﻿ Wattmeter

# Wattmeter

A power meter or power meter is a meter that measures the power ( unit watts). This may be mechanical power, thermal power, electric power or radiation power.

Colloquially, electric power meters, especially electromechanical meter movements, also known as a watt meter.

• 2.1 Calorimetric measurement
• 2.2 Photometric measurement

## Measuring the electrical power

### Active power measurement

To determine the electrical power in an electric circuit, a power meter is not required. When DC voltage is the electrical power

With U - voltage

And I - electric current

The overall performance at DC is thus possible by measurement of current and voltage.

In AC voltage contact instantaneous values ​​of the power in accordance with the instantaneous values ​​of voltage and current. The instantaneous values ​​are in reality often not proportional to each other ( phase shift, distortion of one of the sizes), otherwise you could multiply the measured rms values ​​of current and voltage even when AC voltage to get the power. In the general case one has to distinguish between active power, reactive power and apparent power. The active power is defined by

Or

The latter represents an averaging of the instantaneous power over time represents the integration time must at periodic alternating currents with a fundamental frequency f be an integer multiple of the period T = 1 / f. This possibility is often used by the digital measuring instruments. But places high demands on equipment, can in a wide frequency range and with strong harmonic voltages or containing high harmonic currents ( high crest factor ) work. For analog instrumentation is generally by thermal or mechanical inertia, an integration time T before.

Practically, the measurement task using the following methods be satisfactorily resolved only with AC circuits of low frequency and not too high harmonic content:

• Electromechanical method with electrodynamic measuring equipment. It acts as a moving coil, which moving coil is, however, in the field of an electromagnet through which current of the current path flows. The current through the rotating coil is supplied with a series resistor in the voltage path; the multiplication results from the Lorentz force; the mean value is formed by the mechanical inertia. Such measurement instruments can also withstand higher harmonic content and reactive power components.
• Electronically analogous method: It works on the principle of TDM (Time Division Modulation). The voltage modulates a pulse width; the current modulates a pulse height; is demodulated with a low-pass ( mean value former ), which separates the ac component of the modulation frequency, and measures frequency and provides a direct current signal as a measure of the multiplied sizes.
• Other electronic methods: Four - quadrant multiplier: Some modules (integrated circuits ) containing the function of a four-quadrant multiplier, they are used to high in the kHz range. They work on logarithms, addition and Delogarithmierung. An averaging ( low-pass ) is to be connected.
• Digital power meter: instantaneous values ​​of current and voltage are digitized with a sampling rate as high as possible with analog - to-digital converters and charged in a microprocessor. The devices are often able, in addition to the active power further readings to win (reactive power, crest factor, apparent power, RMS voltage, RMS current, harmonics) and also to allow three-phase measurements.

For the application of non-digital power meter at various measurement tasks and customary for this purpose circuits see active power and reactive power.

For home use, offered cheap devices for measurement of active power and energy metering often have high measurement error at high reactive current or non sinusoidal current.

### Measurement of high-frequency power

In the high-frequency technology, the electric power on the measurement of the current heat in a resistor

Determined by measuring the heating associated with power consumption. The thermal inertia ensures averaging. The ohmic resistance R is a non-glare under RF conditions terminator with the size of the wave impedance (typically 50 Ω ) with low inductance and capacitance or impedance real line segment with a defined resistance 50 Ω. Anechoic statements in waveguides using absorbent wedges.

The increase in temperature is measured by temperature sensors along the path to the heat sink or by the temperature-dependent change in resistance of the sensing resistor itself purpose, it is acted upon by a small direct current measurement. At a defined wave impedance and the correct adjustment ( no standing waves ) to determine the high-frequency power is also possible via a peak - rectification: it come Schottky, germanium or gallium arsenide diodes can be used. The power is here

With

The display must take into account a correction curve, due to the forward voltage of the diodes used.

### Reactive power measurement

Common power meters have a current path and a voltage path. Multiply the instantaneous values ​​of voltage and current and average the instantaneous values ​​of the product. If active power meters are used for the determination of the displacement power factor must be taken to ensure that current and voltage values ​​are measured at 90 ° phase difference between the fundamental frequency. In mechanical devices, for example, in the voltage path, instead of the ohmic series resistor, a circuit can be used with inductive or capacitive series resistors, characterized the voltage measuring signal is compared with the actual voltage phase-shifted by 90 °. According to this principle works such as the Hummel circuit. The distortion power can not be detected in this way.

Digital devices digitize current and voltage (if applicable currents and voltages ) and then calculate the ( different ) reactive and apparent power ( s). With special programs and parameters can be determined approximately in some devices even speed and torque of the connected machine.

The apparent power can be single phase determined by multiplying the rms values ​​of current and voltage.

Power meter for electromagnetic radiation ( microwaves, laser radiation, light) work depending on the measurement task calorimetrically or photometrically.

### Calorimetric measurement

The calorimetric measurement requires an absorber which converts possible the total power into thermal energy. The thermal energy provides heating of a body or a body of liquid with a dependent of their heat capacity temperature gradient. Determining the amount of heat, either by a defined measuring time and measuring the temperature difference before and after the irradiation time, or determining the temperature gradient itself latter process is faster.

Simple device for determining the laser beam power consist of an absorbent coated metal body, a bimetallic thermometer is in the interior. Meanwhile pointer shaft passes through a pipe to a scale whose zero point can be adjusted mechanically. These devices ( jargon: " bone " ) are held for a defined time in the beam after the display has been set to zero. After irradiation time arises directly the power value of the calibrated scale in watts.

Another method is also often referred to incorrectly as calorimetrically: This one measures the temperature difference along a thermal conductor which connects the absorber to a heat sink (heat sink or water cooling). These are located on the thermal bridge temperature sensors. If one uses thermocouples, one obtains directly proportional to the output voltage signal.

Other thermal processes using pyroelectric sensors and bolometers / microbolometer ( bolometer arrays).

### Photometric measurement

The photometric measurement is applied to near-infrared, visible light and ultraviolet low power. It requires a photodiode with a known relationship between wavelength, power density and photocurrent and usually a diffuser and an integrating sphere.

While the link photo current / power must be calibrated frequency-and device-specific, the proportionality between photocurrent and performance over many orders of magnitude is given. The Messgeschwindigleit is very high ( even down to the nanosecond range ).

## Measurement of mechanical power

Mechanical power (usually the power output of engines) can with a Bremszaum, calorimetrically (brake heat) or be measured by means of a coupled to the motor shaft electric generator.

Another possibility of determining the data transmitted from a shaft power is the measurement of the rotation or the braking torque and the angular velocity and its multiplication.