EE T55 MEASUREMENTS AND INSTRUMENTATION

UNIT II: ELECTRICAL MEASURING INSTRUMENTBasic effects of electromechanical instruments–Ammeter and voltmeter–Moving coil–

Moving Iron–Electrodynamo meter and induction type–Extension of range. Wattmeter–Dynamometer and induction type energy meter–induction type–Instrument transformers. Powerfactormeter– Synchroscope –Frequency meter-Digital voltmeter.

Before the operation of an electrical/electronic apparatus can be studied, it is necessary

to have instruments which will indicate the electrical quantities present. The instruments used

to measure these electrical quantities (e.g. current, voltage, resistance, power, etc.) are called

electrical/electronic instruments. These instruments are generally named after the electrical

quantity to be measured. Thus the instruments which measure current, voltage, resistance,

power are called ammeter, voltmeter, ohmmeter, and wattmeter respectively.

These instruments must be reliable and easily read, as well as having little effect on the

circuit to which they are connected. It is important to appreciate the properties of each

instrument and to know the most suitable instrument for a given measurement or the likely

accuracy of a given instrument when used for a particular measurement. Measurement is a

process in which the property of an object or system under consideration is compared to an

accepted standard unit, a standard defined for that particular property.

2.1. MEASURING INSTRUMENTS:

In general, measuring instruments are those electromechanical and electronic devices

usually employed for measurement of both electrical and non-electrical quantities like

current, voltage, resistance, capacitance, inductance, temperature, displacement, etc.

2.1.1. Electromechanical Instruments: These comprises of electrical as well as mechanical system,

the electrical system usually depends upon mechanical meter movements as indicating devices

and the mechanical movement has some inertia, therefore these instruments have a limited time

(and hence, frequency) response e.g. recorders, galvanometers etc.

2.1.2. Electronic Instruments: These days most of the scientist and industrial measurements

require very fast response. The inability of the mechanical and electrical instruments to cope

with such requirements led to the design of today’s electronic instruments and their

associated circuitry. These instruments require the use of semi-conductor devices. Since in

electronic devices, the only movement involved is that of electrons, the response time is

extremely smaller account of very small inertia of electrons. Example of these instruments is

cathode ray oscilloscope, transducers, computers,

The most important use of electronic instruments is their usage in measurement of

non-electrical quantities, where the non-electrical quantity is converted into electrical form

through the use of transducers.

Electronic instruments have the following advantaged over their electrical

counterparts. High sensitizing

A faster response

A greater flexibility

Lower weight

They can monitor remote signal

Lower power consumption and a higher degree of reliability than their mechanical or

purely electrical counterparts.

2.2. METHODS OF INSTRUMENT:

There are a number of ways in which measuring instrument can be classified. One

useful way with electrical and electronic measuring instrument is by the way in which the

measured quantity is displayed and these are broadly divided into two.

Analog instrument

Digital instrument

2.2.1.Analogue Instrument:

An analogue instrument is one in which the magnitude of the measured quantity is

indicated by means of a pointer. Instruments of this category include moving coil instruments,

moving non-instruments, oscilloscope, d.c and a.c bridges, megger etc.

Analog instruments depend for their operation on one of the many effects produced

by current and voltage and this can be classified according to which of the effects is used for

their working. The various effects used are listed in table 2.1

FECT STRUMENTS

agnetic effect mmeter, voltmeters, wattmeter, integrating meters

ating effect mmeters and voltmeters, wattmeter’s

ectromagnetic effect ltmeter

ll effect ux meter, ammeter and poynting vector wattmeter etc

2.2.2. Digital Instruments:

A digital instrument is one whose display is presented in the form of a series of

decimal values. Examples of such devices are digital AVOMETER, frequency counters,

inductance meter etc. the digital instrument have the advantages of indicating, the readings

directly in decimal numbers and therefore errors on account of human factors like error due

to parallax and approximation encounter in the analogue are eliminated. Also power

requirements of digital instruments are considerably smaller.

2.3. FUNCTIONS OF ELECTRICAL/ELECTRONIC INSTRUMENTS:

There is another way in which instruments or measurement systems may be classified.

This classification is based upon the functions they perform. The three main functions

employs in electrical and electronic instruments are explained below:

Indicating instruments

Recording instruments

Integrating instruments

2.3.1.Indicating instruments: These are the instruments which indicate the instantaneous

value of quantity being measured at the time it is being measured. The indication is in the

form of pointer deflection (analogue instrument) or digital readout (digital instrument). In

analogue instruments, a pointer moving over a graduated scale directly gives the value of the

electrical quantity being measured. Ammeters, voltmeters and wattmeters are example of

such instruments.

2.3.2.Recording instruments: Recording instruments are those instruments which give a

continuous record of variations of the electrical quantity being measured over a selected

period of time. The moving system of the instrument carries an inked pen which rests tightly

on a graph chart e.g. recording voltmeter are used in substations to record the variation of

supply voltage during the day. Also recording ammeters are employed in supply stations for

registering the amount of current taken from batteries.

2.3.3. Integrating instruments: These are instruments which measure and register by a set of

dials and pointers, either the total quantity of electricity (in ampere – hours) of the total

amount of electrical energy (in watt hours or kilowatt hours) supplied to a circuit over a

period of time e.g. ampere – hour meters, watt-hour meters, energy meters etc.

In most indicating instruments, three distinct forces are essential for the satisfactory

indicating of the pointer on a dial. These forces are:

A deflecting (or operating) torque

A controlling (or restoring) torque

A damping torque

Deflecting Torque (Td): - It is the torque which deflects the pointer on a calibrated scale

according to the electrical quantity passing through the instrument. This deflecting torque

causes the moving system, and hence the pointer attached to it, to move from its zero

position, i.e. its position when the instrument is disconnected from the supply.

The deflecting torque can be produced by utilizing any of the effects mentioned earlier. Thus

the deflecting system of an instrument converts the electric current or potential into a

mechanical force called deflecting torque.

Controlling Torque (Tc): - It is the torque which controls the movement of the pointer on a

particular scale according to the quantity of electricity passing through it. The controlling

forces are required to control the deflection or rotation and bring the pointer to zero position

when there is no force, or stop the rotation of the disc when there is no power. Without such

a torque, the pointer would swing over to the maximum deflected position irrespective of the

magnitude of current or voltage being measured.

The functions of the controlling system are;

To produce a force equal and opposite to the deflecting torque at the final steady

position of the pointer definite for a particular magnitude of current. In the absence of

a controlling torque, the pointer will shoot (swing) beyond the final steady position

for any magnitude of current and thus the deflection will be indefinite.

To bring the moving system back to zero when the force causing the instrument

moving system to deflect is removed. In the absence of a controlling torque the

pointer will not come back to zero when current is removed.

In indicating instruments, the controlling torque, also called restoring or balancing

torque, is obtained by one of the following two methods:

• Spring control

• Gravity control

Damping Torque: - It is the torque which avoids the vibration of the pointer on a particular

range of scale, such a damping or stabilizing force is necessary to bring the pointer to rest

quickly, otherwise, due to inertia of the moving system, the pointer will oscillate about its

final deflected position for quite sometime before coming to rest in the steady position When

a deflecting torque is applied to the moving system, it deflects and it should come to rest at a

position where the deflecting force is balanced by the controlling torque. The deflecting and

controlling forces are produced by systems which have inertia and, therefore the moving

system cannot immediately settle at its final position but overshoots or swings ahead of it.

Consider fig .2.1. suppose 0 is the equilibrium or final steady position. Because of inertia the

moving system moves to position „a‟. Now for any position „a‟ beyond the equilibrium

position the controlling torque is more than the deflecting torque and hence the moving

system swings back. Due to inertia it cannot settle at „0‟ but swings to a position say „b‟

behind the equilibrium position. At „b‟, the deflecting torque is more than the controlling

force and hence the moving system again swings ahead.

