motors is now common place.
In 1997 it was estimated that 67% of electrical energy in the UK was converted to mechanical energy for utilization [2]. At the same time the motor drive market in Europe was in excess of one billion pounds. The increase in the use of induction motors was largely attributed to major oil and mining companies converting existing diesel and gas powered machinery to run off electricity [2]. Over the past five years however, the area of AC motor control has continued to expand because induction motors are excellent candidates for use in Electric or Hybrid Electric Vehicles.
In this application high performance control schemes are essential. Over the past two decades a great deal of work has been done into techniques such as Field Oriented Control, Direct Torque Control and Space Vector Pulse Width Modulation. Another emerging area of research involves the application of sensorless control. This differs from conventional methods because it doesn’t require mechanical speed or position sensors. Removing these sensors provides a number of advantages such as lower production costs, reduced size and elimination of excess cabling. Sensorless drives are also more suitable for harsh inaccessible environments as they require less maintenance. This undergraduate thesis thoroughly investigated the aforementioned techniques and used them to develop a Field Oriented Control Scheme for use in an Electric Vehicle.
Hardware Overview
The Induction Machine
Three phase induction motors are rugged, cheap to produce and easy to maintain. They can be run at a nearly constant speed from zero to full load. Their design is relatively simple and consists of two main parts, a stationary stator and a rotating rotor. The two main classes of induction motor differ in the way in which their rotors are wound. The rotor windings in a conventional wound rotor are similar to the stator windings and are usually connected in a uniformly distributed Wye [3]. The squirrel cage motor has a very different arrangement to this. A cage rotor consists of bare aluminium bars that are short circuited together by being welded to two aluminium end rings. The motor used in this thesis was a three phase squirrel cage motor.
Despite their benefits, induction machines have one major drawback, which is that that their speed is determined by the frequency of the supply. The reason for this can be better understood when the operating principle of the motor is studied. Unlike DC motors it is difficult to obtain decoupled control of the torque and flux producing components of the stator current. The issue is further complicated because there is no direct access to rotor quantities such as rotor flux and currents [3]. In an induction machine the alternating currents from the three phase source flow through the stator windings producing a rotating stator flux. The speed of rotation of this field is dependent on the number of poles in the motor and the frequency of supply. The field induces a voltage in the rotor bars, which in turn creates a large circulating current. Because the induced rotor current is in the presence of the rotating magnetic field it is subject to Lorentz’s force. The sum of the Lorentz forces on the rotor bars produces a torque that drives the rotor in the direction of the rotating field.
AC Induction Motor Drives
Power electronic devices known as motor drives are used to operate AC motors at frequencies other than that of the supply. These consist of two main sections, a controller to set the operating frequency and a three phase inverter to generate the required sinusoidal three phase system from a DC bus voltage.
Three Phase Voltage Source Inverter
The most common three phase inverter topology is that of a switch mode voltage source inverter. This generates an AC voltage from a DC voltage source when a Pulse Width Modulated waveform is used to switch the MOSFETs in each of the three converter legs. Although the power flow through the device is reversible, it is called an inverter because the predominant power flow is from the DC bus to the three phase AC motor load. Bi-directional power flow is an important feature for motor drives as it allows regenerative breaking i.e. the kinetic energy of the motor and its load is recovered and returned to the grid when the motor slows down [4]. In an AC grid connected motor drive, a second converter is required between the drive and the utility grid, which acts as a rectifier during the motoring mode and an inverter during the breaking mode. An additional benefit is unity power factor (sinusoidal) current flows to or from the grid. In an electric vehicle application, the energy for the DC bus is supplied directly from the batteries, or primary energy source.
Pulse Width Modulation
Although the basic MOSFET circuitry for an inverter may seem simple, accurately switching these devices provides a number of challenges for the power electronics engineer. The most common switching technique is called Pulse Width Modulation (PWM) which involves applying voltages to the gates of the six MOSFETS at different times for varying durations to produce the desired output waveform. In Figure 3.1, Q1 to Q6 represents the six MOSFETS and a,a’,b,b’,c,c’ represent the respective control signals. In practice each switching leg may consist of more than two MOSFETs in order to reduce switching losses by paralleling the on resistance.
Sine-Triangle PWM of Three Phase Inverteres
One commonly used PWM scheme is called carrier based modulation. This uses a carrier frequency usually between 10 to 20 kHz to produce positive and negative pulses of varying frequency and varying width [5]. The pulse widths and spacings are arranged so that their weighted average produces a sine wave. Increasing the number of pulse per half cycle reduces the frequency of the output sine wave whilst, increasing the pulse widths increases the amplitude [1].
In sine-triangle PWM a triangular carrier waveform of frequency fs establishes the inverter switching frequency. This is compared with three sinusoidal control voltages that comprise the three phase system. The output of the comparators produces the switching scheme used to turn particular inverter MOSFETS on or off. These three control voltages have the same frequency as the desired output sine wave which, is commonly referred to as the modulating frequency, f1. The modulation ratio is equal to mf = f1/fs. The value of mf should be an odd integer and preferably a multiple of three in order to cancel out the most dominant harmonics as these are responsible for converter losses [3]. One limitation of the sine triangle method is that it only allows for a limited modulation index, so it doesn’t fully use the DC bus. The modulation index can be increased by using distorted wave forms that contain only triplen (multiples of three) harmonics. These form zero sequence systems where the harmonics cancel out resulting in no iron losses [5] [18].