Moving Iron Instruments

December 29, 2012

        M.I instruments are mainly used for the measurement of alternating currents and voltages, though it can also be used for d.c measurements.
       The general principle of a M.I instrument can be explained under;
           Let a plate or vane of soft iron or of high permeability steel forms the moving element of the system. The iron vane is situated so as, it can move in a magnetic field produced by a stationary coil. The coil is excited by the current or voltage under measurement. When the coil is excited, it becomes an electromagnet and the iron vane moves in such a way so as to increase the flux of the electromagnet. Thus, the vane tries to occupy a position of minimum reluctance. Thus, the force produced is always in such a direction so as to increase the inductance of the coil.
       There are two types of Moving- iron instruments.
                      i. Attraction type:-
             In this type of instrument, a single soft iron vane (moving iron) is mounted on the spindle, and is attracted towards the coil when operating current flows through it.


Deflecting torque equation
          The force F, pulling the soft -iron piece towards the coil is directly proportional to;
a) Field strength H, produced by the coil.
b)pole strength ‘m’ developed in the iron piece.
F α mH
Since, m α H,
F α H2
Instantaneous deflecting torque  α H2
Also, the field strength H = μi
If the permeability(μ) of the iron is assumed constant,
Then,   H α i
Where, i® instantaneous coil current, Ampere
Instantaneous deflecting torque α i2
Average deflecting torque, Td α mean of i2 over a cycle.
Since the instrument is spring controlled,
                                             Tc α θ
In the steady position of deflection, Td = Tc
                                             θ α mean of i2 over a cycle
                                                α I2
                                                
   Since the deflection is proportional to the square of coil current, the scale of such instruments is non-uniform (being crowded in the beginning and spread out near the finishing end of the scale). 
                    ii.Repulsion type:-
                 In this two soft iron vanes are used; one fixed and attached the stationary coil, while the other is movable (moving iron), and mounted on the spindle of the instrument. When operating current flows through the coil, the two vanes are magnetised, developing similar polarity at the same ends. Consequently, repulsion takes place between the vanes and the movable vane causes the pointer to move over the scale.
    Two types
                                         i.radial vane type: - vanes are radial strips of iron.
                                       ii.co-axial vane type:-vanes are sections of coaxial cylinders.
Deflecting torque:-
        The deflecting torque results due to repulsion between the similarly charged soft- iron pieces or vanes. If the two pieces develop pole strength of m1 and m2 respectively, then;
                       Instantaneous deflecting torque α m1m2 α H2

If the permeability of iron is assumed constant, then;  H α i,  where, i is the coil current
                          Instantaneous deflecting torque α i2
 Average deflecting torque, Td α mean of i2 over a cycle.
  Since the instrument is spring controlled, Tc α θ
 In the steady position of deflection, Td = Tc
                                                          θ α mean of i2 over a cycle.
                                                             α  I2
                                                             
     Thus, the deflection is proportional to the square of the coil current.  The scale of the instrument is non- uniform; being crowded in the beginning and spread out near the finish end of the scale. However, the non- linearity of the scale can be corrected to some extent by the accurate shaping and positioning of the iron vanes in relation to the operating coil.
………………………………………………………………………………………………….. Reference:-
(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 315-317
(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 790-792
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Moving Iron Instruments Moving Iron Instruments Reviewed by Bibi Mohanan on December 29, 2012 Rating: 5

PMMC Instruments

December 29, 2012


   These instruments are used either as ammeters or voltmeters and are suitable for d.c work only. PMMC instruments work on the principle that, when a current carrying conductor is placed in a magnetic field, a mechanical force acts on the conductor. The current carrying coil, placed in magnetic field is attached to the moving system. With the movement of the coil, the pointer moves over the scale to indicate the electrical quantity being measured. This type of movement is known as D’ Arsenoval movement.

