Questions and Answers In Measurement & Instrumentation (part-1)

1)    Write down the importance of Measurement and Instrumentation.


         i)Gathering data to improve the next execution of the program.

         ii) Guiding scheduling decisions.

         iii) Adapting to computations while in execution


2)    What is the Need of Instrument?
                The purpose of a measurement system is to link the observer to a process which
                generates information.

3)    Draw the Basic structure of Measurement System?

4)    How the Signal conditioning element differed from the Signal processing Element?

Some non-linear processes like modulation, detection, sampling,
filtering, chopping etc.,are performed on the signal to bring it to the
desired form to be accepted by the next stage of measurement system
This process of conversion is called   signal conditioning element’
The term signal conditioning includes many other functions in addition to Variable conversion & Variable manipulation In fact the element that follows the primary sensing element in any instrument or measurement system is called   signal processing element

5)    What is Sensing Element? Ex

               It is used to sense the original value then the value convertt into suitable level to
           the further process.

        Eg:
              (i)Thermocouple – voltage depends on temperature.

             (ii)Strain gauge – resistance depends on mechanical strain.

             (iii)Orifice plate – pressure drop depends on flow rate.

             (iv)Ultrasonic transducer – electrical output depends on mechanical

              (v)forces (vibrations) acting on the surface of the transducer.


6)    List out the Functional Elements of Measurement System.

           Most of the measurement systems contain three main functional
          elements are i) Primary sensing element ii) Variable conversion element &
           iii) Data presentation element.

7)    Write down the Types of Characteristics

              The performance characteristics of an instrument are mainly divided into two categories:   i) Static characteristics ii) Dynamic characteristics

8)    What is Static Characteristics?

              The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called ‘static characteristics’.

9)    Define Dynamic Characteristics.

             The set of criteria defined for the instruments, which are changes rapidly with time, is called ‘dynamic characteristics’

10)    Compare Repeatability & Reproducibility.

            
 Reproducibility:

It is the degree of closeness with which a given value may be repeatedly measured. It is specified in terms of scale readings over a given period of time.
          
 Repeatability:

               It is defined as the variation of scale reading & random in nature Drift:
               Drift may be classified into three categories: zero drift, span drift or sensitivity
              drift, Zonal drift:

MULTIPLEXER & DEMULTIPLEXER

ENCODER AND DECODER

Transformer Tap Changer

A tap changer is a device fitted to power transformers for regulation of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s. Supply authorities are under obligation to their customers to maintain the supply voltage between certain limits. Tap changers offer variable control to keep the supply voltage within these limits. About 96% of all power transformers today above 10MVA incorporate on load tap changers as a means of voltage regulation.

Tap changers can be on load or off load. On load tap changers generally consist of a diverter switch and a selector switch operating as a unit to effect transfer current from one voltage tap to the next. It was more than 60 years ago on load tap changers were introduced to power transformers as a means of on load voltage control.

Tap changers possess two fundamental features:

(a) Some form of impedance is present to prevent short circuiting of the tapped section,

(b) A duplicate circuit is provided so that the load current can be carried by one circuit whilst switching is being carried out on the other.

The impedance mentioned above can either be resistive or reactive. The tap changer with a resistive type of impedance uses high speed switching, whereas the reactive type uses slow moving switching. High speed resistor switching is now the most popular method used worldwide, and hence it is the method that is reviewed in this report.

The tapped portion of the winding may be located at one of the following locations, depending upon the type of winding:
(a) At the line end of the winding;
(b) In the middle of the winding;
(c) At the star point.

The most common type of arrangements is the last two. This is because they give the least electrical stress between the tap changer and earth; along with subjecting the tapings to less physical and electrical stress from fault currents entering the line terminals. At lower voltages the tap changer may be located at either the low voltage or high voltage windings.

Tap changers can be connected to the primary or secondary side windings of the transformer depending on:

- Current rating of the transformer
- Insulation levels present
- Type of winding within the transformer (eg. Star, delta or autotransformer)
- Position of tap changer in the winding
- Losses associated with different tap changer configurations eg. Coarse tap or reverse winding
- Step voltage and circulating currents
- Cost
- Physical size on-load tap changer

The Consequences Of Transformer Failure

Transformers are one of the more expensive pieces of equipment used in a power system, and the potential consequences of failure can be quite damaging. This has been shown in the past with the political and media attention surrounding blackouts at various locations around the world. Within Australia and New Zealand, the largest cost transformer failures have occurred due to internal winding faults, faulty load tap changers, and failed winding accessories respectively.

