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 timestamped 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:
·
·
·
Voltage
Frequency
Type of plugs and sockets used
2. Measuring the Mains Frequency
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
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%. i.e.
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 mains
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,
2.
or the peak-to-peak voltage is found by multiplying with 2
The frequency used in mains electricity 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
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 realtime. 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 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 Electronics Engineering, 1-4 December, Bursa, TURKEY
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 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.
Fig. 1. Block diagram of the frequency monitoring device
Fig. 2. The near-zero-cross-detector circuit
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 fullwave mains waveform together with the output 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.
147
ELECO 2011 7th International Conference on Electrical and Electronics Engineering, 1-4 December, Bursa, TURKEY
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. 6 The EasyPIC6 Development 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
Frequency (Hz)
49.0
49.2
49.4
49.6
49.8
50.0
50.2
50.4
50.6
50.8
51.0
5. The Software
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 TMR1 of the
microcontroller. With an 8MHz crystal, the counting period is
0.5μs 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:
f =
2 x10 6
count
Counter value
40816
40650
40485
40322
40160
40000
39840
39682
39525
39370
39215
(1)
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 realtime, 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).
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
5.2 The PC Software
The PC software 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.
END
Fig. 7. Operation of the microcontroller software
Fig. 9. Plotting the collected data using Excel
6. Conclusion
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 incircuit-serial programming (ISP) tools provided on the
development board.
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 spreadsheet 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.
149
ELECO 2011 7th International Conference on Electrical and Electronics Engineering, 1-4 December, Bursa, TURKEY
7. References
[1] “Mains Electricity by Country”, Web Site:
http://en.wikipedia.org/wiki/Mains_electricity_by_country
[Accessed on: 23 February, 2011].
[2] R. Elliot, “Zero crossing Detector and Comparators”, Web
Site: http://sound.westhost.com/appnotes/an005.htm,
[Accessed on: 12 January, 2011].
[3] “Visual Basic User Guide”, Microsoft Inc, Web site:
http://www.microsoft.com,[Accessed on: 2 March, 2011].
[4] “Excel User Guide”, Microsoft Inc., Web site:
http://www.microsoft.com, [Accessed on: 3 March, 2011]
[5] “TINA User Manual”, Web Site: http://www.tina.com,
[Accessed on: 20 February, 2011].
[6] “PIC18F4520 Data Sheet”, Web Site:
http://microchip.com, [Accessed on: 10 January 2011].
[7] “EasyPIC6 Data Manual”, Web Site:
http://www.mikroe.com, [Accessed on: 12 March, 2011]
[8] “mikroC User Manual”, Web Site: http://mikroe.com,
[Accessed on: 10 December, 2010].
150