Electrical Technology

Made By:

Muhammad Affan Qamar

Measuring Instruments

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ContentsMeasuring Instruments.........................................................................................................................................................2

1. Galvanometer................................................................................................................................................................2

a. Definition:......................................................................................................................................................................2

b. History:..........................................................................................................................................................................2

c. Basic Principle:..............................................................................................................................................................2

d. Construction:.................................................................................................................................................................2

e. Working:........................................................................................................................................................................3

f. Operation of moving coil:..............................................................................................................................................3

g. Equation:.......................................................................................................................................................................4

h. Uses:..............................................................................................................................................................................4

2. Ammeter.......................................................................................................................................................................5

a. Definition:......................................................................................................................................................................5

b. History:..........................................................................................................................................................................5

c. Principle of Operation:..................................................................................................................................................5

d. Construction:.................................................................................................................................................................6

e. Operation of moving coil:..............................................................................................................................................6

f. Hot-wire Ammeter:.......................................................................................................................................................7

g. Electrodynamometer as Ammeter:...............................................................................................................................7

h. Applications:..................................................................................................................................................................7

3. Voltmeter......................................................................................................................................................................8

a. Definition:......................................................................................................................................................................8

b. Basic Principle of Construction:.....................................................................................................................................8

c. Operation of moving coil:..............................................................................................................................................9

d. Construction:.................................................................................................................................................................9

e. Electrodynamometer as Voltmeter:............................................................................................................................10

f. Uses:............................................................................................................................................................................10

4. Watt meter..................................................................................................................................................................10

a. Definition:....................................................................................................................................................................10

b. Operation of moving coil:............................................................................................................................................11

c. Construction and working:..........................................................................................................................................11

d. Electrodynamometer as Wattmeter:...........................................................................................................................12

e. Wattmeter Connection:...............................................................................................................................................12

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Measuring Instruments

1. Galvanometer

a. Definition: A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.

b. History: The history of galvanometer can be traced back to 1820, when Danish physicist - Hans Christian Oersted noted that a magnetic needle gets deflected as it comes in contact with an electric current. This observation by Oersted went on to become the basic principle of the working of a galvanometer. In the same year, German physicist - Johann Schweigger worked on this principle, and came up with the first galvanometer. The credit for the invention of the first moving-coil galvanometer, which is widely used today, goes to French physicist - Jacques Arsene D'Arsonval. A few years later, Edward Weston made quite a few changes to this design, and improvised it.

c. Basic Principle: Galvanometer works on the principle of conversion of electrical energy into mechanical energy. When a current flows in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional to the current flowing through it.

d. Construction:  The flat rectangular coil of thin enameled insulated wire of suitable number of turns wound on a light nonmetallic or aluminum frame is suspended between the cylindrically concave poles of magnet by a thin

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phosphor bronze strip. One end of the wire of the coil is soldered to strip. The other end of the strip fixed to the frame of the galvanometer and connected to an external terminal. It serves as one leas current lead through which the current enters or leaves the coil. The other end of the wire of the coil is soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of a wire serves as the other current lead. A soft-iron cylinder, coaxial with the pole pieces, is placed within the frame of the coil and is fixed to the body of the galvanometer. In the space between it and the pole pieces, where the coil moves freely, the soft iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.

e. Working: When a current passes through the galvanometer coil, it experiences a magnetic deflecting torque, which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension strip. The twist in the strip produces an electric restoring torque. The coil rotates until the elastic restoring torque due to the strip does not equal and cancels the deflecting magnetic torque, then it attains equilibrium and stops rotating any furthers.

f. Operation of moving coil: The most familiar use is as an analog measuring instrument, often called a meter. It is used to measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval/Weston form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position.