Failure of winding accessories includes loose coil clamping bolts, together with internal winding faults and faulty tap changers. These failures affected on average ten transformers per year during the period 1975 to 1995, incurring repair costs of at least $600,000 per year, together with other associated costs. For example, with several elements drawn from an Australian case study and through discussion with engineers at Pacific Power’s Advanced Technical Center, the cost of a generator unit transformer failing has been conservatively estimated at $5.4M. This figure was considered conservative because there are many other factors that could be added on to this figure that are difficult to determine. It is interesting to note the root of these failures appear to have been predominantly design and manufacturing flaws.

Current Maintenance Strategies of Transformer Tap Changers

During the past years and after a number of visits, meetings, lectures and training courses, the conclusion has been reached that proper organization and execution of OLTC maintenance is found only in very few cases.

The frequency of maintenance to on load tap changers is dependent on the condition of the diverter switch and the necessity to maintain the motor drive unit. Maintenance of the diverter switch should be carried out on a cyclic basis, but on transformers where frequency of tap change is high, maintenance may be necessary before the cyclic maintenance becomes due. A certain period should not be exceeded between inspections. When considering inspection periods, serious consideration should be given to the breaking of circulating current which in some cases may exceed the load current.

The diverter switch and tap selector is the only internal moving parts in a transformer. The diverter switch does the entire on load making and breaking of currents, whereas the tap selector preselects the tap to which the diverter switch will transfer the load current. The tap selector operates off load and therefore needs no maintenance. However experience has shown that in some circumstances inspection of selector switches becomes necessary where contacts become misaligned or contact braids in fact fatigueandbreak.

The next segment is a list taken from on what should be carried out during tap changer maintenance;

- Replace contacts in older type tap changers. Modern tap changers rarely require contact replacement; this depends on the characteristics of the tap changer in question. The frequency of diverter switch and motor drive unit inspections can usually be obtained from manufacturer manuals or previous maintenance experience.

- Measuring and recording contact consumption during inspection will give a reasonably accurate life expectancy of the contacts at that present load condition. Therefore this should be done on a regular basis.

- Transition resistors should be checked for continuity and value as an open circuited resistor can result in excessive contact wear.

- Need to equalize rotation lag between the diverter switch and the motor drive unit to ensure minimum spring energisation in the energy accumulator springs.

- The function of relays, interlocks, limit switches and switches should be checked as well as remote indication of tap position.

- Drive shafts and gearboxes must be inspected for radial and axial wear. A large percentage of tap change failures are as a result of drive shaft faults.

- Replace transformer oil with clean, dry oil. Cleaning is only carried out with transformer oil not solvents. Carbon and copper deposits are generally found on horizontal surfaces of the diverter switch as small convection currents in the oil are established each tap change. This results in the carbon being deposited on top of the diverter.

Why We Use Transformers

Due to the high cost of transmitting electricity at low voltage and high current levels, transformers fulfill a most important role in electrical distribution systems. Utilities distribute electricity over large areas using high voltages, commonly called transmission voltages. Transmission voltages are normally in the 35,000 volt to 50,000 volt range. We know that volts times amps equals watts, and that wires are sized based upon their ability to carry amps. High voltage allows the utility to use small sizes of wire to transmit high levels of power, or watts. You can recognize transmission lines because they are supported by very large steel towers that you see around utility power plants and substations. As this electricity gets closer to its point of use it is converted, through the use of transformers, to a lower voltage normally called distribution voltage. Distribution voltages range from 2,400 to 25,000 volts depending upon the utility. Distribution lines are the ones that feed the pole mount and pad mount transformers located closest to your home or place of business. These transformers convert the distribution voltages to what we call utilization voltages. They are normally below 600 volts and are either single-phase or three-phase and are utilized for operating equipment, including light bulbs and vacuum cleaners in our homes, to motors and elevators where we work. This is the point at which the Dry-Type Distribution Transformer comes into play. It is used to convert the voltage provided by the utility to the voltage we need to operate various equipment.

TYPES OF FLIP FLOPS

Types of flip flops are:

1.    RS flip flop

2.    JK flip flop

3.    D flip flop

TRUTH TABLE 

ASTABLE AND MONOSTABLE MULTIVIBRATOR USING NE555 TIMER

 IC555 is a combination of linear comparators and digital flip flops. The output of comparators is used to set/reset the FF. The output FF circuit is brought out through an amplifier stage. The FF output is also give to a transistor to discharge a timing capacitor. The 555 timer has two basic operational modes: astable and monostable.

