Fill in the gaps with the words in the box. Be ready to speak about analog and digital wattmeters .

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.

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.

3. … the energy, it is necessary to complete the process by integrating over the observation time.

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.

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).

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.

7. Dedicated hardware and software performing instrument testing … into the meter to complete the device with the most advanced features.

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.

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.

10. The power signal at the output of the analog multiplier … to the input of a voltage frequency converter.

Date: 2016-04-22; view: 792