Electrical Measurements

Energy related measurements concerned with electricity production and consumption are usually derived from three elements namely: the electrical current, voltage and resistance of the load. These three variables are directly linked with the energy or power consumption of the load and hence it is common to see a single meter combining these to give a direct reading of the energy consumption.
Multimeters are very useful test instruments. By operating a multi-position switch on the meter they can be quickly and easily set to be a voltmeter, an ammeter or an ohmmeter. Two devices can be used to measure the electrical measurements: analog and digital multimeters as shown below in Figure. 5.3.
Digital and Analog Multimeters Figure 5.3 Digital and Analog Multimeters
As far as power is concerned, the most common unit of power consumption measurement on the electricity meter is the kilowatt hour, which is equal to the amount of energy used by a load of one kilowatt over a period of one hour, or 3,600,000 joules. Some electricity companies use the SI mega joule instead. Modern electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) and finding the product of these to give instantaneous electrical power (watts) which is then integrated against time to give energy used (joules, kilowatt-hours etc). The meters fall into two basic categories, electromechanical and electronic, as shown in Figure. 5.4.
The mechanical electricity meter has every other dial rotating counter-clockwise. The most common type of electricity meter is the Thomson or electromechanical induction watt-hour meter, invented by Elihu Thomson in 1888. it work by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current.
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 true power. The true power divided by the apparent volt-amperes (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatthours. The simple models display that information on LCD. More sophisticated models retain the information over an extended period of time, and can transmit it to field equipment or a central location.
Measurement of Power Figure 5.4 Measurement of Power
Wind speed is the most important factor directly proportional to the power output of a wind turbine.
Various types of anemometer are used to measure the velocity, usually of air.
The ‘cup type’ air speed measurement
(Figure 5.5) is used for free air and has hemispherical cups on arms attached to a rotating shaft. The shape of the cups gives a greater drag on one side than the other and results in a speed of rotation approximately proportional to the air speed. Velocity is found by measuring revolutions over a fixed time.
Cup Type Anemometer Figure 5.5 Cup Type Anemometer
The ‘vane anemometer’
(Figure 5.6) has an axial impeller attached to a handle with extensions and an electrical pick-up which measures the revolutions. A meter with several ranges indicates the velocity.
Vane Anemometer Figure 5.6 Vane Anemometer
The ‘hot-wire’ anemometer’
(Figure 5.7) is a probe terminating in an extremely small heated wire element when subjected to a fluid stream it cools to an extent, which depends on the velocity of the fluid passing. The resulting change in resistance of the element is measured by a bridge circuit and is related to velocity by calibration.
50 Figure 5.7 Hot-Wire Anemometer
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