Astable operation

 When IC555 is to be configured as an astable multivibrator, both the trigger and threshold inputs (pins 2 and 6) to the two comparators are connected together and to the external capacitor. The capacitor charges toward the supply voltage through the two resistors, R1 and R2. The discharge pin (7) connected to the internal transistor is connected to the junction of those two resistors.

When power is first applied to the circuit, the capacitor will be uncharged; therefore, both the trigger and threshold inputs will be near zero volts. The lower comparator sets the control flip-flop causing the output to switch high. That also turns off transistor T1. That allows the capacitor to begin charging through RA and RB.

As soon as the charge on the capacitor reaches 2/3 of the supply voltage,
the upper comparator will trigger causing the flip-flop to reset. That causes the output to switch low. Transistor T1 also conducts. The effect of T1 conducting causes resistor RB to be connected across the external capacitor. Resistor RB is effectively connected to ground through internal transistor T1. The result of that is that the capacitor now begins to discharge through RB.

As soon as the voltage across the capacitor reaches 1/3 of the supply voltage, the lower comparator is triggered. That again causes the control flip-flop to set and the output to go high. Transistor T1 cuts off and again the capacitor begins to charge. That cycle continues to repeat with the capacitor alternately charging and discharging, as the comparators cause the flip-flop to be repeatedly set and reset. The resulting output is a continuous stream of rectangular pulses.

Monostable operation

The trigger input is initially high (about 1/3 of +V). When a negative-going trigger pulse is applied to the trigger input, the threshold on the lower comparator is exceeded. The lower comparator, therefore, sets the flip-flop. That causes T1 to cut off, acting as an open circuit. The setting of the flip-flop also causes a positive-going output level which is the beginning of the output timing pulse.


CIRCUIT DIAGRAM    

ASTABLE MULTIVIBRATOR

MONOSTABLE MULTIVIBRATOR

DESIGN

Astable Multivibrator

T= 0.693(RA+2RB) C

   = 0.7 (RA+2RB) C

Let RA=RB=R

            T= 2.1 RC

Assume R=10K and C = 0.1F for T=2.1msec

Also TON = 0.7(RA+RB)C =1.4msec

        TOFF    = 0.7RBC=0.7msec

Monostable Multivibrator
TON= 1.1 RC

Assume R=10K and C=0.1F for TON=1.1msec



MODEL GRAPH

ASTABLE MULTIVIBRATOR

MONOSTABLE MULTIVIBRATOR

The capacitor now begins to charge through the external resistor. As soon as the charge on the capacitor equal 2/3 of the supply voltage, the upper comparator triggers and resets the control flip-flop. That terminates the output pulse which switches back to zero. At this time, T1 again conducts thereby discharging the capacitor.

Whenever a trigger pulse is applied to the input, the 555 will generate its single-duration output pulse. Depending upon the values of external resistance and capacitance used, the output timing pulse may be adjusted and the duration of the output pulse is approximately equal to T = 1.1 x R x C

INTEGRATOR AND DIFFERENTIATOR

In a differentiator circuit, the output voltage is the differentiation of the input voltage. There are two types of differentiator called passive differentiator and active differentiator. The active differentiator using active components like op-amp.

The output voltage is given by

                                                Vout = - 1/ (RfCf) [dVin / dt]
                              Time constant    = - RfCf

The negative sign indicates that there is a phase shift of 180 degree between input and output. The main advantage of such an active differentiator is the small time constant which gives perfect differentiation.

Sometimes a compensation resistance is needed to connect to the non-inverting terminal to provide the bias compensation. The compensation resistance values is given by Rcomp = (Rf  parallel with R1 ).


CIRCUIT DIAGRAM
INTEGRATOR

 

DESIGN
Integrator design

The output voltage is given by

                                                Vout = - 1/ (RfCf)   Vin (t) + Vo (0)
                              Time constant    = Rf Cf

1)    To find Cf

             The gain value is given by A          = (Rf / R1) / (1 + jω RfCf) ------------------- (1)
            The corner frequency is           fc    = 1 / 2RfCf    -------------------------------- - (2)
Choose,           fc    = 100Hz and
                        Rf    = 10KΩ
By substituting all in equation (2), calculate the value of Cf .