When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current. A useful meter generally contains provision for damping the mechanical resonance of the moving coil and pointer, so that the pointer settles quickly to its position without oscillation

The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage drop of, say, 50 millivolts at full current). Such meters are often calibrated to read some other quantity that can be converted to a current of that magnitude. The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger currents. A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the voltage required to generate a full scale current. A meter can be configured to read other voltages by putting it in a voltage divider circuit. This is generally done by placing a resistor in series with the meter coil. A meter can be used to read resistanceby placing it in series with a known voltage (a battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor adjusted to

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produce full scale deflection. When an unknown resistor is placed in series in the circuit the current will be less than full scale and an appropriately calibrated scale can display the value of the previously-unknown resistor.

Because the pointer of the meter is usually a small distance above the scale of the meter, parallax error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To counter this, some meters include a mirror along the markings of the principal scale. The accuracy of the reading from a mirrored scale is improved by positioning one's head while reading the scale so that the pointer and the reflection of the pointer are aligned; at this point, the operator's eye must be directly above the pointer and any parallax error has been minimized.

g. Equation: The galvanometer is oriented so that the plane of the coil is parallel to the local magnetic meridian, that is the horizontal component BH of the Earth's magnetic field. When a current passes through the galvanometer coil, a second magnetic field B perpendicular to the coil is created, of strength:

Where “I” is the current in amperes, n is the number of turns of the coil and r is the radius of the coil. These two perpendicular magnetic fields add vectorially, and the compass needle points along the direction of their resultant, at an angle of:

From tangent law,  , i.e.

or

Or , where K is called the Reduction Factor of the tangent galvanometer.

One problem with the tangent galvanometer is that its resolution degrades at both high currents and low currents. The maximum resolution is obtained when the value of θ is 45°. When the value of θ is close to 0° or 90°, a large percentage change in the current will only move the needle a few degrees.

h. Uses: Most modern uses for the galvanometer mechanism are in positioning and control systems. A galvanometer mechanism is used for the head positioning servos in hard disk drives. They are also used in laser marking and projection, and in imaging applications such as Optical Coherence Tomography

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(OCT) retinal scanning. Mirror galvanometer systems are used as beam positioning elements in laser optical systems.

Probably the largest use of galvanometers was the D'Arsonval/Weston type movement used in analog meters in electronic equipment.

A major early use for galvanometers was for finding faults in telecommunications cables.

2. Ammeter

a. Definition: An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or micrometers.

b. History: The relation between electric current, magnetic fields and physical forces was first noted by Hans Christian Ørsted who, in 1820, observed a compass needle was deflected from pointing North when a current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called "multipliers".

c. Principle of Operation: An ammeter consists of a coil with very low resistance, when electric current flows through the coil; the coil induces a magnetic field which turns the needle.

Ammeters vary in their operating principles and accuracies. The D’Arsonval-movement ammeter measures direct current with accuracies of from 0.1 to 2.0 percent. The electrodynamics ammeter uses a moving coil rotating in the field produced by a fixed coil. It

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measures direct and alternating current with accuracies of from 0.1 to 0.25 percent. In the thermal ammeter, used primarily to measure.

d. Construction: Since Galvanometer is a very sensitive instrument therefore it can’t measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low resistance known as "shunt" resistance is connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt. In this way a Galvanometer is converted into Ammeter and can measure heavy currents without fully deflected.

Let resistance of galvanometer = Rg and it gives full-scale deflection when current Ig is passed through it. Then,

Vg = IgRg -------(i) Let a shunt of resistance (Rs) is connected in parallel to galvanometer. If total current through the circuit is I.

Then current through shunt:

Is = (I-Ig)    potential difference across the shunt:

Vs= IsRs

or               Vs = (I – Ig)Rs -------(ii)

    But

Vs =Vg

(I - Ig)Rs = IgRg 

e. Operation of moving coil:

The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The modern form of this instrument was developed by Edward Weston (NOT the American photographer!), and uses two spiral springs to provide the restoring force. By maintaining a uniform air gap between the iron core of the instrument and

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the poles of its permanent magnet, the instrument has good linearity and accuracy. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to 10 mill amperes and have linear scales.

f. Hot-wire Ammeter: An ammeter which measures alternating or direct current by sending it through a fine wire, causing the wire to heat and to expand or sag, deflecting a pointer is called hot-wire ammeter . It is also known as thermal ammeter.

