Measurement of electrical quantities

Measurement of electrical quantitiesMade by Ceban DionisieVerified by prof. M. Chiorsac Types of measurementMeasurement:The measurement of a given quantity is an act of comparison between the quantity whose magnitude is unknown and a predefined standard. Since two quantities are compared the result is expressed in the form of numerical values. There are two methods of measurement:Direct methodIndirect method.Direct Measurement:-In direct method the unknown quantity is measured directly such as measurement of current by an ammeter, voltage by voltmeter, resistance by ohm meter, power by wattmeter etc.Indirect Measurement:-In the indirect method of measurement the unknown quantity is determined by measuring other functionally related quantities and calculating the desired quantity rather than measuring it directly with an instrument. For example resistance of a conductor may be determined by measuring voltage across the conductor V, and current flowing through the conductor I, and then calculating it byR=V/I

ClasificationThere are several types of measuring instruments depending on the functioning principle of the devices:Magnetoelectric devicesElectromagnetic devicesElectrodynamic devicesFerro-dynamic devicesElectrostatic devicesInduction devicesAccuracy class

Electrical instruments are in fact divided on the basis of their accuracy rating into the following categories in conformity to IEC standards:

These numbers represent the absolute errors in relation to nominal capacity and are stated as a percentage of nominal capacity.

This means that a 0.5 rating voltmeter with nominal capacity of 200 V must not have at any point of the scale an absolute percentage error that is greater than 0.5%. Thus, whatever the voltage value that is read on the instrument, this read value must not be more than 1 V higher or lower than the real value.

Ammeter

Transformer of current(indirect measurement)When the quantity to be measured is larger than the capacity of the measuring instru- ment, a transformer must be interposed that reduces the quantity and supplies the quantity to the instrument with values that are compatible with its capacity. This methodology is defined as indirect measuring.The measurement conducted via a measuring transformer is defined as an indirect measurement because it does not take place directly on the line under examination. For example, if a current up to 100 A has to be measured with an current that has a capacity of 5 A, a current transformer (CT) has to be interposed with a transformation ratio of 100/5. Scheme of current transformer (CT)

Voltmeter

Voltage transformers(TV)When there is the problem of measuring high voltages or voltages that are greater than the capacity of the instrument, voltage transformers are used (indicated by the letters VT).

WattmeterThewattmeteris an instrument for measuring theelectric power(or the supply rate ofelectrical energy) inwattsof any givencircuit. Electromagnetic wattmeters are used for measurement ofutility frequencyand audio frequency power; other types are required for radio frequency measurements.The traditional analog wattmeter is anelectrodynamicinstrument. The device consists of a pair of fixedcoils, known ascurrent coils, and a movable coil known as thepotential coil.The current coils connected inserieswith the circuit, while the potential coil is connected inparallel. A current flowing through the current coil generates anelectromagnetic fieldaround 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-valueresistorconnected in series with it to reduce the current that flows through it.The result of this arrangement is that on adccircuit, the deflection of the needle is proportional toboththecurrent(I)andthevoltage(V), thus conforming to the equationP=VI.ForAC power, current and voltage may not be in phase, owing to the delaying effects of circuitinductanceorcapacitance. On anaccircuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuringtrue power,P=VIcos. Here, cosrepresents thepower factorwhich shows that the power transmitted may be less than the apparent power obtained

Measurement of three-phase power with two wattmeters

Depending on the electrical charge on phases of three-phase circuit, values can differ on each wattmeter, to determine the active power we need to calculate the algebraic sum of the values shown by the both wattmeters: Measurement of three-phase power with three wattmeters

For three-phase symmetric systems is used the method with one wattmeter:

Star connectionDelta connectionMeasurement of reactive power in three-phase circuits (one wattmeter)

Measurement of reactive power in three-phase circuits (two wattmeter)

Measurement of reactive power in three-phase circuits(three wattmeter)