2)    To find R1  

                    Let   Gain (A) =   1 and substitute all remaining values in equation (1), then               find the value of R1.




MODEL GRAPH
INTEGRATOR

CIRCUIT DIAGRAM
 DIFFERENTIATOR

DESIGN
Differentiator design
The gain value is given by A          = - jω RfC1 / (1 + jω R1C1)2                ------------------- (1)

The lower corner frequency is           fa    = 1 / 2R1C1      ----------------------------------- (2)

The upper corner frequency is           fb    = 1 / 2RfC1      ----------------------------------- (3)

Always assume fa < fb < fc   and Rf C1 < T. Where T is time constant.

Design procedure

1.    Choose fa as the highest frequency of the input signal. i.e. fa    =   100Hz

2.    Choose C1 to be less than 1 micro Farad and calculate the value of R1.

Choose C = 1micro Farad and    from equation (2) and Calculate R1.

3.    Choose fb as 10 times fa which ensures that fa < fb. That is fb =10 fa. Now find Rf.

4.    To find Cf ,  use   RfC1 =  R1C1 and Rcomp  = R1 parallel with Rf .

MODEL GRAPH
DIFFERENTIATOR

INVERTING AND NON INVERTING AMPLIFIERS

1) Inverting amplifier

        In this mode the positive input terminal of the amplifier is grounded and the input signal is applied to the negative input terminal through resistor R .The feedback applied through a resistor from output to the input terminal is negative.

2) Non inverting amplifier

        In this mode the negative input terminal of the input is grounded and the input signal is applied to the positive input terminal through R. The feed back applied through a resistor from output to input terminal is negative.


1.    Gain of the inverting amplifier

     A   = -Rf / Ri
                        V0 = -Avin
                                               V0 / Vin      = -Rf / Ri   ---------------------------- (1)
                                   Assume             A   = 2,
                                   Input voltage   Vin = 1Volts
                                                            R1   = 10 KΩ
Substitute all values in equation (1) and find Rf value.

2.    Gain of the non – inverting amplifier

  Vo / Vin =    (Rf + Ri) / Ri.
                                             A   = 1 + Rf/Ri ----------------- (2)
                                           Assume               A   = 2,
                                                 Input voltage   Vin   = 1Volts
                                                           Rin =    R1     = 10 KΩ

Substitute all values in equation (2) and find Rf value.

INTRODUCTION TO ELECTRIC MOTORS

Electric motors provide the driving power for a large and still increasing
part of our modern industrial economy. The range of sizes and types of
motors is large and the number and diversity of applications continues
to expand. The computer on which this book is typed, for example, has
several electric motors inside, in the cooling fan and in the disk drives.
There is even a little motor that is used to eject the removable disk from
its drive.

All around us there are electrical devices that move things around.
Just about everything in one’s life that whine, whirrs or clicks does so
because an electric motor caused the motion.

At the small end of the power scale are motors that drive the hands
in wristwatches, a job that was formerly done by a mechanical spring
mechanism. At the large end of the power scale are motors, rated in the
hundreds of megawatts (MW), that pump water uphill for energy storage.
Somewhat smaller motors, rated in the range of 12 to 15 MW, have
taken over the job of propulsion for cruise ships—a job formerly done by
steam engines or very large, low speed diesel engines.

The flexibility of electric motors and generators and the possibility of
transmitting electric power from place to place makes the use of electric
motors in many drive mechanisms attractive. Even in situations in which
the prime mover is aboard a vehicle, as in diesel-electric locomotives or
passenger ships, electric transmission has displaced most mechanical
or hydraulic transmission. As well, because electric power can be delivered over sliding contacts, stationary power plants can provide
motive power for rail vehicles. The final drive is, of course, an electric
motor.

The expansion of the use of electric motors’ industrial, commercial
and consumer applications is not at an end. New forms of energy storage
systems, hybrid electric passenger vehicles, and other applications not
yet envisioned will require electric motors, in some cases motors that
have not yet been invented.

This book provides a basic and in-depth explanation for the operation
of several different classes of electric motor. It also contains information
about motor standards and application. The book is mostly concerned
with application of motors, rather than on design or production. It takes,
however, the point of view that good application of a motor must rely on
understanding of its operation.

