Accurate Measurement of the Mains Electricity Frequency
Contents
1. Time critical parts of the program are written in Assembly language so that the pulse edges can be captured quickly and accurately Fig 8 shows a typical display of the measured frequency on the LCD BEGIN Initialise global program variables Configure LCD Configure UART DO FOREVER Wait for low to high transition of pulse Clear timer counter TMR1 Start timer counter TMR1 Wait for low to high transition of pulse Wait for low to high transition of pulse Get timer counter value Calculate the frequency Display frequency on LCD Send frequency to RS232 port Wait 5 seconds ENDDO END Fig 7 Operation of the microcontroller software Fig 8 Typical display of the measured frequency The microcontroller software is based on the popular mikroC 8 language which is a standard C language with additional support for libraries and features for PIC microcontroller programming mikroC offers a Graphical User Interface GUI type design enabling the user to program using the built in editor In addition the compiler offers simulation and in circuit debugging ICD features thus enabling the user to develop and test complex programs easily mikroC is fully compatible with the EasyPIC6 development board so that a compiled program can very easily be downloaded to the program memory of the target microcontroller chip via the in circuit serial programming ISP tools provided on the development board 149 5 2 The PC Software The PC softw2. and time of each data point is available The data can be stored for very long times such as for weeks months or even years 3 The Hardware There are basically two methods used in the literature for frequency measurement The first method which is not accurate involves setting up a time window and calculating the number of cycles within this window The second method which is more accurate and is the one used in this paper involves calculating the period of the waveform Here basically an accurate timer is 146 ELECO 2011 7th International Conference on Electrical and used to measure the period and hence calculate the frequency of the waveform The block diagram of the designed mains frequency monitoring device is shown in Fig 1 The operation of the device is based on a near zero cross detector circuit 2 Mains supply is reduced to 9V using a wall mains adapter As shown in Fig 2 the near zero cross detector circuit is made up of a bridge rectifier and a transistor Full wave rectified mains signal is applied to the base of the transistor The transistor is normally on and its output is low when the signal is high As the signal drops to 0 7V the transistor turns off and the collector voltage rises to the supply voltage 5V generating a pulse Fig 3 shows the rectifier output and the transistor output As shown in the figure three such pulses are obtained during a full period of the mains frequency These pulses are then
3. fed to one of the inputs of a PIC microcontroller The microcontroller starts an accurate timer when a pulse arrives The timer is stopped at the arrival of the third pulse Thus the timer count is proportional to the period and hence to the frequency of the waveform This timer count is converted into real frequency and is displayed on an LCD display In addition the data is sent to a PC using the serial RS232 port A Visual Basic 3 program on the PC receives the frequency data time stamps the data and then stores it in a file The frequency data can be displayed by the Visual Basic program or for more flexibility and statistical analysis it can easily be imported into Excel 4 and analysed or displayed Pzero ector SOCKET Fig 2 The near zero cross detector circuit 147 Electronics Engineering 1 4 December Bursa TURKEY AC Input V Time V Rectifier Output V V Fig 3 Three pulses in one cycle of the waveform Although the near zero cross detector circuit does not detect the exact zero crossing point of the signal it generates pulses at the same near zero points of the waveform thus making it possible to accurately measure the signal period Fig 4 shows simulation of the near zero cross detection circuit The circuit was simulated using the popular TINA circuit simulation suite 5 developed by DesignSoft Fig 4 shows the rectified full wave mains waveform together with the out
4. ELECO 2011 7th International Conference on Electrical and Electronics Engineering 1 4 December Bursa TURKEY Accurate Measurement of the Mains Electricity Frequency Dogan Ibrahim Near East University Faculty of Engineering Lefkosa TRNC dogan neu edu tr Abstract The frequency of the mains electricity supply is very important in many industrial and commercial applications This paper describes the design of a microcontroller based system for measuring the mains frequency accurately The measured frequency is displayed on an LCD and in addition the measured data is sent to a PC continuously for time stamped logging and offline analysis 1 Introduction There are several mains electricity power systems in use around the world 1 These different systems are characterised by their e Voltage e Frequency e Type of plugs and sockets used In general the type of plugs and sockets used is not a problem and passive adapters are available to convert between different varieties as long as the voltage and frequency are correct for the electrical device to be used In general we can divide the mains voltage and frequency usage in the world into four groups 100 127V 50Hz 100 127V 60Hz 220 240V 50Hz 220 240V 60Hz The voltage quoted is the root mean square and the peak voltage can be calculated by multiplying the voltage with ya or the peak to peak voltage is found by multiplying with 2 Jo The frequency used in mains e
