Three Phase Frequency Converter Quratulain Jamil 1, Hafiz Muhammad Ashraf Hayat 2, Haris Masood 3 1 Department of Electrical Engineering Wah Engineering College, University of Wah jamil0265@gmail.com 2 Department of Electrical Engineering Wah Engineering College, University of Wah xmasherz030@gmail.com 3 Department of Electrical Engineering Wah Engineering College, University of Wah haris.masood@wecuw.edu.pk ABSTRACT The radar operates at a frequency of 400Hz but the frequency coming from main is 50Hz. So the project is devised to carter this problem. Secondly, the power supply units that are used to convert three- phase AC voltage from 50Hz to 400Hz are too big in size and are difficult to be carried to far-off places. It is designed to carter this drawback and is made compact yet cost effective. The project includes an AC to DC (converter) circuit and then DC to AC (inverter) circuit by switching DC voltage using MOSFETs or IGBT, to achieve 400Hz.The work cycle begins as the 3 phase AC is converted to DC using rectifier circuit and then capacitor is used to smooth the voltage. This DC voltage is then converted into 400Hz by using a 3 phase inverter circuit. The output of 3 different phases is then supplied to the radar. Keywords: IGBT s, Pulse Generation, Rotary converters, Six-pulse inverter circuit, Threephase inversion. 1. INTRODUCTION A radar operation requires an input of 3 phase AC voltage of 200 Volts 400 Hz rating. The desired rating of the frequency is obtained by power supply units that are attached to the radars. These power supply units work on rotatory converter principles to convert the frequency of the AC voltage coming from main grid (220 V 50 Hz).It consists of large generators and transformers which generates the voltage and frequency and then step down it to the desired level. The basic discrepancy of these power units was their size. The power units are of large sizes due to which they cannot be carried to distant areas of war or abandoned areas. Secondly, the power units used for operating radars are imported from foreign countries. This reason makes the power units highly expensive. Also the manufacturing of the power units is not common in our country and the spare parts are also not easily available and are needed to be imported from other countries in case of faults. Lastly their transport was a big issue because of their size. This need developed the desire of a compact yet efficient frequency converter. By understanding the problems of the prior technology, the three phase frequency converter was devised to be cost effective, compact and efficient. 2. METHODOLOGY The working of three phase frequency converter can be divided into 3 phases: 1
1) Rectification. 2) Three Phase Inverter. 3) Pulse Generation through Arduino. In the first phase of the project the incoming single phase AC voltage (220V 50Hz) from the grid is rectified using a simple bridge rectifier which is based on simple rectification techniques. The AC 220V is converted into 310V DC. Second phase of the project includes arduino, pulses are generated through arduino and is fed to the driver IC which controls the switching of IGBT s. Third phase involves design of a 3 phase inverter that converts single phase into three phases. This is done cascading three legs or 6 MOSFETs/IGBTs in series to obtain the following 3 phase output. The frequency of this output is set by changing the frequency of the MOSFETS/IGBTs. For this purpose arduino is used which controls the input to the driver IC hence controlling the switching of the MOSFETs/IGBTs. The three phases for the working of three phase frequency converter are illustrated in figure 1. and desired frequency is achieved. This DC voltage is fed to an inverter that converts the DC voltages in AC voltages of required frequency and ratings. And lastly, this frequency is used for operating radars. The main drawback of these power supply units is their size. These units are too big in size, as shown in figure 1 due to which they cannot be carried to far-off places especially war zones, deserts and abandoned areas. Hence the desire for a portable frequency converter emerges. 2.1 Rectification Rectification is a process in which alternating current is converted into direct current (unidirectional). The rectification process is a simple process in which AC is converted into pulsating DC using diodes as illustrated in figure 2. Figure 2: Simple rectifier circuit Figure 1: Block Diagram Block diagram illustrates that: Firstly three phase AC supply of 220V is supplied as input to a rectifier. Secondly rectifier converts an input of 3-phase 220V 50Hz AC into 310V DC. Next this DC voltage is fed to a MOSFET/IGBT driver circuit that steps up frequency up to 400Hz using arduino or microcontroller, In the positive cycle, the diode is forward biased i.e. positive voltage occurs across its positive end that allows the current to pass through it. And in negative cycle, current is not allowed to pass as it is reverse biased. The rectification method used is three phase rectification technique. Three-phase diode rectification converts a three-phase AC voltage input into a DC voltage as the output. It is called three phase rectification as it converts Three phase input into three phase output. The circuit and the waveforms at output of three phase rectification as illustrated in figure 3. 2