STATIC & DYNAMIC CHARACTERISTICS OF MEASUREMENT SYSTEM:

The performance characteristics of an instrument are mainly divided into two categories:

i) Static characteristics

ii) Dynamic characteristics

Static characteristics:

The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called ‘static characteristics’.

The various static characteristics are:

i) Accuracy

ii) Precision

iii) Sensitivity

iv) Linearity

v) Reproducibility

vi) Repeatability

vii) Resolution

viii) Threshold

ix) Drift

x) Stability

xi) Tolerance

xii) Range or span

Accuracy:

It is the degree of closeness with which the reading approaches the true value of the quantity to be measured. The accuracy can be expressed in following ways:

a) Point accuracy:

Such accuracy is specified at only one particular point of scale.
It does not give any information about the accuracy at any other Point on the scale.

b) Accuracy as percentage of scale span:

When an instrument as uniform scale, its accuracy may be expressed in terms of scale range.

c) Accuracy as percentage of true value:

The best way to conceive the idea of accuracy is to specify it in
terms of the true value of the quantity being measured. Precision: It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a measure of the degree of agreement within a group of measurements. The precision is composed of two characteristics:

a) Conformity:

Consider a resistor having true value as 2385692 , which is being measured by an ohmmeter. But the reader can read consistently, a value as 2.4 M due to the nonavailability of proper scale. The error created due to the limitation of the scale reading is a precision error.

b) Number of significant figures:

The precision of the measurement is obtained from the number of significant figures, in which the reading is expressed. The significant figures convey the actual information about the magnitude & the measurement  precision of the quantity. The precision can be mathematically expressed as:

Where, P = precision
Xn = Value of nth measurement
Xn = Average value the set of measurement values

Sensitivity:

The sensitivity denotes the smallest change in the measured variable to which the instrument responds. It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity to be measured. Mathematically it is expressed as,

Thus, if the calibration curve is liner, as shown, the sensitivity of the instrument is the slope of the calibration curve. If the calibration curve is not linear as shown, then the sensitivity varies with the input. Inverse sensitivity or deflection factor is defined as the reciprocal of sensitivity. Inverse sensitivity or deflection factor = 1/ sensitivity

Reproducibility:

It is the degree of closeness with which a given value may be repeatedly measured. It is specified in terms of scale readings over a given period of time.

Repeatability:

It is defined as the variation of scale reading & random in nature Drift:
Drift may be classified into three categories:

a) zero drift:

If the whole calibration gradually shifts due to slippage, permanent set, or due to undue warming up of electronic tube circuits, zero drift sets in.

b) span drift or sensitivity drift

If there is proportional change in the indication all along the upward scale, the drifts is called span drift or sensitivity drift.

c) Zonal drift:

In case the drift occurs only a portion of span of an instrument, it is called zonal drift.

Resolution:

If the input is slowly increased from some arbitrary input value, it will again be found that output does not change at all until a certain increment is exceeded. This increment is called resolution.

Threshold:

If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

Stability:

It is the ability of an instrument to retain its performance throughout is
specified operating life.

Tolerance:

The maximum allowable error in the measurement is specified in terms of some value which is called tolerance.

Range or span:

The minimum & maximum values of a quantity for which an instrument is designed to measure is called its range or span.

Dynamic characteristics:

The set of criteria defined for the instruments, which are changes rapidly with time, is called ‘dynamic characteristics’.
The various static characteristics are:
i) Speed of response
ii) Measuring lag
iii) Fidelity
iv) Dynamic error

Speed of response:

It is defined as the rapidity with which a measurement system responds to changes in the measured quantity.

Measuring lag:

It is the retardation or delay in the response of a measurement system to changes in the measured quantity. The measuring lags are of two types:

a) Retardation type:

In this case the response of the measurement system begins immediately after the change in measured quantity has occurred.

b) Time delay lag:

In this case the response of the measurement system begins after a dead time after the application of the input. Fidelity: It is defined as the degree to which a measurement system indicates changes in the measurand quantity without dynamic error.

Dynamic error:

It is the difference between the true value of the quantity changing with time & the value indicated by the measurement system if no static error is assumed. It is also called measurement error.

A GENERALIZED MEASUREMENT SYSTEM:

MEASUREMENTS:

The measurement of a given quantity is essentially an act or the result of comparison between the quantity (whose magnitude is unknown) & a predefined
Standard. Since two quantities are compared, the result is expressed in numerical values.

