Results for types of Instruments

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
……………………………………………………………………………………………………..........

You can buy the text books from below links
Moving Iron Instruments Moving Iron Instruments Reviewed by Sikha 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
……………………………………………………………………………………………………..........






PMMC Instruments PMMC Instruments Reviewed by Sikha 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 Sikha on December 29, 2012 Rating: 5

Instruments- Basics

December 29, 2012

The essential elements of an instrument are;
  1. A detector
  2. An intermediate transfer device
  3. An indicator, recorder or a storage device

 The history of development of instruments shows three phases of instruments.
a)      Mechanical , b) Electrical,  and c) Electronic instruments

a)      Mechanical instruments
  These instruments are very reliable for static and stable conditions. But, these instruments have moving parts that are rigid, heavy, and bulky and hence have a large mass. Mass presents inertia problems and hence, these instruments cannot faithfully follow the rapid changes, which are involved in dynamic measurements.      Thus, the disadvantages of mechanical instruments are;
·         They are unable to respond rapidly to measurements of dynamic and transient conditions.
·         Most of them are a potential source of noise and cause noise pollution.
b)     Electrical instruments
   Electrical method of indicating the output of detectors is more rapid than mechanical methods. But, electrical instruments depends on the mechanical movement of an indicating device, having some inertia and thus have a limited time response (0.5 – 24 s).
c)      Electronic instruments
  These instruments are used for fast responses required for scientific and industrial measurements. They are used for the detection of electromagnetically produced signals such as radio, video, and micro waves, space applications, and computers.     These instruments make use of semiconductor devices. In electronic devices, the only movement involved is that of electrons. Thus, the response time is extremely small on account of very small inertia of electrons.
eg:- A Cathode Ray Oscilloscope (CRO) is capable of following dynamic and transition changes of the order of a few nano seconds (10-9s).
            Advantage:-
                                i.            Can detect very weak signals. ( eg:- In the area of Bio Instrumentation, the bio electric potentials are very weak, ie lower than 1 mV)
                              ii.            They can monitor inaccessible or dangerous locations.
                            iii.            They can be used to measure non electrical quantities.
                            iv.            They have
·          higher sensitivity,
·         Lower weight
·         Lower power consumption
·         Higher degree of reliability compared to electrical or mechanical instruments
………………………………………………………………………………………………….. Reference:- A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 2-3
Instruments- Basics Instruments- Basics Reviewed by Sikha on December 29, 2012 Rating: 5
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