Figure 3: Three Phase Rectification 2.2 Three Phase Inverter A three phase inverter is a device that converts a single phase DC voltage into a three phase AC voltage. The circuit topology for a three phase inverter is cascading three legs of single phase inverters in series with the phase difference of 120 degrees between them. A three phase output can be obtained by a configuration of six transistors and six diodes as illustrated in figure 4 [1]. The type of three-phase inverter used in this frequency converter project is 6-pulse inverter, and will be explained in detail later. transistor conducts for 180 degree [3]. Three transistors in the circuit remain on at any instant of time. The switches cannot be switched on concurrently in any leg of the inverter circuit. This can cause short circuit across the dc link of the voltage supply in the circuit [4]. In this mode of operation three out of six switches are conducting at one time. Each switch (MOSFET/IGBT) is operated with an angle delay of 60 degree. Using this method of conduction we can easily obtain a pure sine wave. The output voltages are illustrated in figure 5 and 6. Figure 5: Phase Voltages Figure 6: Line Voltages Figure 4: Three Phase Inverter Two types of control signals can be applied to transistors 180 degree conduction and 120 degree condition that are [2]: 1. 180 degree conduction 2. 120 degree conduction In the project, 180 conduction mode is used for obtaining three phase output. In 180 degree conduction mode, each 2.2.1 Six pulse inverter circuit The purpose of inverter in our project is to convert the DC 220 volts into a three phase AC voltage signal with the frequency of 400 Hz. The inverter topology used in the project is three phase square wave inverter or 6 pulse inverter circuit as illustrated in figure 7. 3
and MOSFET s. The IGBT s like MOSFET s have relatively high input impedance. And like BJT s have low on-state losses, in case of conduction [5]. The IGBT used in the project is (FGA25NI20ANTD). The reason to use this IGBT among many IGBT s is: Figure 7: Six Pulse Inverter Circuitry There are six MOSFETs/IGBTs used in the circuit as switches. The three phase output is obtained by controlling the switching pattern of these switches. The basic catch in this circuit is that the switches are driven in such a way that the switches of same column are not turned on at the same time. The Proteus simulation of six pulse inverter is illustrated in figure 8. IGBT deals with superior conduction properties. It has better switching performance. It has high avalanche ruggedness and has stress-free parallel operation. The IGBT is appropriate for the resonant applications. It offers soft switching applications. 2.2.1.2 Optocouplers The name indicates that this device is used to couple isolated circuits. It is made of light sensing components. It is used to interconnect two isolated circuits by optical interfacing, using light. The optocoupler 6N137 is used to couple the isolated circuits of 6 pulse inverter and arduino. The main features are: Figure 8: Proteus Simulation of six pulse inverter Components used in Six pulse inverter circuit are: MOSFETs/IGBTs as switches. IR2130 Driver IC. Arduino. Optocoupler (6N137) 2.2.1.1 IGBT s IGBT are fast switching devices. They have both the characteristics of BJT s It has very high speed. The working voltages are double up to -480V. It has logic gate output. Output is strobable because of very high speed photo detector. It has an open collector. Temperature ranges between -40 C to +85. 2.3 Pulse Generation through Arduino The microcontroller board is based on ATmega2560. The arduino ATmega2560 has 54 input and output pins, out of which 14 can be used as PWM. 16 pins are 4
analog input pins. It has four hardware serial ports. It has everything that needs to support the microcontroller. It only needs a PC with a USB cable, and also can be powered by an AC to DC adapter as illustrated in figure 9. A battery can also be needed to get it started. Arduino s main advantage is that it is is very easy to code and interface with other sensors, and LCD etc It is open source which makes it better than microcontrollers. It s easily programmable and has many libraries, drivers and examples available for easy learning, while microcontrollers need complete hardware and software knowledge for making project. Figure 9: Arduino ATmega 2560 Features of the arduino ATmega 2560 are: It compromises microcontroller ATmega2560 Operating Voltages are 5V. Recommended input Voltage are between 7-12V. Input Voltage limits between 6-20V. It consists of 54 pins. It has 16 analog pins. DC Current per I/O Pin is 40 ma DC Current is 50 ma for 3.3V Pin. Flash Memory is 256 KB out of which 8 KB is used by the boot loader 16 MHz is the clock speed. 2.3.1 Advantages of arduino Arduino ATmega 2560 is used to generate pulses that are then fed to the driver IC which is IR2130, which controls the switching of IGBT s. The IGBT s used for switching are FGA25NI20ANTD. Instead of arduino microcontrollers can also be used but there are some advantages of arduino over microcontroller stated as below: 3. DESIGN To make a three phase frequency converter an additional keypad and a LCD has been used along with the arduino. There is a switch button that is used for switching on and off of the whole threephase frequency converter. The required frequency is then entered via keypad and is displayed on the LCD. The value of frequency is then fed to the controller (Arduino and six pulse inverter) which changes the switching frequency of transistor according to our desired frequency. For the controller we set the frequency of 400Hz as reference and all of the other frequencies are first compared with this reference frequency and then we get the desired switching time for the transistor, resulting in our required output frequency. For example, let the time for 400Hz was 2303 milliseconds. We multiplied the 2302 ms with 400 and then divided it with the required frequency entered via keypad e.g. the time for 50Hz is 19708 milliseconds for Arduino controller. So we divided the product of 400 and 2303 with 50 it result it approx. 19708 millisecond.hence through this way we achieved our task and made the VFD. 5