BASIC REQUIREMENTS OF MEASUREMENT:

i) The standard used for comparison purposes must be accurately defined & should be commonly accepted

ii) The apparatus used & the method adopted must be provable.
 
MEASURING INSTRUMENT:

It may be defined as a device for determining the value or magnitude of a quantity or variable.

ELEMENTS OF A GENERALIZED MEASUREMENT SYSTEM:
Most of the measurement systems contain three main functional elements are

i) Primary sensing element

ii) Variable conversion element &

iii) Data presentation element.

Primary sensing element:

•The quantity under measurement makes its first contact with the primary sensing element of a measurement system.

•i.e., the measurand- (the unknown quantity which is to be measured) is first detected by primary sensor which gives the output in a different analogous form

•This output is then converted into an electrical signal by a transducer - (which converts energy from one form to another).

•The first stage of a measurement system is known as a detector

Transducer stage Variable conversion element:

•The output of the primary sensing element may be electrical signal of any form; it may be voltage, a frequency or some other electrical
Parameter

•For the instrument to perform the desired function, it may be necessary to convert this output to some other suitable form.

Variable manipulation element:

•The function of this element is to manipulate the signal presented to it preserving the original nature of the signal

•It is not necessary that a variable manipulation element should follow the variable conversion element

•Some non-linear processes like modulation, detection, sampling,
filtering, chopping etc.,are performed on the signal to bring it to the
desired form to be accepted by the next stage of measurement system

•This process of conversion is called   signal conditioning’

•The term signal conditioning includes many other functions in addition to Variable conversion & Variable manipulation

•In fact the element that follows the primary sensing element in any
instrument or measurement system is called   signal conditioning
element’
 When the elements of an instrument are actually physically separated, it becomes necessary to transmit data from one to another. The element that performs this function is called a data transmission element’.•
The information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring, control, or analysis purposes.

•This function is done by data presentation element


•In case data is to be monitored, visual display devices are needed

•These devices may be analog or digital indicating instruments like
ammeters, voltmeters etc

•In case data is to be recorded, recorders like magnetic tapes, high
speed camera & TV equipment, CRT, printers may be used

•For control & analysis purpose microprocessor or computers may be used.
The final stage in a measurement system is known as   terminating stage’

INSTRUMENT TRANSFORMERS

•    Power measurements are made in high voltage circuits connecting the wattmeter to the circuit through current and potential transformers as shown.

•    The primary winding of the C.T. is connected in series with the load and the secondary winding is connected in series with an ammeter and the current coil of a wattmeter.

•    The primary winding of the potential transformer is connected across the supply lines and a voltmeter and the potential coil circuit of the wattmeter are connected in parallel with the secondary winding of the transformer.

•    One secondary terminal of each transformer and the casings are earthed.

•    The errors in good modem instrument transformers are small and may be ignored for many purposes.

•    However, they must be considered in precision work. Also in some power measurements these errors, if not taken into account, may lead to very inaccurate results.

•    Voltmeters and ammeters are effected by only ratio errors while wattmeters are influenced in addition by phase angle errors.

•    Corrections can be made for these errors if test information is available about the instrument transformers and their burdens.Phasor diagrams for the current and voltages of load, and in the wattmeter coils.

•    some value B, is observed.

Dynamometer Type Three-Phase Wattmeter

•    A dynamometer type three-phase wattmeter consists of two separate wattmeter movements mounted together in one case with the two moving coils mounted on the same spindle.

•    The arrangement is shown in Fig.

•    There are two current coils and two pressure coils.

•    A current coil together with its pressure coil is known as an element. Therefore, a three phase wattmeter has two elements.

•    The connections of two elements of a 3 phase wattmeter are the same as that for two wattmeter method using two single phase wattmeter.

•    The torque on each element is proportional to the power being measured by it.

•    The total torque deflecting the moving system is the sum of the deflecting torque of’ the two elements.

•    Hence the total deflecting torque on the moving system is proportional to the total Power.

•    In order that a 3 phase wattmeter read correctly, there should not be any mutual interference between the two elements.

•    A laminated iron shield may be placed between the two elements to eliminate the mutual effects.

(fig) three phase wattmeter

Ferrodynamic Type Wattmeters

•    Using iron cores for the coils can considerably increase the operating torque.

•    Ferrodynamic wattmeters employ cores of low loss iron so that there is a large increase in the flux density and consequently an increase in operating torque with little loss in accuracy.

•    The fixed coil is wound on a laminated core having pole pieces designed to give a uniform radial field throughout the air gap.

