One of the most important characteristics of an electrical circuit is its power. Using this parameter, the amount of work that the electric current performs in a certain unit of time is determined. All devices included in the circuit must have a power corresponding to the power of the specific network. To measure the power of electric current, a special measuring device is used - a wattmeter.
It is mainly needed in alternating current networks, determining the power of switched-on devices, as well as for testing networks and their individual sections, monitoring and monitoring the operating mode of electrical equipment, and accounting for consumed electricity.
Classification of wattmeters
Before measuring power with a wattmeter, current and voltage are first measured in the area under study. In order to obtain clear summary information, this data should be converted using wattmeters, which can be analog or digital.
For a long time, most of all measurements were carried out with analog devices, which in turn were divided into the categories of indicating and recording. They display the value of active power at a given section of the circuit. A typical representative is considered to be an indicating device with a semicircular scale and a rotating arrow. The scale is marked with a graduation corresponding to the values of the increasing power, which it measures in .
Another type, a digital wattmeter, refers to measuring instruments capable of performing. All such devices are equipped with a display, which, in addition to power, displays current, voltage, and energy consumption readings over a certain period of time. The most advanced devices are connected and allow the received data to be output to a computer located remotely from the measurement site.
Operating principle of an analog wattmeter
The basis of the design of the most common analog wattmeters is the electrodynamic system. Devices of this type make it possible to make the most accurate measurements and obtain the necessary results.
The operating principle of an analog-type wattmeter is based on two interacting coils. The first coil is stationary; its design uses a thick winding wire with a small number of turns and low resistance. This coil is connected in series with the consumer.
The second coil is in motion. A thin conductor with a large number of turns and high resistance is used for its winding. This coil is connected in parallel with the consumer and is equipped with additional resistance to protect against winding short circuits.
When the wattmeter is connected to the network, magnetic fields appear in the windings of its coils, interacting with each other. Due to this interaction, a torque is generated that deflects the moving winding by the calculated angle. This indicator is influenced by the product of current and voltage at a specified point in time.
How does a digital wattmeter work?
The basic principle of operation of a digital wattmeter is to preliminary measure the current and voltage in the test section of the circuit. A current sensor is connected in series to the load consumer, and a voltage sensor is connected in parallel. The main structural element of the sensor is the measuring transformer.
A household wattmeter, widely used at home, works on the same principle. Such a device just needs to be plugged into a power outlet to start the measurement process.
The basis of the device is a microprocessor, which receives the measured parameters of current and voltage, after which the power is calculated. The results obtained are displayed on the screen and simultaneously transmitted to external devices. The microprocessor itself contains elements, including microcontrollers, that allow you to automatically control operating modes and remotely switch measurement limits. With their help, the symbols of the measured quantities are indicated.
When working with high and medium power level converters, calibrate the digital device using a DC power calibrator. Self-calibration of the wattmeter is carried out by an AC power calibrator. All components and elements are powered through a DC power source built inside the measuring device.
The voltage coming from the receiving converter, plugged into the socket, is amplified by the DC amplifier - DC amplifier - to values that make the operation of the ADC - analog-to-digital converter unit - more stable. Next, the voltage proportional to the measured power is converted into a time interval filled with pulses of the reference frequency.
The number of these pulses, proportional to the measured power, will be displayed on the digital readout device. The received data can be entered into a special device designed for information processing.
Measuring device connection diagram
The accuracy of the data obtained will depend on how correctly the wattmeter is connected in a particular section of the circuit. The correct circuit for connecting a wattmeter is as follows: the fixed current coil of the measuring device is connected in series to the load or electricity consumers.
The moving voltage coil is connected in series with an additional resistance, and then this entire section is connected in parallel to the load. The moving part of the wattmeter has a certain angle of rotation, calculated by the formula: α = k2IхIu = k2U/Ru, in which I and Iu are the currents of the serial and parallel coils of the device, respectively.
Since the circuit uses additional resistance, the parallel circuit of the device will have an almost constant resistance (Ru). In this case, the rotation angle will be equal to: α = (k2/Ru)хIхU = k2IхU = k3P. That is, the power of the circuit will be determined precisely by this parameter.
The wattmeter has a uniformly applied measuring scale, made in a one-sided version, when the arrangement of divisions starts from zero to the right. When the electric current in the fixed coil changes its direction, it causes a change in the direction of rotation and torque of the moving coil. If the wattmeter is connected incorrectly and the direction of the current is different, the electronic device will not work.
For these reasons, you should not confuse the terminals that are used for connection. The series winding has a terminal for connection to the power source, called a generator terminal. A parallel circuit is also called a generator circuit and has its own required terminal to connect the section to the wire connected to the series coil.
During normal connection, the currents in the coils of the device from the generator terminals are directed to the non-generator terminals.