5. are reads measured frequencies from the microcontroller inserts the current date and time and then stores the time stamped data in a file on the PC for offline processing This program is based on Visual Basic 6 3 The user starts and stops data logging by clicking the appropriate buttons on the main form of the program Data is stored with the fields being separated with a comma so that it can easily be imported to spreadsheet programs e g Excel for offline analysis For example the file can be opened in Excel and the data can be read into separate columns by specifying that the data fields are separated by commas Fig 9 shows a typical plot of the collected data after it is imported into Excel Here for demonstration purposes the data collection time was about 15 minutes Notice that the absolute date and time information of each record is shown on the horizontal axis j ANH are my Af Aa wa Tha if VV A A R A PA a E a m A xz j 9 Plotting the collected data using Excel 6 Conclusion This paper has described the design of a microcontroller based system for measuring the mains electricity frequency accurately The system is based on using an accurate counter module inside the microcontroller The designed system provides measurement accuracy in excess of 0 001Hz One of the nice things about this design is that the measured frequency data can easily be stored on a PC and then it can be analyzed offline using spreadsh
6. eet programs such as Excel The developed system can be enhanced further by including hardware and software to measure value of the mains voltage in addition to the frequency Thus both the frequency and the voltage variations can be displayed in real time and also they can be stored on the PC for offline analysis Another possible improvement is in the physical PC interface In the design presented here the connection to the PC is via the RS232 serial port Most computers nowadays offer USB ports as standard and they do not have RS232 ports It is possible to use a USB RS232 adapter for connecting the development board to the PC Alternatively a USB hardware and software interface can be designed on the development board for direct connection to the PC ELECO 2011 7th International Conference on Electrical and Electronics Engineering 1 4 December Bursa TURKEY 7 References Mains Electricity by Country Web Site http en wikipedia org wiki Mains electricity by country Accessed on 23 February 2011 R Elliot Zero crossing Detector and Comparators Web Site http sound westhost com appnotes an005 htm Accessed on 12 January 2011 Visual Basic User Guide Microsoft Inc Web site http www microsoft com Accessed on 2 March 2011 Excel User Guide Microsoft Inc Web site http www microsoft com Accessed on 3 March 2011 TINA User Manual Web Site http www tina com Accessed o
7. igger Horizontal Out2 2V Mode Source Time Div Single v 5m 5 Level Position a Sey Mode Storage ass YK Fun Stop x Source Store Erase Ouz z Channel 5 Vertical Duz Z Votts Div Coupling Ee g SI MA XE DX De AC Position YA YB DY iv e a Cursor Data To 7 z Ta 8 onf el 2 2 Gi a ie Fig 4 Simulation of the near zero detector circuit The project was built and tested using the EasyPIC6 microcontroller development board 7 shown in Fig 6 manufactured by mikroElektronika A small breadboard was used to construct the near zero detector circuit and then wire connections were made to the main development board Fig 5 Circuit diagram of the monitoring device 4 Operation of the Circuit The output pulses of the near zero detector circuit are counted using 16 bit timer counter TMRI of the microcontroller With an 8MHz crystal the counting period is 0 5Sus and maximum count is 65535 In a perfect 50Hz signal with 20ms period the maximum count will be 40 000 Table 1 shows the counter values at different frequencies of the mains supply The frequency f of the waveform is then given in Hz by 2x10 count 1 r g 2 E R p e LEOS AME ACTIORTED BY LOOC ONE 75V m mammy femne 4 Jue Ru tutos gncoonooo Promeomcng GC kekekehehene ener ae di a aAA a o Fig 6 The EasyPIC6 Devel
8. lectricity is either 50Hz 20ms period sinusoidal or 60Hz 16 66ms period sinusoidal Some of the appliances we use at home may be affected if the mains voltage is not correct For example the motor speed of some CD players may be affected even though the motor supply voltage is regulated The result of this is that the music can play slightly slower or faster Also some more sensitive appliances such as televisions may not operate correctly if the mains voltage is lowered As the demand on the electricity supply increases the frequency usually drops The electricity suppliers monitor the mains frequency constantly and aim to keep it within the allowed tolerances A change in the mains frequency has negative effects in some of the appliances For example the speed of AC synchronous motors depend upon the supply frequency and any appliance such as a turntable using such a motor will run slower or faster depending upon the changes Also some home or industrial clocks operate by counting the mains pulses and such clocks will run slower or faster depending upon the changes in the supply frequency By law the Electricity Board is required to supply 230V 10 6 i e between 216 2 volts and 253 volts In addition the frequency is required to be maintained at 50Hz 1 Le between 49Hz and 51Hz The European standard supply voltage is 220V and back in 2008 it was decided to change the European standard voltage to 230V and also the UK main