In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current.

Early hot wire ammeters had a silk thread wound around the needle pivot and anchored at one end to a tensioner and the other to the hot wire. A small pulley may have been free to run on the wire element and when the latter heated up from passing current it stretched, allowing the pulley to move toward the needle pivot. This would relax the tension and allow the needle to move across the scale.

The adjusting screw, passing through the lower edge would seem to distort the metal frame in a bow-fashion, causing the hot wire to tighten or slacken, increasing or reducing tension and allowing the pointer to move to read zero.

g. Electrodynamometer as Ammeter: When the electrodynamometer is used as an ammeter, a special type of construction must be used. This is because the large currents that flow through the meter cannot be carried through the moving coils. In the ammeter in view B of figure 3-17, stationary coils a and b are wound of heavier wire to carry up to5.0 amperes. An inductive shunt (XL) is wired in parallel with the moving coils and permits only a small part of the total current to flow through the moving coil. The current flowing through the moving coil is directly proportional to the total current flowing through the instrument. The shunt has the same ratio of reactance to resistance as the moving coil does. Therefore, the instrument will be reasonably correct at frequencies at which it is used if ac currents are to be measured.

h. Applications: The majority of ammeters are either connected in series with the circuit carrying the

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current to be measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In either case, the current passes through the meter or (mostly) through its shunt. They must not be connected to a source of voltage; they are designed for minimal burden, which refers to the voltage drop across the ammeter, which is typically a small fraction of a volt. They are almost a short circuit.

Ordinary Weston-type meter movements can measure only mill amperes at most, because the springs and practical coils can carry only limited currents. To measure larger currents, a resistor called a shunt is placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale deflection (FSD) of 50 mV, so shunts are typically designed to produce a voltage drop of 50 mV when carrying their full rated current.

3. Voltmeter

a. Definition: A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter.

b. Basic Principle of Construction: A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.

One of the design objectives of the instrument is to disturb the circuit as little as possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitiveammeter or microammeter in series with a high resistance.

The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200,000 ohms and at full scale the meter would draw 1 milliampere from the

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circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges.

c. Operation of moving coil:

A current carrying conductor placed in magnetic field. It experiences a force, it is given by F=BIL Where, F=force in Newton B=flux density in teals I=current in ampere L=length of conductor in meter.

A coil of thin wire is mounted on an aluminum frame (spindle) positioned between the poles of a U shaped permanent magnet which is made up of magnetic alloys like alnico.

The coil is pivoted on the jeweled bearing and thus the coil is free to rotate. The current is fed to the coil through spiral springs which are two in numbers. The coil which carries a current, which is to be measured, moves in a strong magnetic field produced by a permanent magnet and a pointer is attached to the spindle which shows the measured value.

When PMMC is used as a voltmeter, the coil is connected in series with high resistance. Rest of the function is same as above. The same moving coil can be used as an ammeter or voltmeter with an interchange of above arrangement.

d. Construction: Since Galvanometer is a very sensitive instrument, therefore it can not measure high potential difference. In order to convert a Galvanometer into voltmeter, a very high resistance known as "series resistance" is connected in series with the galvanometer.

Let resistance of galvanometer = Rg and resistance Rx (high) is connected in series to it. Then combined resistance = (Rg + Rx).

    If potential between the points to be measured = V and if galvanometer gives full-scale deflection, when current "Ig" passes through it. Then,

V = Ig (Rg + Rx) 

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V = IgRg + IgRx V – IgRg = IgRx 

Rx = (V – IgRg)/Ig

    Thus Rx can be found.

e. Electrodynamometer as Voltmeter: When an electrodynamometer is used as a voltmeter, no problems in construction are encountered because the current required is not more than 0.1 ampere. This amount of current can be handled easily by the spiral springs. When the electrodynamometer is used as a voltmeter, its internal connections and construction are as shown in view A of figure 3-17. Fixed coils a and b are wound of fine wire since the current flow through them will not exceed 0.1 ampere. They are connected directly in series with movable coil c and the series current-limiting resistor.

f. Uses: To measure voltages provided to electronics over long cables (voltage drop along long

cables depending on the current). Measure voltages on circuit boards to check the operation of these boards. Set the thresholds on NIM/CAMAC discriminators, & check the voltage offsets in these

units.

