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Module I
Working Principals of Measuring Instruments
-PMMC -> DC
-Moving Iron -> AC & DC
(MI)
-Electrodynamometer -> AC & DC
-Induction -> AC
-Thermal -> AC & DC
-Electrostatic -> DC
Types Of Instruments
(.) Indicating
(..) Intergrating
(...)Recording
Essentials of indicating instrument
It consists of a pointer which moves over a calibrated spring and which is attached to a moving system pivoted and jeweledbearing. The system is subjected to the following torque,
1. Deflecting Torque -> It causes the system to move from its zeroth position. It is provided by one of the effects given below
- Magnetic Effect
- Moving Iron
- Electrodynamic Effect
- Thermal Effect
- Electrostatic Effect
- Induction
2. Controling Torque -> The deflection is indefinite if there was no controling or restoring torque.It opposes deflecting torque & increase with deflection. Without such torque, the pointer will not come to the initial position after each measurement. Controling torque can be provided by
- Spring Control
A & B are coils in opposing directions. One end of each spring is attached to the spindle. The other end are pivoted or fixed.
When there is a deflecting torque, one of the springs unwind itself and other get twisted. The twist produces controlling torque which is directly proportional to the angle of deflection of the moving system. Since deflecting torque Td α I
& controlling torque Tc α θ
For proper functioning of the instrument, Td must be equal to Tc & I must be proportional to θ (Td=Tc & I α θ)
The material used to make spring is usually of phosphor-bronze & should be fairly large so that the deformation caused is less. Spring control instrument is non magnetic, non subjected to appreciable fatigue & temperature coefficient must be less.
Two springs coiled in opposing directions are used to reduce the error offered by temperature variation on the length.
Other materials which can be used for springs are (i) Silicon-Bronze
(ii) Platinum- Silver
(iii) Platinum-Iridium
(iv) Hard rolled Silver
(v) German Silver
Torqu developed due to the spring is given by T = (Ewt³ θ)/ 12 l ----- (1)
E -> Youngs modulu l -> length
w -> width of spring θ -> angle deflected
t -> thickness
The maximum fibre stress, Smax = (6T)/wt² ----------(2)
From (1) & (2), Smax =[6/wt²]x [(Ewt³ θ)/ 12 l]
=> l/t = (E/Smax) x (θ/2)
Advantage
-> Can be used in either vertical or horizontal position.
-> No leveling is needed.
Disadvantage
-> Temperature coefficient of spring results in error in the indication of instruments.
-> In-elastic yield in the spring results in displacement in the zero position of the moving system.
