SYSTEMS IN ELECTRICAL INDICATING INSTRUMENT

ELECTRICAL INDICATING INSTRUMENT:

It indicates the magnitude of an electrical quantity at the time when it is being measured. The indications are given by a pointer moving over a graduated dial.

As defined above, indicating instruments are those which indicate the value of quantity that is being measured at the time at which it is measured. Such instruments consist essentially of a pointer which moves over a calibrated scale & which is attached to a moving system pivoted in bearing. The moving system is sub categorized into three parts. They are:

(1)Deflection system,

(2) Controlling system,

(3)Damping system

(1)   DEFLECTION SYSTEM:

Deflecting system is a system in which deflection torque is produced. A rotating force which tries to deflect the pointer on scale is called deflection torque. The deflecting or operating force is required for moving the pointer -from its zero position.  The deflecting force can be produced, by utilizing any of the effects mentioned below. Thus the deflecting system of an instrument converts the electric current or potential into a mechanical force called deflecting force. The deflecting system thus acts as the prime mover responsible for deflection of the pointer.

The effects they utilize to produce deflecting torque are: (i) magnetic effect (ii) heating effect (iii) electrostatic effect (iv) electromagnetic effect (v) hall effect

(i) Magnetic Effect:  Consider a current carrying conductor of Fig.1 (a), it produces a magnetic field in the anticlockwise direction. We now have a uniform magnetic field as shown in Fig.1 (b). Let the current carrying conductor be placed in this magnetic field. The resultant field is as shown in Fig. 1 (c). This results in distortion of magnetic field causing a force to act from left to right, The reversal of direction of the current will cause a force in the opposite direction, i.e., from right to left, subject to the condition that the direction of the existing field remains the same.   If we form the conductor into a coil, the magnetic field produced by each of the coil will add up and the coil will behave as an imaginary magnet as shown in Fig 2. 

Force of Attraction or Repulsion: Consider a current carrying coil as shown in Fig.2. It produces an imaginary bar magnet When a piece of soft iron which has not been previously magnetized is brought near the end of the coil, it will be attracted by the coil. Therefore, if we pivot the soft iron on a spindle between two bearings and a coil is mounted near it, the iron piece will swing into the coil when the latter is carrying current. The effect is utilized in the attraction type of moving iron instrument. If we have two pieces of soft iron placed near the coil the two will be similarly magnetized and there will be force of repulsion between them. This effect is utilized in repulsion type moving iron instruments. Force between a Current Carrying coil and a Permanent Magnet: Consider the coil of Fig.3. It produces an imaginary bar magnet when carry a current. When a permanent magnet is brought near it, there will be either a force of attraction or repulsion. If the coil is mounted on a spindle between bearings, there will be a movement of the coil. This effect is utilized in permanent magnet moving coil instruments.

Force between   Two Current Carrying Coils: Consider two current carrying coils shown in  Fig 4.For the direction of current shown, the two coils produce unlike poles near each other and thus there is a force of attraction and if one of the coils is movable and the other is fixed, there will be a motion of the movable coil. This effect is utilized in the dynamometer type of instruments. 

 (ii) Thermal Effect: The current to be measured is passed through a small element which heats it. The temperature rise is converted to an emf by a thermocouple attached-to the element. A thermo-couple consists of lengths of two dissimilar electric conductors joined at ends to form a closed loop. If the junctions of the two dissimilar metals are maintained at different temperatures, a current flows through the closed loop. This current can be measured and is indicative of the R.M.S. value of the current flowing through the heater element.

(iii) Electrostatic Effect: When two plates are charged, there is a force exerted between them. This force is used to move one of the plates. The instruments working on this principle are called electrostatic instruments and they are usually voltmeters.

(iv)  Induction Effect: When a non-magnetic conducting pivoted disc or a drum is placed in a magnetic field produced by a system of electromagnets excited by alternating currents, an emf is induced in the disc or drum. If a closed path is provided, the emf forces a current to flow in the disc-or drum. The force produced by the interaction of induced currents and the alternating magnetic fields makes the disc move.   The induction effect is mainly utilized for a.c. energy meters.

(v) Hall Effect: If a strip of conducting material carries current in the presence of a transverse magnetic field as shown in Fig. 5, an emf is produced between two edges of conductor. The magnitude of the voltage depends upon the current, flux density and a property of conductor called "Hall Effect Co-efficient".

2.  CONTROLLING SYSTEM:

It is a system in which controlling torque is produced. This force is required in an indicating instrument in order that the current produces deflection of the pointer proportional to its magnitude. The system producing a controlling force is called a "Controlling System".   The functions of the controlling system are: (i) to produce a force equal and opposite to the deflecting force at the final steady position of pointer in order to make the deflection of the pointer definite for a particular magnitude of current, In the absentee of a controlling system, the pointer will shoot (swing) beyond the final steady position for any magnitude of current and thus the deflection will be indefinite. (ii) To bring the moving system back to zero when the force causing the instrument to deflect is removed

The deflecting system of most of the commercial instruments is mounted on a pivoted spindle, the quantity being measured producing a deflecting torque proportional 3 its magnitude.   There are two types of control systems which are used for such a mounted system: (i) Gravity control.                         (ii) Spring control.

(i) Gravity Control: In this type of control, a small weight is placed on an arm attached to the moving system. The position of this weight is adjustable. This weight produces a controlling torque due to gravity.Fig.7 shows the pointer at zero position. In this case the control torque is zero. Suppose the system deflects through an angle θ as shown in Fig.

The weight acts at a distance l from the center, the component of weight trying to restore the pointer back to zero position is W sin θ. Therefore, controlling torque is: 

 T= r x F------------ (1)

We know that,

r = l =distance from center

F=W sinθ

Substitute above two values in equation (1)

T=Wlsinθ

T=K_gsinθ

Where, K_g=constant

It is obvious that the instruments employing gravity control must be used in vertical position in order that the control may operate. The instruments must be mounted in level position otherwise there will be a very serious zero error. For these reasons, gravity control is not suited for indicating instruments in general and portable instruments in particular.   The system is obsolete now.

(ii) Spring control:

A hair spring attached to the moving system exerts a controlling torque.   The essential requirements for instrument springs are: (i) they should be non-magnetic.  (ii) They should be proof from mechanical fatigue.(iii) Where springs are used to lead current into moving system they should have a small resistance, their cross-sectional area must be sufficient to carry the current without a temperature rise which effects their constant. . They should also have a low resistance temperature co-efficient.

A number of non-magnetic materials like silicon bronze, hard rolled silver or copper, platinum silver, platinum-iridium and German silver have been used but have not been found satisfactory owing to some reason or the other. For most applications phosphor bronze has been the most suitable material except in instruments of low resistance (like Milli voltmeters).   In this case some special bronze alloys having low resistance may be used with some sacrifice in mechanical quality. Flat spiral springs are used in almost all indicating instruments as the space required by these springs is less than for other types.

The controlling torque is thus made proportional to the angle of deflection of the moving system.

Therefore,

T= Kθ

θ= angular deflection; rad

3. DAMPING SYSTEM:

When a deflecting force is applied to the moving system, it deflects and is should come to rest at a position where the deflecting force is balanced by the controlling force. 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. The pointer thus oscillates 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 forces 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 oscillations.