The pointer thus oscillate about its final steady (equilibrium) position with decreasing

amplitude till its kinetic energy (on account of inertia) is dissipated in friction and therefore,

it will settle down at its final steady position. If extra force are not provided to “damp” these

oscillations, the moving system will take a considerable time to settle to the final position

and hence time consumed in taking readings will be very large. Therefore, damping forces

are necessary so that the moving system comes to its equilibrium position rapidly and

smoothly without any oscilations.

Fig.2.1. Damping system

Damping system:

The deflecting torque provides some deflection and controlling torque acts in the

opposite direction to that of deflecting torque. So before coming to the rest, pointer always

oscillates due to inertia, about the equilibrium position. Unless pointer rests, final reading

cannot be obtained. So to bring the pointer to rest within short time, damping system is

required. The system should provide a damping torque when the moving system is in motion.

Damping torque is proportional to the velocity of the moving system but it does not depend

on operating current. It must not affect the controlling torque or increase the friction.

There are three types of damping:

Air – friction damping

Fluid – friction damping

Eddy – current damping

2.4. PERMANENT MAGNET MOVING COIL INSTRUMENTS:

Different instruments are used onboard for measuring several electrical parameters to

analyze and keep these machinery in proper running condition. A permanent magnet moving

coil (PMMC) is one such instrument which is popularly used.

Principle:

When a current carrying conductor is placed in a magnetic field, it experiences a force

and tends to move in the direction as per Fleming’s left hand rule. When a current carrying coil is

placed in the magnetic field produced by permanent magnet, the coil experiences a force and

moves. As the coil is moving and the magnet is permanent, the instrument is called permanent

magnet moving coil instrument. This basic principle is called D’Arsonval principle. The amount

of force experienced by the coil is proportional to the current passing

through the coil.

Fleming left hand rule:

Fig.2.2. Fleming left hand rule

If the first and the second finger and the thumb of the left hand are held so that they

are at right angle to each other, then the thumb shows the direction of the force on the

conductor, the first finger points towards the direction of the magnetic field and the second

finger shows the direction of the current in the wire.

Construction:

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 jewelled 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.

Fig.2.3. PMMC instrument

Working:

The moving coil is either rectangular or circular in shape. It has number of turns of fine

wire. The coil is suspended so that it is free to turn about its vertical axis. The coil is placed in

uniform, horizontal and radial magnetic field of a permanent magnet in the shape of a horse-shoe.

The iron core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron

core, the deflecting torque increase, increasing the sensitivity of the instrument.

When a current flow through the coil, it generates a magnetic field which is

proportional to the current in case of an ammeter.

The deflecting torque is produced by the electromagnetic action of the current in

the coil and the magnetic field.

The controlling torque is provided by two phosphorous bronze flat coiled helical

springs. These springs serve as a flexible connection to the coil conductors.

The damping torque is provided by eddy current damping. It is obtained by

movement of aluminum former, moving in the magnetic field of the permanent

magnet.

The pointer is carried by the spindle and it moves over a graduated scale. The pointer

has light weight, so that it deflects rapidly. The mirror is placed below the pointer to get the

accurate reading by removing the parallax. The weight of the instrument is normally counter

balanced by the weights situated diametrically opposite and rapidly connected to it. The scale

markings of the basic d.c PMMC instruments are usually linearly spaced as the deflecting

torque and hence the pointer deflections are directly proportional to the current passing

through the coil.

Fig.2.4. Cross sectional view of PMMC

Torque equation:

The equation for the developed torque can be obtained from the basic law of

the electromagnetic torque. the deflecting toque is given by

Td = NBIA

Where Td = deflecting torque in N m

B = flux density in air gap Wb/m2

N = number of turns of the coil

A = effective coil area m2

I = current in the moving coil, A

Td = GI

G = NBA = constant

Controlling torque is provided by the springs and is proportional to the

angular deflection of the pointer.

Tc = Kθ

Tc = controlling torque

K = spring constant, Nm/rad

Θ = angular deflection

For the final steady state position

Td = Tc

GI = Kθ

θ = (G/K) I

Thus the deflection is directly proportional to the current passing through the coil. The

pointer deflection can therefore be used to measure current.

Advantages of PMMC:

The various advantages of PMMC are

It has uniform scale

Its torque to weight ratio is very high. So opearating current is small.

Sensitivity is high

It consumes low power of the otder of 25W to 200Mw

It has high accuracy

Instrument is free from hysteresis error

Disadvantages of PMMC

The various disadvantages of PMMC are

PMMC is suitable for direct current measurement only

Ageing of permanent magnet and the control springs introduces the error

The cost is high due to delicate construction and accurate

machining The friction is due to jewel-pivot suspension

Applications:

The PMMC has a variety of uses onboard ship. It can be used as:

Ammeter:

Voltmeter:

Galvanometer

The ohm meter

2.5. Electrodynamic (Dynamometer) Instruments:

These instruments are the modified form of permanent magnet moving coil

instrument in which the operating field is produced, not by a permanent magnet but by a two

air-cored fixed coils placed on either side of the moving coil as seen in fig.2.5.

Electrodynamometer meter movements use stationary coil and moving coils to develop

interacting magnetic fields (that is the electrodynamometer uses two electromagnetic fields

in its operation. One field is created by the current flowing through a pair of series-connected

stationary coils. The other field is caused by current flowing through a movable coil that is

attached to the pivot shaft. If the current in the coils are in the correct directions, the pointer

rotates clockwise. The rotational torque on the movable coil is caused by the opposing

magnetic forces of the three coils.. They respond to alternating current because the a.c.

reverses direction simultaneously in all three coil. and also can operates on direct current and

are used in wattmeter. . Electrodynamometer meters have low sensitivity and high accuracy

Fig.2.5. Dynamometer Instruments

The operating principle of electrodynamics instruments is the interaction between the

currents in the moving coil, mounted on a shaft, and the fixed coils, that is, the deflecting

torque is produced by the reaction between the magnetic field set up by the current in the

moving coils and the magnetic field set up by current in the fixed coil. When the two coils

are energized, their magnetic fields will interact as a result of mechanical force exists

between the coils and the resulting torque will tend to rotate the moving coil and cause the

pointer attached to it to move over the scale. Since there is no iron, the field strength is

proportional to the current in the fixed coil and therefore, the deflecting torque is

proportional to the product of the currents in the fixed coils and the moving coil.

Deflecting Torque The force of attraction or repulsion between the fixed and moving coils is

directly proportional to the product of ampere turns of fixed coils and the moving coils.

i.e. Deflecting torque, Td ∞ NFIF ∞ NMIM Since NF

and NM are constant :

Td = IF IM

This show that the scale of these instruments is not uniform, disadvantage of such a scale is

that the divisions near the start of the scale are small and cannot be read accurately.

Control System: The controlling torque is produced by two control springs, which also act as

leads to the moving coil.

Damping System This system provides for air – damping.

Advantages of Dynamometer Instruments

These instruments can be used for both d.c and a.c measurements.

Since the coil is generally air cored, they are free from eddy current and hysteresis

losses.

They can be use for power measurements.

Disadvantages of Dynamometer nstruments

They have low sensitivity

Such instruments are more expensive than the other types

Because the deflecting torque varies with the square of the current, the scale is not

uniform.

2.6. MOVING IRON INSTRU ENTS:

Moving – Iron instruments depend for their action upon the magnetic effect of current,

and are widely used as indicating instruments. In this type of instrument, the coil is stationary

and the deflection is caused by a soft-iron piece moving in the field produced by the coil. This

type of instrument is principally used for the measurement of alternating currents and voltages,

though it can also be used for D.C measurements but is then liable to small errors due to

permanent magnetism in the iron; there are two basic forms of moving – iron instruments.

