Understanding Electrical Instrument Usage

This question involves matching various types of electrical measuring instruments with the types of electrical signals (DC or AC) they are designed to measure. It tests the knowledge of fundamental principles behind common electrical instruments.

Instrument Type Analysis and Application

Let's break down each instrument type and its typical usage:

• P) Permanent Magnet Moving Coil (PMMC):
  • PMMC instruments use a moving coil suspended in the field of a permanent magnet.
  • The torque produced is proportional to the product of the magnetic field strength and the current flowing through the coil.
  • Since the permanent magnet's field is unidirectional, the torque is also unidirectional.
  • This means PMMC instruments deflect only in one direction and are therefore suitable exclusively for measuring Direct Current (DC) quantities. They cannot accurately measure Alternating Current (AC).
• Q) Moving Iron connected through Current Transformer:
  • Moving Iron (MI) instruments rely on the magnetic effect of current, where either a fixed piece of iron is attracted to or repelled by a magnetized moving iron vane, or vice versa.
  • The torque produced is proportional to the square of the RMS value of the current ($T \propto I_{rms}^2$).
  • MI instruments inherently respond to both AC and DC.
  • However, the crucial part here is "connected through Current Transformer". Current Transformers (CTs) are specialized devices used solely in AC circuits for stepping down high currents to a measurable level.
  • Therefore, an MI instrument used in conjunction with a CT is operating within an AC measurement system, indicating its use for AC only in this context.
• R) Rectifier:
  • Rectifier type instruments are designed to measure AC quantities.
  • They typically employ a rectifier circuit (using diodes) to convert the incoming AC signal into a DC signal.
  • This rectified DC signal is then measured by a DC-sensitive meter movement, commonly a PMMC movement.
  • While the internal mechanism converts AC to DC for measurement, the instrument's primary application is measuring AC.
  • The classification as "DC and AC" might imply that the instrument could potentially measure DC directly if the rectifier component is bypassed or if the input is already DC, alongside its primary function of measuring AC via rectification.
• S) Electrodynamometer:
  • Electrodynamometer instruments operate on the principle of interaction between the magnetic fields produced by fixed coils (field coils) and a moving coil.
  • The torque is proportional to the product of the currents in these coils.
  • For DC measurements, the torque is proportional to the square of the current ($T \propto I^2$).
  • For AC measurements, the average torque is proportional to the square of the RMS value of the current ($T_{avg} \propto I_{rms}^2$).
  • Since the torque depends on the square of the current, it always acts in the same direction, regardless of whether the current is DC or AC.
  • This makes electrodynamometers versatile instruments capable of accurately measuring both DC and AC quantities.

Matching Instruments to Usage Categories

Based on the analysis of each instrument's operating principle and context:

Final Correspondence Summary

The derived matches are:

• Instrument P (Permanent magnet moving coil) is used for 1 (DC only).
• Instrument Q (Moving iron connected through current transformer) is used for 2 (AC only).
• Instrument R (Rectifier) is used for 3 (DC and AC).
• Instrument S (Electrodynamometer) is used for 3 (DC and AC).

This corresponds to the matching P – 1, Q – 2, R – 3, S – 3.