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Deflection Type Instrument

Edwiin
05/16/2025

Definition
Instruments in which the measured quantity generates physical effects that deflect or displace the moving system are known as deflection-type instruments. In other words, these instruments use the deflection of a moving component as the basis for measuring electrical quantities, making them suitable for dynamic condition measurements.
Deflection-type instruments incorporate opposing effects that counteract the displacement of the moving system. These opposing effects are designed such that their magnitude increases with the deflection or displacement caused by the measured quantity. Balance is achieved when the opposing effects equal the force driving the deflection or movement of the moving component.
Example
In a permanent magnet moving coil (PMMC) ammeter, the deflection of the moving element is directly proportional to the current (the measured quantity) passing through it. The deflecting torque \(T_d\) acting on the coil is directly proportional to the current, expressed by the equation:
Td​=GI  Equ(1)
where G is a constant independent of flux density, the area of the moving coil, and the number of turns.
The opposing torque Tc is generated by a spring, which is proportional to the deflection angle θ:
Tc​=Kθ  Equ(2)
where K is the spring constant, dependent on the spring’s material and dimensions.
Under balanced conditions:
Td​=Tc​  Equ(3)
Substituting Td and Tc into Equation (3):
GI = KθI = (K/G)θ
​The measured current thus depends on the deflection angle θ and the meter constants G and K. Current values are directly read from the deflection angle θ, which is calibrated using G and K.
Disadvantages of Deflection-Type Instruments
  • Low Accuracy: These instruments exhibit relatively low measurement accuracy.
  • Reduced Sensitivity: Sensitivity is lower compared to null-type instruments.
  • Calibration Dependency: Measurement accuracy relies on the instrument’s calibration.
Topics
Power Systems Measurement
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