Attraction type

Repulsion type

2.6.1. ATTRACTION TYPE:

The basic working principle of attraction type moving – iron instruments is illustrated in

fig. 2.6. In this system, when current flows through the coil, a magnetic field is produced at its

centre. A soft – iron rod fixed to the spindle becomes magnetized and is pulled inside the coil,

the force of attraction being proportional to the strength of the field inside the coil, which again is

proportional to the strength of the current.

Fig.2.6. Attraction type

Working Principle:

When the current to be measured is passed through the coil, a magnetic field is

produced which attracts the iron rod inwards, thereby deflecting the pointer which moves

over a calibrated scale.

Deflecting Torque: In the attraction – type moving – iron instrument, the deflecting torque

is due to the force of attraction between the field of the coil and the iron disc. The

magnetization of the iron disc is proportional to the field strength H. The force F pulling the

disc inwards is proportional to the magnetization ‘M’ of disc and field strength H.

Deflecting torque (Td) ∞ MH

But M ∞ H , H ∞ I :

Td ∞ I2

Thus, the deflecting torque is proportional to the square of the current passing through the

coil.

Controlling Torque: In the above instrument the controlling torque is achieved by gravity

control, but now spring control is used almost universally.

Damping Torque: The dampingg of the moving system is obtained by air damp ing, in

which a light aluminum piston moves freely inside the curved cylinder closed at one end.

The resistance offered by air in escaping from the restricted space around the pi ston

effectively damps out any oscillations.

2.6.2. REPULSION TYPE:

It consists of a fixed coil inside which two soft iron and are arranged parallel to one

another and along the axis of th e coil (as shown in fig. 2.7). One of these rods A , is fixed to

the coil frame, while the other rod B is moving and is mounted on the spindle. T he moving

rod carries a pointer which moves over a calibrated scale. In this type of movement, the coil

which receives the current to be meas ured is stationary. The field set up by the coil

magnetizes two iron vanes, which then becom es temporary magnets. Since the same field m

agnetizes both vanes, both vanes have the s ame magnetizes polarity. Consequently, there is

a force of repulsion between the two vanes. One of the vanes (stationary vane) is attached to

the coil form. The other vane (the mov ing vane) is mounted on the pivot shaft to w hich the

meter pointer is attached. Thus, the m agnetic force of repulsion forces the moving vane

away from the stationary vane. Of course, this force is offset by the counter torgue of the

spiral springs attached to the pivot shaft. The greater the current through the coil in, the

strnger the magnetic repelling force; thus, the fartheer the moving vane rotates and the more

current the pointer indicates. The iron vane meter m ovement can operate on either a.c or d.c

Fig.2.7. Repulsion type

Working Principle:

When the current to be measured is passed through the fixed coil, it set up its own

magnetic field which magnetizes the two rods with same polarity so that they repel one

another, with the result that the pointer is deflect and causes the pointer to move from zero

position. The force of repulsion is approximately proportional to the square of the current

passing through the coil.

Deflecting Torque: The deflecting torque results due to the repulsion between the two

similarly magnetized (charged) soft iron rods. Therefore,

Instantaneous torque ∞ repulsive force and repulsive force

∞ to the product of pole strengths M1 and M2 of two vanes.

Pole strengths are ∞ magnetizing force ‘H’ of the coil and

H ∞ current passing through the coil .

Therefore, the instantaneous torque, which is the deflecting torque, is given as

Instantaneous torque ∞ I2 i.e. Td ∞ I2

Hence, deflecting torque is proportional to the square of the current when used in an A.C

circuit; the instrument reads the r.m.s value of the electrical quantity.

Controlling Torque: In this type of instrument, controlling torque is obtained either with a

spring or by gravity. In figure 2.7. spring has been used for the controlling torque.

Damping Torque: In this type of instrument, pneumatic type damping is used. Eddy current

cannot be employed because the presence of a permanent magnet, required for such a

purpose, would affect the deflection and hence the ready of the instrument.

Advantage of Moving – Iron Instruments:

Following are the advantages of moving – iron instruments

Cheap, robust and give reliable service

Usable in both a.c and d.c circuits.

Disadvantages and Limitations of Moving – Iron Instrument

Have non-linear scale

Cannot be calibrated with a high degree of precision for d.c on account of the affect

of hysteresis in the iron vanes

The instrument will always have to be put in the vertical position if it uses gravity

control.

Applications:

The moving – iron instruments are primarily used for a.c measurement such as, alternating

currents and voltages.

2.8. DC VOLTMETERS:

By adding a series resistance or a multiplier, the D'Arsonval movement can be

converted into a dc voltmeter. The series resistance Rs or the multiplier limits the current

through the meter, so as not to exceed the full-scale deflection current IFSD (Fig. 2.8.).

The value of the multiplier resistance required to extend the voltage range is

calculated as follows:

Im = deflection current of the movement

Rm = internal resistance of the movement

Figure 2.8. For DC voltmeter

Rs = multiplier series resistance

V = full-range voltage of the instrument

Direct current voltmeters are available up to 500 V. The multiplier resistance is built

into the meter. For higher voltage ranges, Rm is mounted separately.

2.8.1. Multirange Voltmeter:

A voltmeter with different ranges can be obtained by connecting a number of

multipliers. This is shown in Fig. 2.9.

R1 is the multiplier resistance for the voltage range V1

R2 is the multiplier resistance for the voltage range V2 and so on.

Figure 2.9. Multirange voltmeter

To determine the value of R1, R2,…, etc., the following equations are used:

The multiplier resistances can also be connected in series as shown in Fig.2.10. The

advantage with this system is that all multipliers except the first have standard prevention

values and can be obtained commercially.

Figure 2.10. More practical arrangement of multiplier resistors in the multirange voltmeter

The resistance offered by the voltmeter for each range is expressed as the sensitivity of the

voltmeter. It is expressed in O/V. It is also called the ohm-per-volt rating of the voltmeter.

An ideal voltmeter should have infinite input resistance. When the voltmeter is connected

across two points, it shunts the circuit or source. Therefore, the net resistance decreases. Due to

this, the voltage measured will be less than the actual voltage. This is known as the loading

effect.

2.9. ELECTRONIC VOLTMETER (FOR DC):

Electronic voltmeters, in general, consist of the following:

1. A potential divider network to reduce the input in case it is high, to make it suitable,

to give as input to the amplifier.

2. An amplifier circuit to enhance the signal so that the sensitivity and resolution of

measurement improve.

3. A rectifier and filter circuit in case the meter for deflection is a DC meter.

Usually, the electronic circuits generate a current proportional to the quantity

being measured. Many digital instruments have auxiliary provisions to make permanent

records of measurement results using printers or magnetic tape recorders. Instruments

with PC compatibility are the new type of instruments developed so far. The general

block schematic of an electronic DC voltmeter is shown in fig.2.11.

Figure 2.11. Block diagram of an electronic DC voltmeter

The potential divider network is nothing but a series of resistors to alternate the value

of input in case it is large. If the signal magnitude is small and needs no attenuation, it is

passed directly. The signal is amplified by the DC amplifier and then given to the meter. The

meter is calibrated in terms of the parameter to be measured. The DC amplifier used can be

1. Direct-coupled amplifier.

2. Chopper-type DC amplifier.

Direct-coupled amplifiers are preferred because they are economical. A typical circuit for

a FET input electronic DC voltmeter is shown in Fig.2.12. The DC input voltage is applied to a

Range attenuator, which is a potential divider network. It is calibrated on the front panel control.

The attenuator is necessary to provide input voltage levels, which the DC amplifier can take. The

input stage of the amplifier consists of a JFET. It provides high input impedance and isolates the

meter circuit from the input. Hence, loading of the input on a circuit under test can be prevented.

