CAREER EPISODE 2
A. INTRODUCTION
CE 2.1
The work I carried out as my academic project during my bachelor of engineering in Electronic Engineering from Karachi Institute of Economics and Technology was titled as “Design of Pure Sine Wave Inverter”. Details of the same have been presented in the mentioned episode. I began to work for this task on this date and completed it on this date.
B. BACKGROUND
CE 2.2
The development of a pure sine wave inverter was taken for project consideration, as pure sine wave inverter provides a minimum level of harmonics and better noise reduction. For the design, the boost converter was used for stepping up the dc voltage, and then pulse width modulation would convert the DC voltage to the AC voltage. The Arduino Nano microcontroller was used for control function and generation of PWM signal. This signal would get an isolation and then the complementary PWM wouild be generated with the insertion of deadtime, which would be then given to Half-bridge gate driver for Mosfet operation for one side.
CE 2.3
The list of objectives that were a point of focus for this project is mentioned below:
To design the circuit diagram to convert of DC into AC required for the proposed inverter system
To generate the sine PWM signal with automated changeover and feedback loop
To reduce the harmonics and ensure the safety and optimize the performance of inverters
CE 2.4
This was the academic project that I did in a team with me as a team leader overseeing the project conduct, planning and managing the project. Along with the duration of the project, I executed different tasks to accomplish the project. I oversaw the entire managerial works which involved preparation of a proposal, planning the project, scheduling, time management, team management, preparation of project report, etc. I designed the working circuit diagram for the pure sine wave generator circuit, performed different calculations to make appropriate choices of the components to be used. I assembled the circuit and programmed the microcontroller for the generation of the PWM signal. I was involved in the testing of the system.
CE 2.5
The following hierarchical diagram shows the hierarchy of the organization being maintained during which the project was carried out. My position is shown highlighted in the chart below:
Fig: Organizational chart
CE 2.6
Different tasks required to be performed for the execution and accomplishment of the project. Those tasks are mention below,
To design the power supply for components like optocoupler, microcontroller, gate driver, etc.
To program the microcontroller for a generation of PWM signal
To provide the isolation between the generated PWM signals using HCPL 2630
To implement the LC filter circuit in order to obtain required frequency output
To insert the dead time for the safe operation of Mosfet
To implement logic gates for the design of the functioning of the circuit
C. PERSONAL ENGINEERING ACTIVITY
CE 2.7
I designed the circuit that used the combination of H-bridge circuit, LC filter circuit, a gate driver circuit, optocoupler circuit and delays circuit for the proposed pure sine wave inverter system. I took 60V DC for the inverter design and stepped it up using the boost converter. I generated tri-level PWM by comparing it with a reference sine wave with a modified triangular wave by using a microcontroller. I used Optocoupler in order to isolate the PWM signal, and 2nd order low pass filter circuitry for maintaining the actual signal by filtering out unwanted frequency. I introduced a delay between the switching of Mosfets for safe operation. For powering up the gate driver, delay circuit and optoisolator, I used another power source using 7812IC to get 12V DC and 7805 to get 5V dc.
CE 2.8
I used my understanding of the various principles of electronics engineering I gained from various academic subjects in this project, like electronic devices and circuits, power electronics, combinational logic circuits, digital signal processing, linear integrated circuits, and devices, etc. I used my understanding of microcontroller architecture and pin configuration for pin assignments, programming the microcontroller, interfacing, etc. I reviewed IEC 60617 standards for the reliability of the electronic components used in the project. I reviewed the IEEE 1801-2013 which is an IEEE Standard that specifies the utilisation of IC for the reliability of the electronic circuit used in the system, and also I referred to IEEE C57.18.10-1998 Standard as well.
CE 2.9
CE 2.9.1
I started reviewing the literature on types of inverters, studying their design, advantages, applications, drawbacks, comparing in reference to distortions, harmonics, and efficiency. I found that harmonic content was lowest for the pure sine wave inverter and have distortion very low for the waveform generated using PWM techniques. I then proceeded with designing the proposed inverter system by preparing the block diagram that contained all the functional blocks, working circuitry diagrams that showed the entire connection among the various components used in the system and the process flow on how the system was supposed to work. I determined the project requirements i.e. the components for the development of the system.
Fig: Block diagram of the proposed PSWI system using a microcontroller
CE 2.9.2
As shown, I used Atmega328 microcontroller of Arduino Nano board for generation of PWM signal through PWM pins of the Arduino Nano, and applied dead time circuitry for modifying the PWM signals, and passed on to gate circuit where the gate circuit would amplify the low power input to high current input that I supplied as the input for the MOSFET gates of the H-bridge. I implemented the LC filter circuit for getting the desired frequency output. Starting with the hardware design, I first worked on generation of PWM signal, which is a function of Timer register and prescale, crystal oscillator and PWM resolution, given as . I used Timer 2 for two PWM signals differing in duty cycles however having the same frequency and connected it to the pins 3 & 11 of Arduino nano. In order to attain the desired max. the resolution, I changed the timer’s Prescaler as carrier frequency was 16MHz as per output filter design.
CE 2.9.3
From the list of prescale values, I selected 1, which resulted in a frequency, , as this frequency, was appropriate for the reduced filter size and for generation of sinewave signal having less Harmonics. The PWM signals of 31.25 kHz frequency had to be converted to Sinewave PWM, which is a function of the duty cycle. As Arduino nano would be slow with processing power to calculate mathematical sine functions, I stored the pre-calculated value of sine in a look-up table so the controller would pick up the value from it than calculating sine function each time. For the generation of the table, I used Matlab command: , where . I used Timer 1 interrupt for updating the duty cycle from this table consisting of the half-sine wave from 0-180deg so that the index was reset when the last value was reached and the PWM channel was switched. The system required power supply of own for powering up opto-isolator, gate driver and a logic gate. I used a step down transformer, a rectifier circuit, and a capacitance filter to convert the 220V AC supply to DC voltage of 15.5V. For the gate driver, I required 12V supply which I obtained through 7812 voltage regulator IC. I also needed 5V for the logic gates, so I used the 7805 voltage regulator IC.
