MICROCONTROLLER BASED THREE PHASE INVERTER Project index: PRJ 012 By SANG GIDEON KIPCHIRCHIR F17/2161/2004 Supervisor: Dr.-Ing. W. Mwema Examiner: Mr. Ogaba
OBJECTIVE INTRODUCTION This project presents a design that will attempt to convert 12 V DC power from a solar panel to a three phase 120 V AC power at 50 Hz. The design is based on CMOS logic inverters made up of power MOSFETs and a microcontroller. DEFINITION An inverter circuit is used to convert DC power to AC power. The AC power produced can run regular AC appliances.
THE NEED FOR AN INVERTER Conversion of DC power from solar panels to drive AC loads. Uninterruptible power supplies. High voltage DC power transmission. Electric vehicle drives.
INVERTER DESIGN A CMOS logic inverter was implemented using power MOSFETs. Two sets of CMOS logic inverter circuits are used. There gates are driven by the antiphase signals generated a microcontroller. With the use of microcontroller, the output frequency could be altered with easy since its software manipulation. To produce an ac signal, current is made to flow in one direction for half a period then reversed in the next half period. The duration of the period determines the output frequency.
when the gate inputs of transistors Q1 and Q3 are L level (0 volts), and the inputs of transistors Q2 and Q4 are H level (5 volts), transistors Q1 and Q4 are turned ON while transistors Q2 and Q3 are OFF. Therefore, the electric current flows through the direction of A to B on the primary coil of the transformer. Q1 L Q3 N-MOS OFF A T1 B +12 V Q2 P-MOS OFF H Q4
Considering when the gate inputs of transistor Q5 and Q6 are H level and the inputs of transistors Q7 and Q8 are L level. Transistors Q6 and Q7 are ON while transistors Q5 and Q8 are OFF. Therefore, the electric current flows through the direction of B to A on the primary coil of the transformer. Q5 P-MOS OFF H Q6 A T2 B +12 V Q7 L Q8 N-MOS OFF
Gate drive signals The gate drive signals were generated by the AVR microcontroller. The ATtiny26L AVR microcontroller was chosen as the most appropriate source of gating signal because it has the following characteristics: It has an internal oscillator with frequencies ranging from 1 MHz to 8 MHz Most of its instructions are single clock cycle execution therefore executes faster. It is programmed by connecting some of its pins directly to some pins of the computer parallel port.
The desired output frequency is 50Hz hence a period of 0.02 seconds. To obtain the three phase square wave AC signal, the three phases must be 120 out of phase. For the three phase waveforms, at every one sixth of the period, one of the three waveforms will either be changing from high to low or from low to high. This is achieved using a progressive delay of one sixth of the period R1, Y1 and B1 and corresponding anti-phase signals R2, Y2 and B2 are taken from the pins of the microcontroller. R1 0 R2 Y1 Y2 B1 0 0 B2 0 3 6 9 12 15 18 21 24
The microcontroller frequency used is 1MHz, implying that one machine cycle takes 1 microsecond. A delay of 3333 microseconds, was to be created but instead, the two 8-bit registers were loaded with a value 3580 and decrementing the value while monitoring the content of the register. The value was attained after several trials and took care of the time taken to execute the various instructions. When the value is zero, then the microcontroller clears one pin and sets another pin and the value loaded to the registers and decremented again.
Switching circuit The switching circuit for each phase consists of two CMOS logic inverters with their gates driven by two anti-phase signals from the microcontroller. The design is based on the saturated switch approach where high efficiency is achieved because transistors dissipate very little power. 0 12 V 9 Q1 Q5 Q9 R1 1 Y1 18 B1 20 Q3 Q6 Q10 0 0 0 Q2 Q7 Q11 5 7 R2 2 Y2 19 B2 8 10 3 Q4 Q8 Q12 0 0 4 6 0 T1 T2 T3 IRON_CORE_XFORMER IRON_CORE_XFORMER IRON_CORE_XFORMER
IMPLEMENTATION IRF9540 PMOS and IRF830 NMOS power MOSFETs were used in the actual implementation of the CMOS logic inverter mainly because they have a freewheeling diode internally connected between their drain and source An LM7805 voltage regulator was used to power the microcontroller. Its input voltage was 12V from the laboratory power supply and the output was a stable 5.1V.
Gate drive circuit The output of the microcontroller was a square wave of amplitude voltage 2.2V. This voltage could not drive the gates for the CMOS logic inverter because the threshold voltage for the MOSFETs is 4.5V. For efficient switching of the MOSFETs, the gate drive voltage need to be in the range of 10-20 V. A BJT circuit was used for the gate drive. connection from the microcontroller output pins R2 R1 1kΩ Q1 connetion to the MOSFET gate 10kΩ BC107BP
Circuit implementation Microcontroller Gate drive circuits CMOS logic inverters Transformers
RESULTS OBTAINED The anti-phase square waveforms generated by the microcontroller pins were connected to the gates of the CMOS logic inverter. Both waveforms have the same frequency and duty cycle of 50%.
Waveforms obtained had 120 phase difference between red and yellow phase. The two waveforms were fed to the transformer primary windings and had same frequency and duty cycle of 50%.
The waveforms of the red and blue phases were similarly obtained, and had relative phase of 120. The two waveforms were fed to the transformer primary windings and had same frequency and duty cycle of 50%.
The waveforms of the yellow and blue phases were similarly obtained, and had relative phase of 120. The two waveforms were fed to the transformer primary windings and had same frequency and duty cycle of 50%.
CONCLUSION The output voltages obtained at the secondary coil of each transformer was 118 V AC at a frequency of 50 Hz. The three phases of the inverter implemented gave same values in terms of voltage and frequency. The current that the inverter can draw from the source will depend on the load to be driven.
RECOMMENDATION FOR FUTURE WORK The following recommendations are suggested for better performance, To obtain a proper sinusoidal ac power output, harmonic reduction methods should be employed. To ensure high switching speed of order of 100 nanoseconds, a proper charging and discharging circuit should be provided to every CMOS logic inverter gate.
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