    Construction:-
         It consists of a light rectangular coil of many turns of fine wire wound on an aluminium former inside which is an iron core as shown in fig. The coil is delicately pivoted upon jewel bearings and is mounted between the poles of a permanent horse shoe magnet. Two soft-iron pole pieces are attached to these poles to concentrate the magnetic field. The current is led in to and out of the coils by means of two control hair- springs, one above and other below the coil, as shown in fig (b). These springs also provide the controlling torque. The damping torque is provided by eddy currents induced in the aluminium former as the coil moves from one position to another.
Working:-
    When the instrument is connected in the circuit to measure current or voltage, the operating current flows through the coil. Since the current carrying coil is placed in the magnetic field of the permanent magnet, a mechanical torque acts on it. As a result of this torque, the pointer attached to the moving system moves in clockwise direction over the graduated scale to indicate the value of current or voltage being measured.
       This type of instruments can be used to measure direct current only. This is because, since the direction of the field of permanent magnet is same, the deflecting torque also gets reversed, when the current in the coil reverses. Consequently, the pointer will try to deflect below zero. Deflection in the reverse direction can be prevented by a “stop” spring.


Deflecting torque equation:-
       The magnetic field in the air gap is radial due to the presence of soft iron core. Thus, the conductors of the coil will move at right angles to the field. When the current is passed through the coil, forces act on its both sides which produce the deflecting torque.

                 Let,        B = flux density, Wb/m2
                                l = length or depth of coil, m
                               b = breadth of the coil.
                               N = no. of turns of the coil.
   If a current of ‘I’ Amperes flows in the coil, then the force acting on each coil side is given by,
                 Force on each coil side, F = BIlN Newtons.
 Deflecting torque, Td = Force × perpendicular distance
                                    = (BIlN) × b
                               Td = BINA Newton metre.
Where, A = l × b, the area of the coil in m2.
                         Thus, Td α I
The instrument is spring controlled so that, Tc α θ
The pointer will comes to rest at a position, where Td =Tc
Therefore,    θ α I
     Thus, the deflection is directly proportional to the operating current. Hence, such instruments have uniform scale.
Advantages:-
a) Uniform scale.ie, evenly divided scale.
b) Very effective eddy current damping.
c) High efficiency.
d) Require little power for their operation.
e) No hysteresis loss (as the magnetic field is constant).
f) External stray fields have little effects on the readings (as the operating magnetic field is very strong).
g) Very accurate and reliable.
Disadvantages:-
a) Cannot be used for a.c measurements.
b) More expensive (about 50%) than the moving iron instruments because of their accurate design.
c) Some errors are caused due to variations (with time or temperature) either in the strength of permanent magnet or in the control spring.
Applications:-
a) In the measurement of direct currents and voltages.
b)In d.c galvanometers to detect small currents.
c)In Ballistic galvanometers used for measuring changes of magnetic flux linkages.
………………………………………………………………………………………………….. Reference:-
(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 294
(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 775-777
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PMMC Instruments PMMC Instruments Reviewed by Bibi Mohanan on December 29, 2012 Rating: 5

Types of instruments

December 29, 2012

Types of instruments


(1) Permanent Magnet Moving Coil (PMMC) instruments .
(2) Moving iron instruments.
(3) Electrodynamometer instruments.
(4) Thermal instruments
(5) Induction instruments.
(6) Electrostatic type instruments.
(7) Rectifier type instruments.

    The PMMC instruments can be used for direct measurement only, and the induction type for alternating current measurements only.

    The moving iron(MI) and moving coil(MC) types both depend for their action upon the magnetic effect of current. The MI instruments can be used for either direct or alternating current measurements, and is the cheapest.

   Electrodynamometer type of instruments can be used both on a.c as well as on d.c. They are useful as “transfer instruments” , as their calibration for both d.c and a.c is the same.

   The calibration for d.c and a.c is same for the thermal instruments also. They are particularly suited for a.c measurements. This is because, the deflection of the thermal instruments depends directly upon the heating effect of the alternating current. ie, upon the rms value of the current.

   For the electrostatic instruments, the electrostatic principle is only directly applicable to voltage measurements. They have the advantage that, their power consumption is extremely small.

