| LCD reading circuit true power device volt-amperes coils needle voltage current resistor power equipment |
The traditional analog wattmeter is an electrodynamic instrument. The consists of a pair of fixed , known as current coils, and a movable coil known as the potential coil.
The current coils connected in series with the , while the potential coil is connected in parallel. Also, on analog wattmeters, the potential coil carries a that moves over a scale to indicate the measurement. A flowing through the current coil generates an electromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a high-value connected in series with it to reduce the current that flows through it.
The result of this arrangement is that on a dc circuit, the deflection of the needle is proportional to both the current and the , thus conforming to the equation W=VA or P=VI. On an ac circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true , and possibly (depending on load characteristics) showing a different to that obtained by simply multiplying the readings showing on a stand-alone voltmeter and a stand-alone ammeter in the same circuit.
A modern digital electronic wattmeter/energy meter samples the voltage and current thousands of times a second. The average of the instantaneous voltage multiplied by the current is the . The true power divided by the apparent (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The simple models display that information on . More sophisticated models retain the information over an extended period of time, and can transmit it to field or a central location.
11. Choose the correct form:
1. The acting torque causes the disk... around its axis, this rotation … a dynamic equilibrium by balancing the torque Cm of the voltage and current coils and the reacting torque generated by permanent magnet.
| a) to rotate, reaches | b) rotation, to reach | c) rotating, reach |
2. The basic blocks of the conditioning system for a dc energy meter… from a voltage divider for the voltage input, and a shunt for the current input.
| a) formed | b) forming | c) are formed |
3. … the energy, it is necessary to complete the process by integrating over the observation time.
| a) calculating | b) to calculate | c) having calculated |
4. The most advanced solution for energy measurement can be found in all-digital meters where both the voltage and current signals… before any other processing.
| a) are sampled | b) have been sampled | c) sampled |
5. Accuracy of energy meters … defined by means of relative parameters (in percent) … from a testing process by powering the instrument with a constant (nominal) voltage signal and a variable current signal (5, 10, 20, 50, 100, 120% of the nominal value).
| a) are, obtaining | b) is, obtained | c) is, to obtain |
6. Sometimes, the system is equipped with a DSP capable of providing hardware resources… real-time evaluation of complex parameters (i.e., signal transforms) of the signal and energy measurement.
| a) to implement | b) implementing | c) implemented |
7. Dedicated hardware and software performing instrument testing … into the meter to complete the device with the most advanced features.
| a) were also integrated | b) are also integrated | c) also integrated |
8. The rotor position is shown by a mechanical counter (similar to the system mounted on the induction energy meters)… the total number of complete rotations performed by the system, proportional to the energy of the system under measurement.
| a) indicating | b) to indicate | c) indicate |
9. Electronic-Analog Energy Meters with Digital Output… the product of the two input signals (both voltages) through an analog multiplier that ... a voltage output proportional to the power of the input signals.
| a) providing, evaluates | b) to provide, evaluating | c) provide, evaluates |
10. The power signal at the output of the analog multiplier … to the input of a voltage frequency converter.
| a) is applied | b) was applied | c) applied |
Read and translate text B.
TEXT B
Static Energy Meters
The development of electronic multipliers led to their use in energy meters that directly multiply voltage by current. In their first version, electronic multipliers used analog components (operational amplifiers, resistors, capacitors, etc.), while recent devices use digital components and programmable logic systems. Voltage and current signals are processed to obtain a signal proportional to the real power flowing into the line. The result is integrated over the observation time in order to calculate the measured energy. The devices based on these components are completely static (i.e., they do not have any moving parts). Moreover, because these electronic components have a frequency range from dc to high frequencies, instruments based on them can be applied to dc, ac, or distorted power systems (some care must be taken in order to provide a correct sampling of signals in all-digital systems).
There are many different prototypes in this class of energy meters. The first realizations were based on analog multipliers and, even if they were not able to replace the traditional induction energy meters, they represented a good solution for all those applications where an increased accuracy was required (up to 0.2%). Now, more sophisticated digital instruments are under design and development, based on dedicated structures mainly implementing DSPs (digital signal processors) as powerful tools for numerical computation and sigma-delta analog-to-digital converters (ADCs) in order to optimize the conversion process.
Many of these instruments can be analyzed by means of the following functional descriptions.
Дата: 2016-10-02, просмотров: 157.