The two BJTs form a direct-coupled DC amplifier, driving a meter movement. The transistor Q2

is in a common-base configuration. It provides the voltage gain. Transistor Q1 is in a common-

collector (CC) configuration. Its voltage gain is less than one, but provides a large current gain.

The input to the meter is the amplified version of actual input. The output from the collector of

Q2 is directly coupled to the base input of Q1.

Hence, it is named direct-coupled amplifier. The output current from the emitter of

Q1 since it is in the CC configuration is given to the meter. Zero adjustment of the meter can

be done with the help ofR2. Full-scale adjustment can be done with Rs. The gain in the

amplifier allows the instrument to be used for the measurement of even millivolts.

Figure 2.12. Circuit diagram for an electronic DC voltmeter

This circuit has the added advantage that accidental high voltages do not damage the

instrument because amplifier saturation limits the maximum current through the meter.

A chopper-type DC amplifier is used to avoid drift problems. These are used in high-

sensitive instruments. Zero adjustment is done using R2 to make the transistor Q2 to go to

cut-off.

2.10. ELECTRONIC VOLTMETER (FOR AC)

Electronic AC voltmeters are similar to electronic DC voltmeters except that the signal is

rectified before being amplified. The block schematic is shown in Fig. 2.13.

Figure 2.13. Block diagram for an electronic AC voltmeter

However, designing a DC amplifier is more difficult and expensive, because the input

signal right from 0 Hz is to be amplified. The input AC signal is first amplified and then

rectification and filtering are done and a DC meter is used for deflection. Therefore,

sometimes an AC amplifier is found to be more convenient. The block schematic of an AC

voltmeter is shown in Fig. 2.14.

Figure 2.14. Block diagram for an electronic AC voltmeter (another version)

AC voltmeters are subdivided into three categories:

Average reading voltmeter.

Peak responding voltmeter.

True RMS responding voltmeter.

The difference between average and peak responding voltmeters is only in filter circuits.

2.10.1. Average Reading Voltmeter:

The simplified circuit diagram is shown in Fig.2.15. This meter reads the average value

of a positive half cycle or a negative half cycle of the AC input. It depends on the position of

diode D. In the above figure, the diode conducts during the positive half cycle of the AC input. It

provides half wave rectification of the input. The average value of the positive half cycle is

developed across the resistor R. This is applied to the DC amplifier and meter. If the diode

position is reversed, the meter indicates the voltage across the negative half.

Figure 2.15. Circuit diagram for an average reading voltmeter

2.10.2. Peak Responding Voltmeter:

2.10.2.1. Peak reading voltmeter:

A circuit diagram for a peak reading voltmeter is shown in Fig.2.16.

Figure 2.16. Circuit diagram for a peak reading voltmeter

In this circuit, since the diode D is connected as shown in Fig. 2.16., it gets forward

biased during the positive half cycle. Therefore, the capacitor gets charged to the positive

peak. The charge cannot leak off rapidly because of the one-way conduction of the diode.

The voltage across the meter stays near the peak value of the input. The value of resistor R is

greater than the forward resistance of the diode but less than the reverse resistance.

Therefore, when the diode is forward biased, it gets charged through the diode. When the

diode is reverse biased, the discharge path is through resistor R. If the position of the diode is

reversed, the meter reads negative peak value of the input.

Peak reading voltmeters or Peak detectors are used in coaxial configurations to

measure signals up to 40 GHz, by keeping the diode and the capacitor in a probe without

applying to the amplifier or meter.

In the average reading circuits, the input is full-wave rectified (FWR), and the low-

pass filtering characteristic of the meter movement is used to extract the average value.

The rms reading meter circuit approximates the required square law parabola with a

few straight line segments in the fashion of a diode function generator. The voltage applied

to the meter is only an average value. However, the scale is calibrated for rms value.

2.10.2.2. Peak-To-Peak Detector:

In the peak detector instrument, only the positive peak or negative peak of the input will be

measured. If the input waveform is not symmetrical, the positive and negative peak values will

be different. An unsymmetrical waveform causes turnover error in meters. This is overcome in

peak-to-peak reading voltmeters. The circuit diagram for this is shown in Fig.2.17.

Figure 2.17. Circuit diagram for a peak-to-peak reading voltmeter

The detection efficiency of the peak-to-peak detector is twice that of a peak

responding meter. During the negative half cycle of the AC input, diode D1 becomes forward

biased. C1 charges up to approximately the negative peak voltage. When voltage V1 goes

positive, D1 is reverse biased and D2 becomes forward biased. The change on C1 is gradually

transferred to C2 during the initial transient period.

When the circuit is in steady-state operation, the output voltage is the sum of the

voltage developed across C1 during the negative portion of the input V1 and positive peak of

V1, which is equal to the peak-to-peak input voltage. C1 and C2 must be large enough, so that

the voltage does not change appreciably across C2 during one period of the input voltage and

the voltage across C1 does not appreciably change in the process of recharging C2.

Peak-to-peak detectors are used for non-sinusoidal waveforms or complex waveforms.

These are used in communication systems for modulated waves.

2.10.3.True Rms-Responding Voltmeter:

To measure the values of complex AC inputs, true rms-responding voltmeters are to be

used. The heating power of a given input signal is proportional to the square of the rms value of

the voltage. Sensing this power, this meter produces a deflection using a thermocouple.

Thermocouple outputs are non-linear, in general. This difficulty is overcome by placing two

thermocouples in the heating environment. The schematic of this meter is shown in Fig.2.18.

Figure 2.18. Block diagram of a true rms-reading voltmeter.

The measuring and balancing thermocouples are located in the same thermal

environment. The unknown AC input voltage is amplified and applied to the heating element of

the measuring thermocouple. The thermocouple in the input side is the measuring thermocouple.

The thermocouple in the feedback path is the balancing thermocouple. They are similar

thermocouples and form a complementary pair. Therefore, the non-linearity due to the measuring

thermocouple is cancelled by the similar non-linearity of the balancing thermocouple. The heater

coil gets heated due to the AC input given. The measuring thermocouple produces a voltage,

which upsets the balance of the bridge. The imbalance voltage is amplified by the DC amplifier

and feedback to the heating element of the balancing thermocouple. Bridge balance is restored

when the feedback current delivers sufficient heat to the balancing thermocouple, so that the

voltage output of both thermocouples are the same.

The DC current in the heating element of the feedback thermocouple is equal to the

AC current in the input thermocouple. Therefore, this DC current is directly proportional to

the effective or rms value of the input voltage. This is indicated on the meter movement in

the output circuit of the DC amplifier. Thus, the true rms value of the AC input can be

measured irrespective of the shape of the input.

Laboratory-type meters can measure inputs with a crest factor of 10/1.

Voltage range: 100 µV to 300 V.

Frequency range: 10 Hz to 10 MHz.

2.11. DIGITAL VOLTMETERS:

The digital voltmeter commonly refers to as DVM is an instrument use in the

measurement of both a.c and d.c voltages and displayed in a simple discrete numeral, instead of

the pointer deflection on a continuous scale as in analog devices.

This has provide numerous advantage to users as it

Reduces human error due to reading, interpolation and parallax error

It provides a faster readout result and is more compactable to other devices than its

analog counter parts

With the present existence of integral circuit, its cost is greatly reduced as well as its

power consumption. Though they are more expensive than their analog counter parts

Its accuracy level is quite high about ὠ0.005% of reading

The stability of DVM is high since its power consumption is low

Digital voltmeter are quite versatile and can also be use in the measurement of resistance

and current by using suitable means of conversion. The basic stages in producing a digital

display in DVM are

Sampling

Encoding

Display

The digital voltmeter can be considered to be basically just an analogue to digital

converter connected to a counter and a display unit. The voltmeter to be measured, an analogue

quantity, is a sampled at some instant of time and converted by the ADC to a digital signal, i.e a

series of pulses with the number of the pulses being related to the size of the analogue voltmeter.