Electronic wattmeters based on electronic voltmeters there are parametric and modulation types. Parametric wattmeters are divided into direct and indirect conversion wattmeters.
Principle of operation parametric wattmeter ov with direct transformation is based on the implementation of a functional dependence of the form:
Thus, as a result of performing the specified mathematical operations with two signals, it is possible to obtain their product, which is what is required when measuring signal power. For this purpose, the current is first converted into voltage, and the squaring of the signal values is carried out using functional converters.
Rice. 9.5 Block diagram of a quadrature wattmeter.
Modulation wattmeters are based on double modulation of pulse signals (pulse width - PWM and pulse amplitude - PAM)).
IN electricity meters Time sharing uses a unique but accurate method of measuring electrical power. This device has two channels. One channel is an electronic switch that passes or does not pass the input signal Y (or the reversed input signal Y) to the low-pass filter. The state of the key is controlled by the output signal of the second channel with the “closed”/“open” time interval ratio proportional to its input signal. The average signal at the filter output is equal to the time average of the product of the two input signals. If one input signal is proportional to the load voltage and the other is proportional to the load current, then the output voltage is proportional to the power consumed by the load.
The error of such industrial counters is 0.02% at frequencies up to 3 kHz (laboratory ones are about only 0.0001%). As high-precision instruments, they are used as reference meters for checking working measuring instruments.
Sampling wattmeters and meters electric power meters are based on the principle of a digital voltmeter, but have two input channels that sample current and voltage signals in parallel. Each sample value representing the instantaneous values of the voltage signal at the time of sampling is multiplied by the corresponding sample value of the current signal obtained at the same time. The time average of such products is the power in watts:
.
An adder that accumulates the products of discrete values over time gives the total electricity in watt-hours. The error of electricity meters can be as little as 0.01%.
Induction counter is nothing more than a low-power AC electric motor with two windings - a current winding and a voltage winding. A conductive disk placed between the windings rotates under the influence of a torque proportional to the power consumed. This torque is balanced by currents induced in the disk by a permanent magnet, so that the rotation speed of the disk is proportional to the power consumption. The number of revolutions of the disk for a given time is proportional to the total electricity received by the consumer during this time. The number of revolutions of the disk is counted by a mechanical counter, which shows electricity in kilowatt-hours. Devices of this type are widely used as household electricity meters. Their error is usually 0.5%; They have a long service life at any permissible current levels.
The presence of two coils in an electrodynamic device and the possibility of connecting them to two different circuits allows these devices to be used for measuring the power of electric current, i.e., like wattmeters.
From the expression for the angle of rotation of the moving system of an electrodynamic device (2.12) it follows that if the fixed coil is connected in series with the load z (Fig. 2-12), and the additional resistance Pod is connected in series with the moving coil so that this coil can be connected in parallel with the load , then the current in the moving coil is equal to
where is the coil resistance; U - load voltage; - power constant of this device; P - power consumed by the load. Such a device is called a wattmeter. Its scale is uniform.
To measure electrical power in alternating current circuits, active and reactive power wattmeters are used.
Active power wattmeter. If an active additional resistance is included in the moving coil circuit so that the total resistance of this circuit R is equal to
then at voltage both in the network and at current i in the load
the current in the moving coil is equal to
The instantaneous value of the torque in this case is equal to
and the average value of this moment over the period
Therefore, a wattmeter with an active additional resistance in the moving coil circuit measures the active power of the alternating current circuit.
The conclusion obtained has a simple physical explanation. In fact, if an ammeter, voltmeter and wattmeter are included in a circuit with inductance (Fig. 2-13), then, since the moving system of the voltmeter rotates under the influence of only the applied voltage, regardless of the phase of this voltage (more precisely, under the influence of the current in the coil , proportional to the applied voltage), and the moving part of the ammeter rotates under the influence of only the current in the coil, regardless of the phase of this current. As for the moving part (coil) of the wattmeter, it rotates only when the currents in both coils are not zero, otherwise there will be no interaction. But in the circuit under consideration, the moving coil current is maximum when the current in circuit i is zero, and vice versa. The device will not show anything. This was to be expected, since the load either stores energy in the magnetic field or returns it to the network.
From the graph of the currents of this circuit with inductance (Fig. 2-14) it follows that the currents coincide in direction (on the graph - on one side of the time axis) only during two (every other) quarters of the period per period, and in the other two quarters period, the currents have opposite directions. This means that the direction of the torque changes four times per period. Therefore, the moving system of the wattmeter during the period will experience the action of four impulses of equal value, but opposite in direction, and the device will not show anything, since the torque acting on the moving system is determined by its average value over the period.
If the shift angle between the currents is small (Fig. 2-15), then during the period the positive values of the torque greatly exceed the negative ones (in time and in values) and the moving wattmeter system will rotate under the influence of the average
values in response to the active power consumed by a given load.