     The induction principle is more generally used for Watt- hour meters than for ammeters and voltmeters.


Types of instruments Types of instruments Reviewed by Bibi Mohanan on December 29, 2012 Rating: 5

Essentials of Indicating instruments

December 29, 2012


            An indicating instrument essentially consists of a moving system pivoted in jewel bearings. A pointer is attached to the moving system which indicates the electrical quantity to be measured, on a graduated scale. In order to ensure the proper operation of the indicating instruments , the following three torques are required.
  1. Deflecting (or operating) torque.
  2. Controlling (or restoring) torque.
  3. Damping torque
The deflecting torque is produced by utilising the various effects (magnetic effect, induction effect, thermal effect, hall effect) of electric current or voltage, and causes the moving system and hence the pointer to move from  zero position.
The controlling torque is produced by spring or gravity and opposes the deflecting torque. The pointer comes to rest at a position, where these two opposing torques are equal.
Damping torque is provided by air friction or eddy currents. It ensures that, the pointer comes to the final position, without oscillations, thus enabling accurate and quick readings to  be taken.
(i)                 Deflecting torque(Td) :-
The deflecting torque causes the moving system to move from zero position to indicate the value of the electrical quantity being measured on a graduated scale. The actual method of producing the deflecting torque depends upon the type of instrument.
(ii)               Controlling torque(Tc):-
If the deflecting torque were acting alone, the pointer will continue to move indefenitely and would swing over to the maximum deflected position irrespective of the magnitude of the electrical quantity to be measured. This necessitates providing some form of controlling or opposing torque. This controlling torque should increase with the deflection of the moving system. The pointer will be brought to rest at a position where the two opposing torques are equal. ie, Td = Tc .
The controlling torque performs two functions.
a)      It increases with the deflection of the moving system so that, the final position of the pointer on the scale will be according to the magnitude of the electrical quantity to be measured.
b)      It brings the pointer back to zero position, when the deflecting torque is removed. If it were not provided, the pointer once deflected would not return to zero position on removing the deflecting torque.
 The controlling torque can be provided,
Ø  by using one or more springs
Ø  by the weight of moving parts.
(iii)              Damping torque (Tdamp):-
If the moving system is acted upon by deflecting and controlling torques alone, then due to inertia, the pointer will oscillate about its final deflected position for some time before coming to rest. This oscillation makes it difficult to obtain quick and accurate reading. Inorder to avoid these oscillations of the pointer and to bring it quickly to its final deflected position, a damping torque is provided in the indicating instruments. The damping do not affect the stationary pointer, as the damping torque acts only when the pointer is in motion and always opposes the motion.
The damping torque in indicating instruments can be provided by,
Ø  Air- friction
Ø  Fluid friction
Ø  Eddy currents

The behaviour of the moving system is decided by the degree of damping. The fig.  given below shows the graph for under damping, over damping, and critical damping.


  Under damped moving system:- The pointer will oscillate about the final position for some time, before coming to rest.
Over damped:-  The pointer will become slow and lethargic.
Critically damped/ dead beat:- The degree of damping is so that, the pointer comes up to the correct reading quickly without passing beyond it or oscillating about it.
………………………………………………………………………………………………….. Reference:-
(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 225
(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 769- 773
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Essentials of Indicating instruments Essentials of Indicating instruments Reviewed by Bibi Mohanan on December 29, 2012 Rating: 5

Standards and their classification

December 29, 2012

Standards and their classification


            A standard is a physical representation of a unit of measurement. It is a piece of equipment having a known measure of physical quantity. They are used for the measurements of other physical quantities by comparison methods.
            Standards of measurements can be classified in to;
                                            i.            International standards
                                          ii.            Primary standards
                                        iii.            Secondary standards
                                        iv.            Working standards

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  1. International standards

        These are defined on the basis of international agreement. They represent the units of measurements which are closest to the possible accuracy attainable with present day technological and scientific methods. International standards are checked and evaluated regularly against absolute measurements in terms of the fundamental units. These standards are maintained at the International Bureau of Weights and Measures and are not available to the ordinary user of measuring instruments for the purposes of calibration or comparison.
            The international unit of length was defined in 1960 in terms of the wavelength of Krypton-86. The metre is equal to 1,650,763.73 wavelengths in vaccum of the orange-red light radiation of the Krypton-86 atom in its transition between levels 2 p10 and 5d5.
            In 1983, the metre was redefined. The metre is now, the length travelled by light in vaccum in a time interval of 1/299792458 sec.