4. Watt meter

a. Definition: The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.

An instrument which measures electrical energy in watt hours (electricity meter or energy analyser) is essentially a wattmeter which accumulates or averages readings; many such instruments measure and can display many parameters and can be used where a wattmeter is needed: volts,

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current in amperes, apparent instantaneous power, actual power, power factor, energy in [k]Wh over a period of time, and cost of electricity consumed.

b. Operation of moving coil: Probably the most useful instrument in the measurement of ac power at commercial frequencies is the indicating (deflecting) electrodynamic wattmeter. It is similar in principle to the double-coil dc ammeter or voltmeter in that it depends on the interaction of the fields of two sets of coils, one fixed and the other movable. The moving coil is suspended, or pivoted, so that it is free to rotate through a limited angle about an axis perpendicular to that of the fixed coils. As a single-phase wattmeter, the moving (potential) coil, usually constructed of fine wire, carries a current proportional to the voltage applied to the measured circuit, and the fixed (current) coils carry the load current. This arrangement of coils is due to the practical necessity of designing current coils of relatively heavy conductors to carry large values of current. The potential coil can be lighter because the operating current is limited to low values.

The electrostatic force between two conductors is proportional to the product of the square of the potential difference between them and the rate of change of capacitance with displacement. A differential electrostatic instrument may therefore be used to construct a quarter-squares wattmeter. In spite of the problems of matching the capacitance changes of the two elements and the small forces available, electrostatic watt meters were used as standards for many years.

c. Construction and working: The traditional analog wattmeter is an electrodynamics’ instrument. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil.

The current coils connected in series with the circuit, while the potential coil is connected in parallel. Also, on analog wattmeter, the potential coil carries a needle that moves over a scale to indicate the measurement. A current 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 resistor 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 voltage, thus conforming to the equation W=VA or P=VI.

The result of this arrangement is that on a dc circuit, the deflection of the needle is proportional to both the current and the voltage, thus conforming to the equation W=VA or P=VI.

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For AC power, current and voltage may not be in step, owing to the delaying effects of circuit inductance or capacitance. On an ac circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true power, P=VI cos φ. Here, cos φ represents the power factor which shows that the power transmitted may be less than the apparent power obtained by multiplying the readings of a voltmeter and ammeter in the same circuit.

d. Electrodynamometer as Wattmeter: Electrodynamometer type wattmeter is an electrical instrument which is used to measure power. It consists of two coils-Fixed coils and Moving coil. The fixed coils or the field coils are connected in series with the load and therefore carry the current flowing through the circuit. Hence, they are also called as the current coils (C.C) of the wattmeter. Where as the moving coil is connected across the voltage, and hence carry a current proportional to the voltage. This coil is also called as the pressure coil (P.C) of the wattmeter.

The control used in these types of instruments is the spring control. And the damping used is the air damping. Here, eddy current damping is avoided, since it might distort the weak operating magnetic field. These types of instruments use mirror type scales and knife edge pointers and thus helps removing parallax. And the deflection is directly proportional to the power being measured. One of the major disadvantage of this wattmeter is that, the reading of the wattmeter is affected by the change in temperature.

e. Wattmeter Connection: The current coil of the wattmeter is connected in series with the circuit (load), and the voltage coil is connected across the line. When line current flows through the current coil of a wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to the line current. The voltage coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for this connection is to make the voltage-coil circuit of the meter as purely resistive as possible. As a result, current in the voltage circuit is practically in phase with line voltage. Therefore, when voltage is impressed on the voltage circuit, current is proportional to and in phase with the line voltage. Figure 3-19 shows the proper way to connect a wattmeter into a circuit.