These pulses are counted by a counter and display as a series of digits.

fig.2.19. Block diagram of a DVM

The digital voltmeter (DVM) attains the required measurement by converting the analog

input signal into digital, and when necessary, by discrete-time processing of the converted

values. The measurement result is presented in a digital form that can take the form of a

digital front panel display or a digital output signal. The digital output signal can be coded as

a decimal BCD code, or a binary code. The main factors that characterize DVM s are speed,

automatic operation and programmability. In particular, they present offer the best

combination of speed, automatically operated system. When a DVM is directly interfaced to

a digital signal processing system and used to convert the analog input voltage into a

sequence of sampled values. It is usually called an analog to digital converter (ADC). It is

basically differ in the following ways:

Number of measurement ranges

Number of digits

Accuracy

Speed if reading

Operating principle:

The basic requirements ranges of most DVMs are either 1V or 10V. it is however

possible, with an appropriate preamplifier stage to obtain full scale as low as 0.1V. if an

appropriate voltage divider is used, it is also possible to obtain full scale value as high as

1000V.

If the digital presentation takes the form of a digital front-panel display, the measurement result

is presented as a decimal number, with a number of digits that typically ranges from 3 to

6. If the digital representation typically ranges from 8 to 16, though 18 bit ADCs are

available. The accuracy of the DVM is usually correlated to its resolution. Indeed, assigning

an uncertainty lower than the 0.1% of the range to a three digit DVM makes no sense, since

this is the displayed resolution of the instrument. A poorer accuracy makes the three digit

resolution quite useless. A six digit DVM can feature an uncertainty range, for short periods

of time in controlled environments, as low as the 0.0015% of reading or 0.0002% of full

range. The speed of a DVM can be as high as 1000 readings per second. When the ADC is

considered, the conversion rate is taken into account instead of the speed of reading.

Characteristic of the DVMS

Input range: + 1.000000V to 1,000000V,with automatic range selection and overload

indication.

Absolute accuracy: as high as+ 0.005% of the reading.

Stability: short term, 0.002% of the reading for 24 hrs period; long tern, 0.008% of

the reading for a 6-months period.

Resolution: 1 part in 106 (1 v can be read on the 1V input range).

Calibration: internal calibration standard allows calibration independent of the

measuring circuit; derived from stabilized reference source

Output characteristic: output is uniform of digital for further processing or recording

Input characteristics: input resistance typical 10m , input capacitance typically 40pF.

Advantages:

Since the development of integrated circuit (Ic) modules, power requirements and

cost of the DSVM have been drastically reduce, so that DVMs can actively compete with

conventional analog instruments, both in portability and price.

Principle of Operation:

There are five main methods used in the construction of a digital voltmeter for

conversion of an analog signal to a digital one. These are.

Successive approximate

Ramp or voltage to time conversion

Integrating type or voltage frequency method

Dual slope techniques

Recalculating remainder

2.11.1. Successive Approximation Method:

This is the fastest and one of the most stable and basic analog to digital conversion

techniques. Instruments using this method work automatically in a similar manner to the operator

of a normal laboratory d.c potentiometer. In the successive of normal approximation (dvm) seen

in fig.2.20. below. The blocks diagram consist of a voltage divider network, with coarse and five

steps is connected via read or transistor switches to a voltage comparator (the equivalent of the

potentiometer operator galvanometer), which compares the internal voltage with the unknown.

The output of the comparator feeds the logic circuits which control the steps on the voltage

divider network. A measurement sequence usually selects the largest steps of the internal voltage

first, the magnitude of the steps decreases until the null point is reached.

Fig.2.20.Block Diagram of successive Approximation Method

2.11.2. Ramp or Voltage to Time Conversion Techniques:

The operating principle of a ramp method is to measure the time it takes for a linear

ramp voltage to rise from OV to the level of input voltage or to decrease from the level of the

input to OV. This time interval is measured with an electronic time interval counter and the

count is displayed as a number of digits on electronic indicating tubes of the output readout

of the voltmeter as shown in fig. 2.21.

Fig.2.21. Block diagram of a ramp DVM

Fig.2.22.Timing Diagram Showing Voltage to Time Conversion

At the start of the measurement cycle, a ramp voltage is initiated, this voltage can be

positive or negative going. The negative going ramp is shown in fig.2.22. this is continuously

compared with the unknown input voltage, when the input voltage is equal to the unknown

voltage, a coincidence circuit or comparator generates a pulse which opens a gate. This gate

is shown in the fig. the ramp voltage continues to decrease with time until it finally reaches

OV. At this instant another comparator called ground comparator generates pulse and closes

the gate. An oscillator generates clock pulses which are allowed to pass through the gate to a

number of decade counting units(DCUS) which totalize th pulses passed through the gate.

The decimal number displayed by the indicating tubes associated with the DCUS is a

measure of the magnitude of the input voltage.

2.11.3. Voltage To Frequency (Integrating Type):

Fig.2.23. Block diagram of voltage to frequency converter

This method consist of an oscillator whose frequency depends on the input voltage,

thus its precisely related to the difference in the input voltage levels. Its mode of operation is

fundamentally different but uses the ramp principle to count. It operates such that the

frequency generated by the voltage to frequency counter passes through the gate which

remains open for a certain pre-determine time interval (as set by fixed time generator).the

pulse are counted and scaled then displayed as representing the input signal.

The errors of (DVM) using technique are dependent on

The accuracy and linearity of the voltage to frequency conversion, which is not as

inherently stable or accurate as this successive approximation method

The precision of the time interval over which the frequency measurement is made,

which may be small by using crystal control. The internal reference on calibration

voltage .

2.11.4. Dual slope technique:

In this method of analog-digital conversion, an attempt is made to combine the

advantage and remove the disadvantage of the two proceeding methods. For while, the actual

measurement is a voltage to time conversion, the same time is constant and can be arranged

to reject power line noise. Thus the unknown voltage is determined by a two stage operation.

The first stage of which occurs a fixed time T = 1/f mains frequency, during a which

capacitor is charged at a rate proportional to the input voltage as shown in fig.2.24.

Fig 2.24. Dual slope technique

At the end of time T then 1/f to the operational amplifier is switched to a reference

voltage of opposite polarity to the input voltage and the capacitor discharged at a constant

rate going the time internal, for pulses to flow the clock is directly proportional to the

magnitude of the input voltage e.g t αV1 The errors on this technique are also dependent on

this frequency but is affected by

The input or reference switch characteristic

The voltage and leakage characteristic of the operational amplifier

The comparator characteristics

The reference voltage

The major advantage and reasons for wide use of the dual slope technique is its

inherent rejection of supply frequency interference. Additionally good accuracy and stability

are possible but the reading rate is limited to half the power line frequency thus excluding the

use of the technique from high speed data acquisition system.

2.25. Block diagram of dual slope technique

2.12. WATTMETER:

A wattmeter, as its name implies, measure electric power given to or develop by an

electronic apparatus or circuit. A wattmeter is hardly over required in a d.c circuit because

power (P = VI) can be easily determined from voltmeter and ammeter readings. However, in

an a.c circuit, such a computation is generally speaking impossible. It is because in an a.c

circuit, power (P = VI Cos θ) depends not only on voltage and current but also on the phase

shift between them. Therefore, a wattmeter is necessary for a.c power measurement.The

wattmeter shows a reading which is proportional to the product of the current through its

current coli, the p.d across its potential or pressure coil and cosine of the angle between this

voltage and current.

The “wattmeter” is an indicating type instruments, generally used for power

measurement of the electrical circuit .

A wattmeter consists of

a low resistance current coil which is inserted in series with the line carrying the

current and

a high resistance pressure coil which is connected across the two points whose

potential difference is to be measured.