So, the wattmeter shows the active power consumed from the network.
Reactive power wattmeter. In this wattmeter, an inductive additional resistance (Fig. 2-16) is specially switched on in series with the moving coil such that
Let an applied voltage act in the circuit and the load create a current
Then the instantaneous value of the torque is
After substitution and transformations we get:
The average value of the torque over the period is
It follows that a wattmeter with inductive reactance in a moving coil circuit shows the reactive power of an alternating current circuit. This conclusion is explained simply: in the case, for example, of a purely inductive load, when energy is not irretrievably consumed from the network, such a circuit artificially shifts the phase of the current in the moving coil to coincide with the phase of the current in the stationary coil, so the wattmeter shows the value of reactive power.
So, an electrodynamic wattmeter has two coils: one is a current coil, connected in series with the load, the other is a voltage coil, connected in parallel with the load, the power consumption of which must be measured.
To correctly turn on the device (so that the arrow deviates in the desired direction), one of the terminals of its windings is marked with an asterisk; these terminals of the wattmeter are called generator terminals. They should be connected to the load terminal that is connected to the generator (mains).
electro-labs.com
Everyone has probably ever thought about the question of how much a particular household electrical appliance consumes. For example, how much energy does a TV consume in standby mode? How does the energy consumption of a refrigerator change in different operating modes? For these purposes, you will need an AC wattmeter, and in this article we will look in detail at the design of one of the device options (Figure 1).
| Picture 1. | |
It makes no sense to develop such devices for direct current due to the fact that in this case everything is very simply calculated using known laws and mathematical formulas, and only an ammeter is required from the measuring instruments. For alternating current, everything is a little more complicated and previously analog wattmeters for alternating current, although they provided high accuracy, were difficult to manufacture, not to mention digital wattmeters and the ability to assemble such devices at home. Modern technologies and element base make it possible to design multifunctional devices at minimal cost. Cheap microcontrollers (MCUs) with rich peripherals and powerful computing capabilities significantly simplify the creation of various automation and control systems. Integrated precision analog peripherals, and in some microcontrollers a digital signal processing subsystem, make it possible to develop multifunctional measuring instruments.
The digital wattmeter, the design of which we will consider, is designed to measure the power consumption of devices connected to an alternating voltage network of 207 - 235 V / 50 Hz. The main element of the wattmeter is an 8-bit PIC microcontroller of the company series, which, using external ADCs, measures the current flowing through the load, the voltage across the load, calculates the effective value of the voltage (effective value) in the network, the effective value of the current and the average value of power consumption. All specified parameters are displayed on a two-line character LCD indicator.
The device does not have a separate power source. A built-in mains power supply is used, due to which the microcontroller part of the device is completely isolated from analog nodes that are under mains voltage.
Schematic diagram
The circuit diagram and PCB design were developed in the free SoloPCB tools design environment. The schematic diagram of the device is shown in Figure 2. A complete list of components used is given in Table 2.
To calculate power consumption, we need to know the voltage across the load and the current consumed by the load. The voltage to be measured is the AC mains voltage, so it must be taken into account that it can be in the range of 207 V - 253 V. In order to increase the accuracy of measurements, it is necessary to measure the mains voltage rather than using a fixed average value of 230 V in calculations .
The mains power lines are connected to connector J1 (AC IN, AC input). The analog node for measuring network voltage consists of a resistive divider (R1, R2 R3), a precision reference voltage source (U3) and an ADC (U5). A resistive divider connected between phase and neutral is designed to reduce voltage scaling by a factor R1/(R1+R2+R3)=1/201. This way we lower the peak voltage of ±320V to the level of ±1.59V. We then use the REF03() voltage reference to offset this voltage up by 2.5V, resulting in a range of ±320V corresponding to the ADC input range of 0.91 V - 4.09 V.
After scaling and biasing, the voltage across resistor R2 is read by an A/D converter (U5) MCP3202 (Microchip) and transmitted in 12-bit format via the SPI interface to the microcontroller. To isolate the microcontroller from analog nodes, high-speed optocouplers HCPL-0630 are used. The second ADC channel is used to measure the 2.5 V reference voltage - this value will be used as a correction factor in the calculations.
The AC, neutral and ground lines from J1 are directly connected to the output connector J2 (AC OUT), the phase line passes through the current sensor (U4) ACS712-20A company. This is a low-noise analog current sensor based on the Hall effect with galvanic isolation from the measured line and the ability to measure direct and alternating current. To improve noise characteristics and measurement accuracy, there is a terminal for connecting a filter capacitor. At zero current, the output voltage of the sensor is 2.5 V. When current flows through the IP+ and IP- terminals, the output voltage of the sensor changes in accordance with a scaling factor of 100 mV/A, therefore, with a flowing current of +20 A, the output voltage will be 4.5 V and 0.5 V at current -20 A. The analog value of the current sensor is converted into digital form using another MCP3202 ADC chip.