4. RESULTS AND DISCUSSIONS The project s main goal is to convert conventional frequency of 50 Hz into desirable frequency of 400 Hz, as many machines and devices operate on this frequency like radars. For calculation of results a digital oscilloscope is used. The digital oscilloscope shows the desired waveforms of line to line and line to neutral voltages of a three-phase 6 pulse inverter circuit. Frequency and other parameters like time period, rise time, fall time and RMS voltages are also determined. The project is capable of obtaining desired frequencies other than only the reference frequency i.e. is 400Hz. We will obtain calculations for different frequencies for verifying our results accuracy. 4.1 CASE 1 (50 Hz) The desired frequency is fed to the arduino through the keypad feature and the oscilloscope displays the required frequency that is converted with help of 6 pulse driver circuitry and VFD together. In case 1, the desired frequency is50 Hz and the oscilloscope shows the required line voltage waveform and measures parameters for e.g. 49.26 Hz. Conversion in micro second: 0.0203*1000*1000= 20,300 µs. Now the difference between input full time and output full time is due to the delay added by the arduino [4]. Figure 10 (a) and (b) illustrate the line voltages and phase voltages respectively, while (c) shows the measured parameters, with help of a digital oscilloscope. Figure 10 (a): Line voltages Figure 10 (b): Phase voltages Formula can be used to calculate the time period of the desired frequency by general formula: T=1/f Input=50Hz Full time=1/50= 0.02sec. Conversion in micro second: 0.02*1000*1000= 20,000 µs. Output= 49.26Hz Full time=1/49.26=0.0203 Figure 10 (c): Measured Parameters 6
4.2 CASE 2 (400 Hz) The desired frequency is fed to the arduino through the keypad feature and the oscilloscope displays the required frequency that is converted with help of 6 pulse driver circuitry and VFD together. In case 1, the desired frequency is50 Hz and the oscilloscope shows the required line voltage waveform and measures parameters for e.g. 362.32 Hz. Formula can be used to calculate the time period of the desired frequency by general formula: T=1/f Figure 11 (b): Phase Voltages Input=400Hz Full time=1/400= 0.0025sec. Conversion in micro second: 0.0025 *1000*1000 = 2500µs. Output=362.32 Hz Full time=1/362.32= 0.002759sec. Conversion in micro second: 0.002759 *1000*1000 = 2760µs. Now the difference between input full time and output full time is due to the delay added by the arduino. Figure 11(a) and (b) illustrate the line voltages and phase voltages respectively, while (c) shows the measured parameters, with help of a digital oscilloscope. Figure 11 (c): Measured Parameters 5. FUTURE RECOMMENDATIONS The Project has a tendency to be upgraded in near future by integrating it with any Renewable energy resources i.e. Solar, Wind etc. Moreover, portability of project can also be improved and more work can also be done to make it a more marketable and standalone product in foreseeable future. 6. CONCLUSION Figure 11 (a): Line voltages This project was chosen to solve the problem of rotary converters, are used to convert frequency for radars, induction motors and in aircrafts. The main goal of the project was to design and fabricate a frequency converter that could convert frequency from 50 Hz to 400 Hz and is 7
feasible, cost effective and most importantly concise and compact so that it can be easily carried to places of need. As the rotary converters are of very large size the repair, maintenance and transport of it was a major issue. The frequency converter in the project was designed to achieve frequency of 400 Hz AC three-phase. As for the milestones achieved at the end of the project, not only all the goals set in the start were achieved successfully but some additional tasks were also performed and tested successfully. The additional tasks included installation of LCD and keypad with arduino. As for the learning process, a lot was learnt during the course of the project. The project helped have a clearer concept of some major phenomenon in the field of electronics. It also helped in building a more practical approach and the problems relating to implementations of different circuit elements and topologies like transistor switching, DC-DC converters, gate drivers, Microcontrollers and high power MOSFETs, and frequency drives. Lastly, the project helped us in developing skills on working on our own and in establishing technical skills in field of engineering and technology... Conference on Mechatronics, 4(11), pp. 3-7. [3] Muhammad H. Rashid. 2003. Power Electronics Handbook, 3 rd Edition, Pearson. [4] Saied, Mohamed H., M. Z. Mostafa, T. M.Abdel- Moneim, and H. A. Yousef. 2012. New 13- space vector diagram for the three-phase six switches voltage source inverter, IEEE International Symposium on Industrial Electronics, 9(12), pp. 402-407 [5] Robert W. Erickson, Dragan Maksimovic. 2001. Fundamentals of Power Electronics, 2nd edition, Kluwer Academic Publishers. REFERENCES [1] Bose, Upama, K. Divya, Vallathur Jyothi, and Sreejith S. 2014. Performance analysis of four switch three-phase inverter-fed induction motor drive, Power and Energy Systems Conference: Towards Sustainable Energy, 2(1), pp.1-6. [2] S Satar, Mohamad Nasrul Abdul, and Dahaman Ishak. 2011. Application of Proteus VSM in modeling brushless DC motor drives, 4 th International 8