The wattmeter require polarity markings so that the current in the stationary coils will

be in the correct direction relative to the current in the movable coil There are two principle

types of wattmeter viz:

Dynamometer Wattmeter – for both d.c and a.c

power Induction Wattmeter – for a.c power only.

2.12.1. DYNAMOMETER:

Wattmeter design:

Power in an electric circuit is the product (multiplication) of voltage and current, so

any meter designed to measure power must account for both of these variables. A special

meter movement designed especially for power measurement is called the dynamometer

movement, and is similar to a D'Arsonval in that a lightweight coil of wire is attached to the

pointer mechanism. However, unlike the D'Arsonval movement, another (stationary) coil is

used instead of a permanent magnet to provide the magnetic field for the moving coil to react

against. The moving coil is generally energized by the voltage in the circuit, while the

stationary coil is generally energized by the current in the circuit. A dynamometer movement

connected in a circuit looks something like this:

Fig 2.26.(a) connection diagram of dynamometrer wattment

The top (horizontal) coil of wire measures load current in fig2.26.(a) as while the

bottom (vertical) coil measures load voltage. Just like the lightweight moving coils of

voltmeter movements, the (moving) voltage coil of a dynamometer is typically connected in

series with a range resistor so that full load voltage is not applied to it. Likewise, the

(stationary) current coil of a dynamometer may have precision shunt resistors to divide the

load current around it.

With custom-built dynamometer movements, shunt resistors are less likely to be

needed because the stationary coil can be constructed with as heavy of wire as needed

without impacting meter response, unlike the moving coil which must be constructed of

lightweight wire for minimum inertia.

Fig 2.26.(b) schematic diagram of dynamometer wattmeter

(Wattmeters are often designed around dynamometer meter movements, which employ both

voltage and current coils to move a needle.)

The dynamometer wattmeter is most commonly used to measure power in a.c

circuits. It works on the dynamometer principle i.e. mechanical force exists between two

current carrying conductors or coils. The wattmeter use an electrodynamometer movement

because the meter reads true power regardless of the value of angle θ.. Figure 3.3(b) shows

the circuit diagram of the electrodynamometer wattmeter

Fig.2.27.a Schematic diagram Fig.2.27.b connection circuit

Operation:

When the wattmeter is connected in the circuit to measure power (see figure 2.27.b),

the current (stationary coil) which is wound with a larger-diameter wire carries the load

current and potential (moving coil) coil carries current proportional to the load voltage.

Due to currents in the coils, mechanical force exists between them. The result is that

movable coil moves the pointer over the scale. The pointer comes to rest at a position when

deflecting torque is equal to the controlling torque. The moving coil is used to detect the

magnitude of the circuit voltage. The stationary coils are referred to as the current coils. The

circuit current is detected by the current coils, which are connected in series with the load.

The stationary current is wound with larger diameter. This keeps the resistance that is in

series with the load as low as possible. The moving coil is wound with thin wire to keep it as

high as possible. Since the movable coil responds to voltage, it has a multiplier (a high non-

inductive resistance) connected in series with the moving coil to limit the current flowing

through the moving coil to a small value, usually up to 100mA. Such instruments can be

used for the measurement of d.c as well as a.c power.

Deflection torque: We shall now prove that deflecting torque is proportional to load power.

Consider that the wattmeter is connected in a d.c circuit to measure power as shown in (fig

2.27.b).

The power taken by the load is VI1.

Deflecting torque, Td ∞ I1 I1.

Since I2 is directly proportional to V :

Deflecting torque, Td ∞ VI

∞ load power

And if the system is spring controlled then θ ∞ power.

The above statements refers to average power, but in the case of a.c Td ∞ VI CosΦ

Where Φ is the phase difference between the current and voltage.

Two ways of connecting wattmeters:

There are two alternative methods of connecting a wattmeter in a circuit. These are

shown in fig below. Due to these connections, errors are introduced in the measurement

among to power loss in the current coil and the pressure coil.

Fig 2.28. Wattmeter connections

In the connection of fig.2.28.

The pressure coil is connected on the supply side (i.e cc on the load side) and therefore

the voltage applied to the pressure coil is the voltage across the load plus the voltage

drop across the current coil. Thus the wattmeter measures the power loss in its current

coil in addition to the power consumed by load.

Power indicated by wattmeter = power consumed by load + power loss in current

coil I2 RC = PL + PC In connection

The current coil is on supply side and, therefore it carries the pressure coil current plus

the load current. Hence the wattmeter reads the power consumed by the load plus the

power loss in pressure coil.

Power indicated by wattmeter = power consumed by load + power loss in pressure coil

(V2/Rp).

If the load current is small, the voltage drop in the current coil is small, so that

connection of fig. (a) introduces a very small as compared with the load current and hence

power loss in pressure coil will be very small as compared with the load power and,

therefore, connection of fig (b) is preferable.

Advantages Of Dynamometer Wattmeter:

Such instruments can be made to give a very high degree of accuracy. Hence, they

are used as a standard for calibrated purposes.

They are equally accurate on d.c as well as a.c measurements.

It can be used on both a.c and d.c supply, for any waveform of voltage and current,

and is not restricted to sinusoidal waveforms.

Disadvantages Of Dynamometer Wattmeter:

At low power factor, the inductance of the voltage coil causes serious error unless

special precautions are taken to reduce this effect.

2.12.2. INDUCTION WATTMETER:

This induction type wattmeter can be used to measure a.c power only in contrast to

dynamometer wattmeter which can be used to measure d.c as well as a.c power. However, it

differs from induction instrument in so far that two separate coils are used to produce the

rotating magnetic field in place of one coil with phase split arrangement. Figure.2.29.shows

the arrangement of the various part of an induction wattmeter.

Operation:

When the wattmeter is connected in the circuit to measure a.c power, the shunt magnet

carries current proportional to the supply voltage and the series magnet carries the load current.

The two fluxes produced by the magnets induce eddy currents in the aluminum disc.

Fig.2.29. Induction Wattmeter

The interaction between the fluxes and eddy currents produces the deflecting torques

on the disc, causing the pointer connected to the moving system to move over the scale. The

pointer comes to rest at a position where deflecting torque is equal to the controlling torque.

Let V = supply voltage,

Iv = current carried by shunt magnet

Ic = current carried by series magnet

Cos Φ = lagging power factor of the loads.

The current Iv in the shunt magnet lags the supply voltage by 900 and so does the flux

ΦV produced by it. This current IC in the series magnet is the load current and hence lags behind

the supply voltage V by Φ. The flux, Φc produced by this current (i.e. Ic) is in phase with it.

It is clear that phase angle θ between the two fluxes is 90 – Φ

i.e. θ = 90 – Φ :

Td ∞ Φ V Φ C sinθ

∞ VI (sin 90 – Φ)

∞ VI (-sin Φ)

∞ VI Cos Φ

∞ a.c power.

Since the instrument is spring controlled

Tc ∞ θ For steady deflected position,

Td = Tc θ = a.c power

Hence such instruments have uniform scale

Advantage Of Induction Wattmeter

They have a uniform scale

They are free from the effects of stray fields

They provide very good damping

Disadvantages Of Induction Wattmeter

They can be used to measure a.c power only

They cause series error due to temperature variation

They have high power consumption

Induction wattmeters have their chief application as panel instruments where

the variations in frequency are not too much.

2.13. SINGLE PHASE ENERGY METER:

Induction type instruments are most commonly used as energy meters. Energy meter

is an integrating instrument which measures quantity of electricity. Induction type of energy

meters are universally used for domestic and industrial applications.

Energy meter is an instrument which measures electrical energy. It is also known as

watt-hour (Wh) meter. It is an integrating device. There are several types of energy meters

single phase induction type energy meter are very commonly used to measure electrical

energy consumed domestic and commercial installation. lectrical energy is measured in kilo

watt-hours (kWh) by this energy meter.

