This paper presents an advanced three phase inverter topology the Source Inverter and its control using microcontroller 89C52. Inverter employs second order filter network at front end which provides unique buck-boost feature for inverter. The inverter can be controlled by any traditional PWM method. Here the modified maximum constant boost PWM method is utilized for inverter control. The microcontroller 89C52 is used to generate PWM pulses and to control operation of inverter. The complete hardware is designed to drive the three phase induction motor. The hardware design involves the design of control circuit, driver circuit, Source network, main inverter bridge, power supply etc. The inverter is implemented and tested to verify the inverter concept. The desired three phase PWM signals are generated by using control circuit and detailed hardware results are presented.
CHAPTER ONE
INTRODUCTION
Over the year‟s induction motor (IM) has been utilized as a workhorse in the industry due to its easy build, high robustness, and generally satisfactory efficiency. By tradition, 6-switch 3-phase inverters have been widely used for variable speed IM drives. The investigated the performance of a 4-switch, 3-phase inverter fed cost effective induction motor in real time, which has been implemented by vector control . A standard three-phase voltage source inverter utilizes three legs [six-switch three-phase voltage source inverter , with a pair of complementary power switches per phase. A reduced switch count voltage source inverter [four switch three-phase voltage source inverter uses only two legs, with four switches. Several articles report on structure regarding inverter performance and switch control. This paper presents a general method to generate pulse width modulated (pwm) signals for control of four-switch, three phase voltage source inverters, even when there are voltage oscillations across the two dc-link capacitors. The method is based on the so called space vector modulation, and includes the scalar version. This permits to implement all alternatives, thus allowing for a fair comparison of the different modulation techniques. The proposed method provides a simple way to select either three, or four vectors to synthesize the desired output voltage during the switching period. In the proposed approach, the selection between three or four vectors is parameterized by a single variable .The influence of different switching patterns on output voltage symmetry, current waveform, switching frequency and common mode voltage is examined. induction motor can only run efficiently at ow slips, i.e. close to the synchronous speed of the rotating field. The best method of speed control must therefore provide for continuous smooth variation of the synchronous speed, which in turn calls for variation of the supply frequency. This is achieved using an inverter to supply the motor.
Variable frequency inverter-fed induction motor drives are used in ratings up to hundreds of kilowatts. Standard 50 Hz or 60 Hz motors are often used, and the inverter output frequency typically covers the range from around 5-10 Hz up to perhaps 120 Hz. This is sufficient to give at least a 10:1 speed range with a top speed of twice the normal (mains frequency) operating speed. The majority of inverters are 3-phase input and 3-phase output, but single-phase input versions are available up to about 5 kW, and some very small inverters (usually less than 1 kW) are specifically intended for use with single-phase motors.
A fundamental aspect of any converter, which is often overlooked, is the instantaneous energy balance. In principle, for any balanced three-phase load, the total load power remains constant from instant to instant, so if it was possible to build an ideal 3-phase input, 3-phase output converter, there would be no need for the converter to include any energy storage elements. In practice, all converters require some energy storage (in capacitors or inductors), but these are relatively small when the input is 3-phase because the energy balance is good. However, as mentioned above, many small and medium power converters are supplied from single-phase mains. In this case, the instantaneous input power is zero at least twice per cycle of the mains (because the voltage and current go through zero every half-cycle). If the motor is 3-phase (and thus draws power at a constant rate), it is obviously necessary to store sufficient energy in the converter to supply the motor during the brief intervals when the load power is greater than the input power. This explains why the most bulky components in many small and medium power inverters are electrolytic capacitors.
The majority of inverters used in motor drives are voltage source inverters (VSI), in which the output voltage to the motor is controlled to suit the operating conditions of the motor. Current source inverters (CSI) are still used, particularly for large applications, but will not be discussed here.
1.2 OBJECTIVE OF THE PROJECT
The objective of this work is to provide for continuous smooth variation of the synchronous speed, which in turn calls for variation of the supply frequency of an induction motor. This is achieved using an inverter to supply the motor using programmable control. A complete speed control scheme which includes tacho (speed) feedback.
1.3 SIGNIFICANCE OF THE PROJECT
The long standing desire to be able to adjust the speed of AC induction motors electronically became a reality in the early 1980’s. Called Adjustable Speed Drives, Variable Frequency Drives or just Inverters, they caught on quickly due to the many advantages they offer.
Equipment builders and plant engineers quickly saw the advantages of matching the machine’s speed to process needs and variables. Modern drives can control starting currents, maintain precise set speeds, quickly change speeds, control reversing and quickly stop. This makes machinery of all types more productive, improves quality and more flexible with quick change over to run different materials with a minimum of downtime. Multiple machines can be coordinated to have the right number of parts come together in the right place, and at the right time.
Another great advantage is the potential for energy savings. Variable torque loads like fans and pumps that vary their output by mechanical means can now adjust the motor’s speed and reduce energy input by 25% to 50% with the simple addition of an inverter. They can also continuously adjust the speed to maintain a desired situation under varying system conditions.
Inverters have taken their place in machinery of all kinds: metalworking, woodworking, chemical processing, water treatment, conveyors, heating, cooling, refrigeration, hoists, and just about every industrial or commercial process. The advantages are so obvious and numerous that today there are millions of them in use and will continue to grow well into the next century.
1.4 LIMITATION OF THE PROJECT
Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at a constant speed unless you use a variable-frequency drive; the speed of DC motors is much easier to control simply by turning the supply voltage up or down. Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. Unlike DC motors, they can't be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That's because they need a changing magnetic field to turn the rotor.
Early inverters caused significantly higher temperature rise in the motor, and mismatched could easily burn out the motor. As new transistor devices and software attempted to minimize this effect, they introduced other stresses on the motor’s insulation system. It is time to design motors specifically to operate on these new power sources. New IGBT, PWM inverters can output very high switching frequencies, very rapid changes in voltage, and transient voltage spikes that can burn pin holes in the motors insulation causing short circuits and premature motor failure.
Insulation systems must be improved to prevent this cause of unscheduled and costly downtime. Other issues are increased motor noise and bearing failure.
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