•    The moving coil is asymmetrically pivoted and is placed over a hook shaped pole piece.

•    This type of construction permits the use of a long scale up to about 270° and gives a deflecting torque which is almost proportional to the average power.

•    With this construction there is a tendency on the part of the pressure coil to creep (move further on the hook) when only the pressure coil is energized.

•    This is due to the fact that a coil tries to take up a position where it links with maximum flux. The creep causes errors and a compensating coil is put to compensate for this voltage creep.

•    The use of ferromagnetic core makes it possible to employ a robust construction for the moving element.

•    Also the Instrument is less sensitive to external magnetic fields.

•    On the other hand, this construction introduces non-linearity of magnetization curve and introduction of large eddy current  & hysteresis losses in the core.

Ferrodynamic Type Wattmeters

•    Using iron cores for the coils can considerably increase the operating torque.

•    Ferrodynamic wattmeters employ cores of low loss iron so that there is a large increase in the flux density and consequently an increase in operating torque with little loss in accuracy.

•    The fixed coil is wound on a laminated core having pole pieces designed to give a uniform radial field throughout the air gap.

•    The moving coil is asymmetrically pivoted and is placed over a hook shaped pole piece.

•    This type of construction permits the use of a long scale up to about 270° and gives a deflecting torque which is almost proportional to the average power.

•    With this construction there is a tendency on the part of the pressure coil to creep (move further on the hook) when only the pressure coil is energized.

•    This is due to the fact that a coil tries to take up a position where it links with maximum flux. The creep causes errors and a compensating coil is put to compensate for this voltage creep.

•    The use of ferromagnetic core makes it possible to employ a robust construction for the moving element.

•    Also the Instrument is less sensitive to external magnetic fields.

•    On the other hand, this construction introduces non-linearity of magnetization curve and introduction of large eddy current  & hysteresis losses in the core.

Electrodynamometer Wattmeters

•    These instruments are similar in design and construction to electrodynamometer type ammeters and voltmeters.

•    The two coils are connected in different circuits for measurement of power.

•    The fixed coils or “field coils” arc connected in series with the load and so carry the current in the circuit.

•    The fixed coils, therefore, form the current coil or simply C.C. of the wattmeter.

•    The moving coil is connected across the voltage and, therefore, carries a current proportional to the voltage.

•    A high non-inductive resistance is connected in series with the moving coil to limit the current to a small value.

•    Since the moving coil carries a current proportional to the voltage, it is called the ‘‘pressure coil’’ or “voltage coil” or simply called P.C. of the wattmeter.

Construction of Electrodynamometer Wattmeter

 Fixed Coils

•    The fixed coils carry the current of the circuit.

•     They are divided into two halves.

•    The reason for using fixed coils as current coils is that they can be made more massive and can be easily constructed to carry considerable current since they present no problem of leading the current in or out.

•    The fixed coils are wound with heavy wire. This wire is stranded or laminated especially when carrying heavy currents in order to avoid eddy current losses in conductors.

•    The fixed coils of earlier wattmeters were designed to carry a current of 100 A but modem designs usually limit the maximum current ranges of wattmeters to about 20 A.

•     For power measurements involving large load currents, it is usually better to use a 5 A wattmeter in conjunction with a current transformer of suitable range.

(fig) Dynamometer wattmeter

Moving Coil

•    The moving coil is mounted on a pivoted spindle and is entirely embraced by the fixed spindle & is entirely embraced b the fixed current coils.

•    Spring control is used for the movement.

•    The use of moving coil as pressure coil is a natural consequence of design requirements.

•    Since the current of the moving coil is carried by the instrument springs, it is limited to values, which can be carried safely by springs.

Control

•    Spring control is used for the instrument.


Damping

•    Air friction damping is used.

•    The moving system carries a light aluminium vane which moves in a sector shaped box.

•    Electromagnetic or eddy current damping is not used as introduction of a permanent magnet (for damping purposes) will greatly distort the weak operating magnetic field.
Scales and Pointers

•    They are equipped with mirror type scales and knife edge pointers to remove reading errors due to parallax.

Theory of Electrodynamometer Watt-meters

Errors in electrodynamometer

i) Errors due to inductance effects

ii) Stray magnetic field errors

iii) Eddy current errors

iv) Temperature error

Single Phase Induction Type Energy Meters

Construction of Induction Type Energy Meters

There are four main parts of the operating mechanism

(i)    Driving system

(ii)    Moving system

(iii)    Braking system 

(iv)    Registering system

Driving system

•    The driving system of the meter consists of two electro-magnets.