Construction:

A single phase induction type energy meter consists of driving system, moving system,

braking system and registering system. Each of the systems is briefly explained below.

Driving system: - This system of the energy meter consists of two silicon steel laminated

electromagnets. 1 & 2 as shown in fig.1The electromagnet M1 is called the series magnet and the

electromagnet 2 is called the shunt magnet. The series magnet M1 carries a coil consisting of a

few turns of thick wire. This coil is called the current coil (CC) and it is connected in series with

the circuit. The load current flows through this coil. The shunt magnet M2 carries a coil

consisting many turns of thin wire. This coil is called the voltage coil (VC) and is connected

across the supply it consist of current proportional to the supply voltage. Short circuited copper

bands are provided on the lower part of the central limb of the shunt magnet. By adjusting the

position of these loops the shunt magnet flux can be made to lag behind the supply voltage

exactly 900. These copper bands are called power factor compensator (PFC). A copper shading

band is provided on each outer limb of the shunt magnet (fc1 &fc2) these band provides

frictional compensation.

Moving system: - The moving system consists of a thin aluminum disc mounted on a spindle and

is placed in the air gap between the series and the shunt magnets. It cuts the flux of both the

magnet forces are produced by the fluxes of each of the magnets with the eddy current induced in

the disc by the flux of the other magnets. Both these forces act on the disc. These two forces

constitute a deflecting torque.

Braking system: - The braking system consists of a permanent magnet called brake magnet. It is

placed near the edge of the disc as the disc rotates in the field of brake magnet eddy current are

induced in it. These eddies current react with the flux and exert a torque. This torque acts in

direction so that it opposes the motion of disc. The braking torque is proportional to the speed of

the disc.

Registering system: - the disc spindle is connected to a counting mechanism this mechanism

records a number which is proportional to the number of revolutions of the disc the counter is

calibrated to indicate the energy consumed directly in kilo watts-hour (kWh).

Fig.2.30. single phase energy meter

Working:-

Since the pressure coil is carried by shunt magnet M2 which is connected across the

supply, it carries current proportional to the voltage. Series magnet M1 carries current coil which

carries the load current. Both these coils produce alternating fluxes Φ1 and Φ2 respectively.

These fluxes are proportional to the currents in their coil. Part of each of these fluxes link with

the disc and induced emf in it. Due to these emf eddy current are induced in the disc. The eddy

current induced by the electromagnet M2 react with magnetic field produced by M1. Also eddy

current induced by electromagnet M1 react with magnetic field produced by M2. Thus the each

portion of the disc experiences a mechanical force and due to motor action, disc rotates. The

speed of the disc is controlled by the C shaped magnet called braking magnet. When disc rotates

in the air gap, eddy currents are induced in disc which oppose the cause producing them. Hence

braking torque Tb is generated. This is proportional to the speed N of disc. By adjusting position

of this magnet, desired speed of disc is obtained.

Spindle is connected to recording mechanism through gears which record the energy supplied.

Let V=Supply voltage

I=Load current lagging behind V by Φ

Cos Φ = Load Power Factor (Lagging)

Ish = Current setup by Φsh in disc

Ise = Current setup by Φse in disc

Phasor diagram will be as follows:

Fig.2.31. phasor diagram

Instantaneous deflecting torque

Td α (ψsh ise –ψse ise) where ψ & i are instantaneous

values Average deflecting toque

Td α [Φsh Ise cosΦ – Φse Ish cos(180 Φ)] where Φ & I are RMS values

Td α [Φsh Ise cosΦ + Φse Ish cosΦ]

Td α [Φsh Ise + Φse Ish] cosΦ

We know Φsh α V, Ise α I, Φse α , sh α V

So Td α [VI+VI] cosΦ

Td α VIcosΦ α Power

Now braking torque is proportional to speed N which disc rotates.

Td α N

Tb α Td

Multiplying both side by t, Nt α VI1tcosΦ α Pt.

Number of revolution in time t α energy supplied.

Advantages:

The various types of induction energy meter are:

Its construction is simple and strong

It is cheap in cost

It has high torque to weight ratio, so frictional errors are less and we can get accurate

reading.

It has more accuracy

It requires less maintenance

Its range can be extended with the help of instrument transformer

Disadvantages:

It can be used for only ac circuits

Creeping can cause errors

Lack of symmetry in magnetic circuit may cause errors.

2.14. THREE PHASE ENERGY METER:

In a three phase, four wire system, the measurement of energy is to be carried out by a

three phase energy meter. For three phase, three wire system, the energy measurement can be

carried out by two element energy meter, the connections of which are similar to the

connections of two wattmeter for power measurement in a three phase, three wire system. So

these meters are classified as

Three element energy meter

Two element energy meter

2.14.1.THREE ELEMENT ENERGY METER:

This meter consists of three elements. The construction of an individual element is

similar to that of the single phase energy meter. The pressure colis are denoted as P1, P2 and

P3. The current coils are denoted as C1, C2, C3. All the elements are mounted in a vertical

line in common case and have a common spindle, gearing and registering mechanism. The

coils are connected in such a manner that the net torque produced is sum of the torques due

to all the three elements. These are employed for three phase, four wire system where fourth

wire is a neutral line.

The current coils are connected in series with the lines while pressure coils are

connected across a line and a neutral as shown in fig.

Fig.2.32.three element energy meter

One unit of three element, three phase element is always cheaper than the three units

of single phase energy meter. But due to interaction between eddy currents produced by one

element with the flux produced by another element, there may be errors in the measurement

by three phase energy meter. Such errors may be reduced by suitable adjustments.

2.14.2.TWO ELEMENT ENERGY METER:

Fig shows a two element energy meter and a simplified connection diagram

Fig.2.33.. Two element energy meter

This energy meter is used for three phase, three wire systems. The meter si provided

with two discs each for an element. The shunt magnet is carrying pressure coil while a series

magnet carries a current coil. The pressure coils are connected in parallel and the current coil

in series. The connections are similar to the connections of two wattmeters for power

measurement in three phases, three wire system. Torque is produced in same manner as in a

single phase energy meter, in each element. The total torque on the registering mechanism

connected to moving system, is sum of the torques of the individual elements.

Adjustment in energy meter:

The adjustment are required in the energymeters so that they read accurately with

minimum possible errors.

Main speed adjustment: The measurement of energy is dependent on the speed of the

rotating disc. For accurate measurement, speed of the disc must be also proportionate. The

speed of the meter can be adjusted by means of changing the effective radius of the braking

magnet. Moving the braking magnet in the direction of the spindle, decrease the value of the

effective radius, decrease the braking torque.. this increases the speed of the torque.while the

movement of the braking magnet in the outward direction, decrease the speed of the disc.

Power factor adjustment: it is absolutely necessary that meter should measure correctly for

all power factor conditions of the loads. This is possible when the flux produced due to

current in the pressure coil lags the applied voltage by 900. But the iron loss and resistance

winding do not allow the flux to lag by exact 900 with respect to the voltage.

Friction adjustment: Inspite of proper design of the bearings and registering the mechanism,

there is bound to exist some friction. Due to this speed of the meter gets affected which cause

the error in the measurement of energy. To compensate for this, a metallic loop is provided

between central limb of shunt magnet and the disc.

Creep adjustment: it is seen that, without any current through current coil. Disc rotates due

to the supply voltage exciting its pressure coil. This is called creeping. Tis creeping may be

because of overfriction compensation.

