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In which instrument the deflecting torque depends on the frequency

  • In Induction type instrument instrument the deflecting torque depends on the frequency.
  • In induction type instruments, the deflecting torque is proportional to the square of the current flowing through the instrument coil.
  • At low frequencies, the current through the coil is almost in-phase with the applied voltage, resulting in a high current and a correspondingly high deflecting torque.
  • As the frequency increases, the current through the coil lags behind the applied voltage due to the inductance of the coil, resulting in a reduced current and deflecting torque.
  • At very high frequencies, the current through the coil is greatly reduced, resulting in a negligible deflecting torque.
  • Therefore, while the frequency of the current flowing through the coil does affect the current and, indirectly, the deflecting torque in induction type instruments.
  • To compensate for this frequency error, a non-inductive shunt can be connected in parallel with the instrument coil.
  • The shunt provides a low-resistance path for the high-frequency components of the current, effectively reducing the effective impedance of the instrument coil at high frequencies and reducing the frequency error in the measurement.