The current sensor has a measuring range of ±20 A, but due to the current limitations of the connectors and fuse holder, the AC current sensing unit is protected by a 16 A in-line fuse.
A transformer power supply is used to power the analog nodes and microcontroller part (Figure 3). The transformer has two identical secondary windings, from which an alternating voltage of 6 V is removed. Next, the voltage is rectified and stabilized using a microcircuit (U1, U2) with a standard switching circuit. LEDs D2 and D3 are designed to indicate the supply voltage.
The wattmeter uses an 8-bit PIC18F252 microcontroller. It reads voltage and current values, calculates their rms values and averages the power consumption. An LCD indicator is connected directly to the MK, which displays the specified values. Both 4- and 8-bit operating modes can be used. To work with external ADCs, an SPI interface module integrated into the MK is used. Although the circuit uses a 20 MHz crystal, the microcontroller is clocked at 5 MHz. For programming the microcontroller, an ICSP connector (J3) is provided (Figure 4).
Table 1. List of components used.
| Designation in the diagram |
Name, denomination |
Frame, note |
| U1, U2 | 78L05 | SOT-89 |
| U3 | REF03 | SO-8 |
| U4 | ACS712-20A | SO-8 |
| U5, U10 | MCP3202-BI/SN | SO-8 |
| U6, U7, U8 | HCPL-0630 | SO-8 |
| U9 | PIC18F252-I/SO | SO-28 |
| BR1, BR2 | Diode bridge | 800 V / 1 A |
| TR1 | Transformer HR-E3013051 |
2 × 6 V, 1.5 VA |
| LCD1 | TC1602D | Two-line LCD indicator |
| C1, C18 | 470 µF 25 V | 10 mm × 10 mm |
| C2, C17 | 100 µF 16 V | 6.3 mm × 5.4 mm |
| C11, C12 | 22 pF 50 V | smd 0805, ceramics |
| C9 | 1 nF 50 V | smd 0805, ceramics |
| C2, C4, C5, C6, C7, C8,C10, C13, C22, C14, C15, C16, C17, C20 |
100 nF 50 V | smd 0805, ceramics |
| C21 | 1 µF 25 V | smd 1206, ceramics |
| R16 | 0 ohm | smd 0805, 1% |
| R2, R3 | 1 MOhm | |
| R5, R6, R17 | 1 kOhm | |
| R1, R14, R15, R18, R19 |
10 kOhm | |
| R7, R8, R9, R13 | 2.5 kOhm | |
| R4, R10, R11, R12 | 330 Ohm | |
| D2, D3 | Red LED | smd 0805 |
| D1 | Schottky diode | 1 A / 40 V, SMA housing |
| Y1 | Quartz crystal 20 MHz | |
| F1 | Fuse holder | For superficial installation |
| J1, J2 | Screw terminal block 1×3 | pitch 5.2 mm |
| J3 | Pin connector 1x5 | pitch 2.5 mm |
Printed circuit board
The PCB design was also done in the SoloPCB environment. Designing the instrument as a portable device was a good idea, with the PCB outline being designed in Autocad and then exported to the SoloPCB environment (Figure 5).
The printed conductors of the power lines (phase, neutral, ground) connecting the input (AC IN) and output (AC OUT) connectors are made as wide as possible, all blocking capacitors are located as close as possible to the microcircuits. The analog (AGND) and digital ground (DGND) buses are separate. All components are located on the top layer.
Note:
When designing the circuit and PCB in the SoloPCB environment, some elements that were missing in the libraries were created manually. A library of these elements is included in the archive with project files, which you can download in the downloads section.
Microcontroller program
As we noted above, the microcontroller reads voltage and current values every 1 ms and accumulates 40 measurements of each parameter, which corresponds to two periods for a frequency of 50 Hz. The RMS values and power consumption are then calculated. The 1 ms period is generated using the built-in Timer A, operating in 16-bit mode with an overflow interrupt signal.
After receiving all samples, the effective (rms) values of voltage and current are calculated using the formula:
It should be noted that the resulting samples also contain the phase relationship between voltage and current. Thus, the active power of alternating current, which is calculated by the formula ( V×I×cosθ) can be obtained by calculating the average power using the following formula:
All calculated values are displayed on the LCD indicator screen. To work with the indicator, the lcd.h library for the CCS C compiler is used.
The figures below show measurements using a digital wattmeter: Figure 6 - power consumption of the soldering station in heating mode, Figure 7 - 2 kW water heater.
Downloads
Listing of the source code of the microcontroller program (CCS C compiler) -
SoloPCB project files (schematic, printed circuit board, element libraries) -
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