2.15. MAGNETIC MEASUREMENTS:

The measurements of various properties of a magnetic material are called magnetic

measurements. The magnetic materials play a very important role in the operation of

electrical machines hence measurement of various characteristics of a magnetic materials is

important from the point of view of designing and manufacturing of electrical machines. The

magnetic measurements include,

Measurement of flux density B in a specimen of ferro magnetic material

Measurement of magnetizing force H, producing the flux density B, in air

Determination of B-H curve and the hysteresis loop

Determination of eddy current and the hysteresis losses

Testing of permanent magnets

For such magnetic measurements following test are performed:

1. D.C. Tests: these are used to determine B-H curve and hysteresis loop of ferro-

magnetic materials. The direct current is used to have variable mmf and fluxmeter is

used to measure the flux density. A ballistic galvanometer can be used to measure

flux density. Such test are called ballistic tests.

2. A.C.Tests: when a ferro-magnetic material is subjected to a cycle of magnetization and

demagnetization then the eddy current and hysteresis losses occur. Hence alternating

current is used to determine the iron losses, having provision of a variable frequency and

form factor. Such tests are carried out at power, audio or radio frequencies.

3. Steady State Tests: the flux in the airgap plays an important role in the operation of

various electrical equipments. Such a flux is measured using steady state test. Such

test give steady state value of the flux in the airgap of a magnetic material.

The results of the magnetic measurements are not very accurate because of

following reasons:

The magnetic materials are non-homogeneous

The condition at the time of calculations are different than the conditions

existing at the time of testing of magnetic material Various group of test

specimens have no uniformity

2.16. DIGITAL FREQUENCY METER:

Frequency can be measured with a variety of electric and electronic devices.

Electronically, frequency can be measured with such devices as digital frequency counter

and heterodyne frequency meters. These devices are capable of measuring a wide range of

frequencies extending to hundreds of MHz. Electric frequency meters can only measure a

narrow range of frequencies in the power frequency range.

A digital frequency meter measures an unknown frequency by counting the number

of cycles the frequency produces in precisely controlled period of time. The counter circuit is

incremented one count for each cycle. At the end of the time period, the final count, which

represents the frequency is displayed by the digital readout. For the next sampling of the

unknown frequency, the counter is cleared, the time period is started over and the final count

in the counter is again displayed.

If the measured frequency is stable, the readout does not change from sample to

sample. Because the range switch selects the time period and places the decimal point in the

readout, the indicated frequency is in the units specified by the range switch.

When the time period is 1ms, the readout is KHz and the range switch indicated KHz.

For example, if the count at the end of the 1ms period is 100, the unknown frequency must

be 100KHz, because 100 counts per ms is equal to 100000 counts per second.

The signal whose frequency is to be measured is converted into a train of pulses, one

pulse for each cycle of the signal. Then the number of pulse appearing in a definite interval

of time is counted by means of an electronic counter. Since the pulses represent the cycles of

unknown signal the number of appearing on the counter is direct indication of frequency of

the unknown signal. Since the electronic counters are extremely fast, the frequency of high

frequency signal may be known.

The block diagram of the basic of a digital frequency meter as shown in fig. the

unknown frequency signal is fed to a Schmitt trigger. In a Schmitt trigger, the signal is

converted into a square wave with very fast rise and fall times, then differential and clipped.

As a result the output from a Schmitt trigger is a train of pulses, one pulse, for each cycle of

the signal.

Fig.2.34. Digital frequency meter

The output pulses from the Schmitt trigger are fed to start-stop gate. When this gate

open(start) the input pulses pass through this gate and are fed to an electronic counter which

starts registering the input pulses when the gate is closed(stop), the input of pulses to counter

ceases and it stops counting.

The counter displays the number of pulses that have passed through it in the time

interval between start and stop. If this interval is known, the pulse rate and hence the

frequency of the input signal can be known.Suppose F is the frequency of unknown signal,

Nis the number of counts displayed by counter and its time interval between start and stop of

gate. Therefore frequency of unknown signal can be given as

F = Nit (Hz)

2.17. PHASE METER:

The phase meter is a device which measures the phase difference between the two

signals. In the simplest method of phase measurement, one of the two signals is used as s

reference signal. A zero center galvanometer is used for deflection. When the signal is

applied, the indicating galvanometer deflects depending on the phase relationship of that

signal with the reference signal.

The fig2.35, 2.36. shows the phase sensitive detector. Ie. Phase meter.

Fig.2.35.

Vs = signal voltage

Vr = reference voltage

Fig.2.36.

For the first half cycle, the instantaneous polarity of Vr causes the rectified current to

flow through the rectifier diode D1. This produces positive voltage drop across R1 as shown

in fig.2.1. Due to this meter deflects to the right.

In the second half cycle, the polarity of Vr changes. thus the current flows through

diode D2. This causes a positive voltage drop across R2 as shown in fig.(b). the meter

dedflects to the left.

Infact as these two deflections are equal and opposite, the average deflection of a

meter over a full cycle is zero, when the input Vs = 0.

The two diodes D1 and D2 are providing the rectifier action.

Now consider that the input signal voltage Vs is applied. This voltage helps or

opposes the voltage Vr, depending upon whether it is in phase or out of phase with it.

If it is in phase with Vr the signal voltage will help the instantaneous ac voltage in the

upper half of the transformer secondary. This will produce the large current through diode

D1 hence large dc output voltage across R1. As long as Vs is less than Vr, the diode D2 will

not conduct. Thus voltage in upper half is Vs + Vr while in lower half it is Vs - Vr .

In the next half cycle, Vs polarity is reversed. D1 now stops conducting. Vs will

oppose the instantaneous ac voltage to produce small dc voltage across R2. The

galvanometer deflects to the right indicating that the Vs is in phase with Vr.

If Vs is 180o out of phase, then the two voltages aids each other in the lower half on

the transformer secondary. Hus the galvanometer deflects to the left proportional to the

magnitude of the input signal.

As the method uses voltage addition principle, it is called as voltage addition method

of phase measurement.

The advantage of this method are:

The method is very simple

The value of the input signal voltage Vs can be calculated.

The disadvantage of the methods are:

o The phase difference of 180 or in phase condition only can be detected. Other phase angles cannot be

measured.

To overcome these disadvantages the digital phase meter is used.

2.17.1. DIGITAL PHASE METER:

This meter uses two flip flops. The two signals of the same frequency are applied to

the meter. In this meter, both the signals are shaped to a square waveform, without any

change in their phase relationship, which is required to be measured.

The function of two flip flops is that the one enables the output control gate while the

disenables the output control gate. The number of pulses allowed to pass during enabling and

disabiling the gate are counted which are proportional to the phase difference between the

two signals.

The schematic block diagram of the digital phase meter is shown in the fig,.

Fig.2.37. Digital phase meter

It consists of two preamplifiers, zero crossing detectors, J-K flip flops and an output

control gate. The phases of the two input signals are Φo and Φx. The frequency of the input

is same.

As Φo signal increases in the positive half cycle, when it crosses zero, the zero crossing

detector senses the zero crossing and changes its state. This causes first J-K flip flop to be set (1)

and its output Q becomes high. This enables the AND gate and thus output gate is enabled which

allows the clock pulses to fed directly to the counter. Now Φx is having a certain phase

difference with respect to Φo . this means it will cross zero, after the Φo crosses the zero. This

zero crossing of Φo is detected by second zero crossing detector which causes its state to change.

Thus the output of second J-K flip flop goes high. This is connected to the clear input of first J-K

flip flop. This resets the first J-K flip flop and output Q of J-L flip flop one goes low (0). Due to

this AND gate is disabled and counter stops counting.

Fig.2.38. phase measurement waveform

The number of pulses counted while enabling and disabling of AND gate is directly

proportional to the phase difference.

The display unit displays accurately the phase difference between two signals. For

accurate measurement s, if input signal frequency is f, then the clock frequency must be 360

times the input frequency.

The advantages of the method are:

High accuracy

And phase angle difference can be detected and measured.

The speed of operation is fast

The circuit is simple to design.

The disadvantage of the method are:

Both inputs must have same frequency

It is difficult to measure small phase differences

For very accurate results the clock frequency should be 360 times the input

frequency.