9. n 20 February 2011 PIC18F4520 Data Sheet Web Site http microchip com Accessed on 10 January 2011 EasyPIC6 Data Manual Web Site http www mikroe com Accessed on 12 March 2011 mikroC User Manual Web Site http mikroe com Accessed on 10 December 2010 150
10. opment Board By considering that a difference of one count can be measured the accuracy of the frequency measurement is then given by approximately 0 001Hz or 0 002 Table 1 Frequency and counter values 5 The Software The software consists of the microcontroller software or the measuring software and the PC software or the data logging software 5 1 The Microcontroller Software Fig 7 shows operation of the microcontroller software in the form of a Program Description Language PDL Counter TMR1 is cleared and internal counting starts on the high to low transition of the first input pulse on pin RC2 see Fig 5 The counting continues until the third pulse is detected on pin RC2 and stops on the high to low transition of the third pulse i e after a complete cycle of the waveform is received The frequency is then calculated using the equation given in 1 above Floating point calculations are used in the program for high accuracy 148 ELECO 2011 7th International Conference on Electrical and Electronics Engineering 1 4 December Bursa TURKEY The calculated frequency is displayed on the LCD in real time as well as it is sent to the PC over the serial link The PC receives the frequency adds date and time data to each record and then stores each record in a file for offline analysis This process is repeated forever with a 5 second delay between each measurement this time can easily be changed if required
11. put pulses of the near zero detector circuit on a virtual oscilloscope of TINA TINA is a general purpose low cost Spice based simulation program that can be used to simulate complex analog and digital circuits In addition microcontroller systems can also be simulated using TINA The circuit to be simulated is drawn using the built in graphical editor of TINA A large library of standard components are provided by the package In addition models of other components can be extracted from the manufacturers data sheets over the Internet and they can be loaded into TINA The package provides virtual instruments such as oscilloscopes and spectrum analyzers that enable the user to observe the waveforms A compatible hardware kit can also be purchased and used with TINA for real time simulation work Fig 5 shows full circuit diagram of the device A PIC18F4520 microcontroller 6 is used in the design with the timing provided with an 8MHz crystal PORT B of the microcontroller is connected to a 2x16 character LCD display UART output pin RC6 is connected to a MAX232 type RS232 TTL voltage level converter chip and then to the PC serial port via a 9 pin D type connector Output pulses of the near zero detector circuit are fed to port pin RC2 of the microcontroller ELECO 2011 7th International Conference on Electrical and Electronics Engineering 1 4 December Bursa TURKEY Oscilloscope Virtual J xq _ e Tr
12. s voltage to 230V Although this sounded easy in theory in practise it was too costly and uneconomic to change all the supply equipment to give 230V Instead the voltage limits were changed in UK and the UK voltage was kept at 240V which is within the allowed limits of 230V 10 6 At the same time the European standard was left at 220V 2 Measuring the Mains Frequency The expected variation of the mains frequency in UK is between 49Hz and 51Hz The mains frequency can in practise be measured using a simple frequency counter But here the problem is that we need to measure very small changes in the order of less than 0 01 and the cost of frequency counters to measure such small changes are rather high In addition we usually want to log the variations of the mains frequency over long periods of time and then to analyse these changes by for example plotting the results Most low cost frequency counters are not capable of logging the frequency changes In this paper the design of a microcontroller based mains frequency measuring and logging device is given The device has an LCD display that shows the frequency changes in real time In addition the frequency data is sent to a PC over the RS232 serial port and stored in a file on the PC The stored data can easily be analysed for example by plotting it or by using a statistical analysis package such as Excel The stored data is time stamped by the PC software so that the actual date
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