•    The core of these electromagnets is made up of silicon steel laminations.

•    The load current excites the coil of one of the electromagnets. This coil is called the current coil.

•    The coil of second electromagnet is connected across the supply and, therefore, carries a current proportional to the supply voltage. This coil is called the pressure coil.

•    Consequently the two electromagnets are known as series and shunt magnets respectively.

•    Copper shading bands are provided on the central limb.

•    The position of these bands is adjustable.

•    The function of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage.

Moving System

•    This consists of an aluminum disc mounted on a light alloy shaft.

•    This disc is positioned in the air gap between series and shunt magnets.

•    The upper bearing of the rotor (moving system) is a steel pin located in a hole in the bearing cap fixed to the top of the shaft.

•    The rotor runs on a hardened steel pivot, screwed to the foot of the shaft.

•    A jewel bearing supports the pivot.

•    A pinion engages the shaft with the counting or registering mechanism.

(Fig) single-phase energy meter

Braking System

•    A permanent magnet positioned near the edge of the aluminium disc forms the braking system.

•    The aluminium disc moves in the field of this magnet and thus provides a braking torque.

•    The position of the permanent magnet is adjustable, and therefore braking torque can be adjusted by shifting the permanent magnet to different radial positions as explained earlier.

(fig) Pointer type                                        (fig) cyclometer register

Registering (counting) Mechanism

•    The function of a registering or counting mechanism is to record continuously a number, which is proportional to the revolutions made by the moving system.

•    By a suitable system, a train of reduction gears the pinion on the rotor shaft drives a series of five or six pointers.

•    These rotate on round dials, which are marked with ten equal divisions.

•    The pointer type of register is shown in Fig. Cyclo-meter register as shown in Fig can also he used.

Errors in Single Phase Energy Meters

The errors caused by the driving system are

(i)    Incorrect magnitude of fluxes.

(ii)    Incorrect phase angles.

(iii)    Lack of Symmetry in magnetic circuit.

The errors caused by the braking system are

i)    Changes in strength of brake magnet

ii)    Changes in disc resistance

iii)    Abnormal friction

iv)    Self braking effect

Potentiometric Type Digital Voltmeter

• A potentiometric type of DVM employs voltage comparison technique. In this DVM the unknown voltage is compared with reference voltage whose value is fixed by the setting of the calibrated potentiometer.

• The potentiometer setting is changed to obtain balance (i.e. null conditions).

• When null conditions are obtained the value of the unknown voltage, is indicated by the dial setting of the potentiometer.

• In potentiometric type DVMs, the balance is not obtained manually but is arrived at automatically.

• Thus, this DVM is in fact a self- balancing potentiometer.

• The potentiometric DVM is provided with a readout which displays the voltage being measured.

(Fig.) Basic block diagram of a potentiometric DVM.

•    The block diagram of basic circuit of a potentiometric DVM is shown.

•    The unknown voltage is filtered and attenuated to suitable level.

•    This input voltage is applied to a comparator (also known as error detector).

•    This error detector may be chopper.

•    The reference voltage is obtained from a fixed voltage source.

•    This voltage is applied to a potentiometer.

•    The value of the feedback voltage depends up the position of the sliding contact.

•    The feedback voltage is also applied to the comparator.

•    The unknown voltage and the feedback voltages are compared in the comparator.

•    The output voltage of the comparator is the difference of the above two voltages.

•    The difference of voltage is called the error signal.

•     The error signal is amplified and is fed to a potentiometer adjustment device, which moves the sliding contact of the potentiometer.

•     This magnitude by which the sliding contact moves depends upon the magnitude of the error signal.

•    The direction of movement of slider depends upon whether the feedback voltage is larger or the input voltage is larger.

•    The sliding contact moves to such a place where the feedback voltage equals the unknown voltage.

•    In that case, there will not be any error voltage and hence there will be no input to the device adjusting the position of the sliding contact and therefore it (sliding contact) will come to rest.

•    The position of the potentiometer adjustment device at this point is indicated in numerical form on the digital readout device associated with it.

Since the position at which no voltage appears at potentiometer adjustment device is the one where the unknown voltage equals the feedback voltage, the reading of readout device indicates the value of unknown voltage.