CE 2.9.4
I calculated the update time for Timer1 from the requirement of updating 250 values in 10ms i.e. I set the Timer1 delay 40µsec to update the duty cycle and switch the PWM channel after 10msec to change the polarity of the sinewave. The output waveform from the microcontroller is shown below:
Fig: SPWM waveform from the microcontroller (SPWM1-pin3-blue, SPWM2-pin11-yellow)
I then used HCPL 2630 for isolating the PWM signals, where I provided the microcontroller ground on one side with isolation between the inverter ground on the other side. Then, for obtaining the complementary signal for these PWM signals, I used NOT gate, so that Q1 was on when Q2 was off and vice-versa, and similar operation for Q3 and Q4 was achieved. Then, I utilized AND gate to designing a circuit that added a dead time delay of 4µs to the SPWM signals. The output signals were found out to be this nature,
CE 2.9.5
For the generation of high voltage, I used the bootstrap circuit. I connected the PWM signals of the microcontroller to the IR2110 gate driver circuit where two PWM signals were connected to the High-Input and Low-Input pins of the IR2110. With the use of the gate circuit, I turned the low power SPWM signals to high current signals which I served as inputs for the high powered MOSFETs of the H-bridge. I connected the Gate circuit’s output to H-bridge. With a combination of four MOSFET gates, the H-bridge converted the SPWM signals to sine waves with the switching configuration of gates that is shown below.
Higher left | Higher right | Lower left | Lower right | Voltage across load |
On | Off | Off | On | Positive |
Off | On | On | Off | Negative |
On | On | Off | Off | Zero potential |
Off | Off | On | On | Zero potential |
Table: H-bridge switch states
CE 2.9.6
With a successful switching configuration of H-bridge, I achieved a sine wave with a frequency of 31.25 kHz. I used an LC resonator circuit of 50 Hz cut-off frequency to act as a low pass filter and suppress the carrier frequency i.e. 31.25 kHz. I chose a 2.2µF capacitor and 1.2mH inductor to set the filter to 3.1KHz. I also added a varistor to the LC circuit to protect the load from the voltage spike. The final output obtained was:
Fig: Sine wave inverter output
CE 2.10 TECHNICAL PROBLEMS AND SOLUTIONS
I encountered a challenge during the grounding of the microcontroller as when I grounded the optocoupler used for isolation of PWM signals, a connecting path was formed which allowed passing of some current, which could have caused the damage to the microcontroller or Optocoupler device. In order to overcome this issue, I provided individual grounds to the microcontroller and optocoupler.
In order to obtain 50Hz sinewave, I used 2nd order low pass filter circuit to restrict the 31.25KHz carrier frequency. This consisted of a capacitor and inductor between H-bridge. Here, for obtaining the complementary signal of Q1 to provide to Q2 so that if Q1 was off, Q2 would be on, I used Not gate. But here, there was a challenge presented for alternate switching between these switches, which would cause a short circuit in case both were in ON state. In order to avoid this, I used delay circuitry between Q1 and Q2, which consisted of AND gate, capacitor, and Diode, for the safety of the system.
CE 2.11 CREATIVE WORK
It wouldn’t be unfair to say that the entire project was a result of creative work for designing of the pure sine wave inverter system. Like, rather than going with comparators summer circuitry for generation of PWM signal, I used a microcontroller for the purpose, which not only reduced the circuit complexity but also gave accurate PWM signal having varying duty cycle for achieving SPWM. Then, with the processing capacity of the microcontroller used in the project, it would have a long time to perform complex trigonometry functions of sine, so I added the look-up table containing the pre-calculated value of sine function. Also, I used a varistor to avoid a voltage spike that could have occurred across the load in the low pass LC filter.
CE 2.12
I maintained commendable synchronization with my project guide who I shared the signs of progress made in the project period and gained ideas on making the project more innovative and tackling the challenges that were encountered in the project. I made sure there were no communication loopholes in the team and so no one felt hesitated to share their ideas and thoughts on the project. I organized team meetings and held group discussions for the suggestions for the betterment of the project. I received motivation and technical support from the stakeholders as well. I was helped by my faculty staff and laboratory staff with the availability of the electronic components and other different issues.
D. SUMMARY
CE 2.13
The pure sine wave inverter system was developed successfully using a microcontroller for the generation of PWM signal with varying duty cycle, and SPWM with fewer harmonics. For the isolation of the PWM signals, optocoupler was used and delay circuitry was used to provide dead time delay. For filtering the carrier frequency of 31.2KHz from the obtained SPWM signal, the LC filter was used. The readings obtained from the CRO was satisfactory.
CE 2.14
I referred to the working diagram for the proposed pure sine wave inverter system and implemented the hardware as per the circuit. Here, I used a microcontroller to generate PWM signals, and I used the HCPL2630 optoisolator isolation of microcontroller from high voltages and noisy systems which has 2 channels and has a high speed. I provided the isolator ground from the other side than the microcontroller for the isolation needed.
CE 2.15
I gained a deeper technical understanding of various electronic components and microcontroller applications by conducting this project. My knowledge of Matlab and Arduino coding was also made better from this project about designing the sine wave inverter. I got to know about the project management skills and how to exercise the managerial role for the management of the team by exhibiting proper coordination skills, team leadership skills, etc.