  1. Primary standards (Absolute standards)

The Primary standards are the absolute standards which can be used as the ultimate reference standards. These standards are maintained by National Standards Laboratories in different parts of the world. The primary standards which represent the fundamental units are independently calibrated by absolute measurements at each of the national laboratories. One of the main functions of the primary standards is the verifications and calibration of secondary standards.
The primary standards are very few in number. These standards have the highest possible accuracy and stability.
The primary unit of mass is a prototype kilogramme kept at National Physical Laboratories of every country. This has an accuracy of 1 part in 108.
  1. Secondary standards
    The secondary standards are the basic reference standards used in industrial measurement laboratories. They are sent periodically to the national standards laboratories for calibration and comparison against primary standards. The secondary standards are sent back to the industry by the National laboratories with a certification regarding their measured values in terms of primary standards.
            The secondary standards of mass are kept by industrial laboratories. These standards have an accuracy of 1 ppm and are checked against the primary standards.

  1. Working standards

These standards are used to check and calibrate general laboratory instrument for their accuracy and performance.
The working standards of mass and length are available in a wide range of values so that, they suit any kind of application. The working standards of mass have an accuracy of 5 ppm and are checked against the secondary standards. The working standards of length are usually precision gauge blocks made up of steel. These blocks have two parallel surfaces and the distance between the two surfaces is specified. They have an accuracy of 1 ppm.

Classification of Instruments

            Instruments can be broadly classified in to
  1. Absolute instruments
  2. Secondary instruments
Absolute instruments give the magnitude of the quantity under measurement in terms of physical constants of the instruments.
e.g:- Tangent galvanometer, Rayleigh’s current balance.
In secondary instruments, the quantity under measurement can only be measured by observing the output of the instrument. The secondary instruments should be calibrated by comparing with an absolute instrument or another secondary instrument which has already been calibrated against an absolute instrument.
e.g:- Voltmeter, pressure gauge.
The secondary instruments are  the commonly  used instruments compared to the absolute instruments.
Electrical measuring instruments may be classified according to their functions as;
(i)                 Indicating instruments (ii)Integrating instruments (iii) Recording instruments

  1. Indicating instruments:-

These instruments directly indicate the value of the electrical quantity at the time when it is being measured. In these instruments, a pointer moving over a graduated scale directly gives the value of the electrical quantity being measured.
e.g:- Ammeter, voltmeter, wattmeter.

  1. Integrating instruments

The instruments which measure the total quantity of electricity (in Ampere hours ) or electrical energy (in Watt hours) in a given time are called integrating instruments. In such instruments, there are a set of dials and pointers which register the total quantity of electricity or electrical energy supplied to the load.
 e.g:- Ampere- Hour meter, Watt-hour meter.

  1. Recording instruments

Recording instruments give a continuous record of the variations of the electrical quantity to be measured. A recording instrument is merely an indicating instrument with a pen attached to its pointer. The pen rests lightly on a chart wrapped over a drum moving with a slow uniform speed. The motion of the drum is in a direction perpendicular to the direction of the pointer. The path traced out by the pen indicates the manner in which the quantity being measured, has varied during the time of the record.
e.g:- Recording voltmeters, Recording ammeters in supply stations.
………………………………………………………………………………………………….. Reference:-
(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 181-182]
(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 768
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Standards and their classification Standards and their classification Reviewed by Bibi Mohanan on December 29, 2012 Rating: 5
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