Permanent Magnet Instruments Overview

Permanent magnet type instruments are a crucial category of electrical measuring devices. These instruments operate on the fundamental principle of the interaction between a magnetic field produced by a permanent magnet and the magnetic field generated by the current flowing through a coil. The most common example of such an instrument is the Permanent Magnet Moving Coil (PMMC) instrument.

D.C. Operation Principle of Permanent Magnet Instruments

Permanent magnet type instruments, particularly the PMMC type, are designed specifically for measuring direct current (D.C.) quantities. Here’s why:

• Magnetic Field Interaction: In a PMMC instrument, a coil is placed within the strong and uniform magnetic field of a permanent magnet. When a direct current flows through the coil, it produces its own magnetic field.
• Torque Production: The interaction between the permanent magnet's field and the coil's field creates a deflecting torque. This torque causes the coil to rotate, and a pointer attached to the coil moves across a calibrated scale. The magnitude of the deflection is directly proportional to the magnitude of the D.C. current flowing through the coil.
• Directional Consistency: For D.C., the current flows in one constant direction, meaning the magnetic field produced by the coil is also constant in direction. This ensures a steady, unidirectional deflecting torque, leading to a stable and accurate reading on the scale.

A.C. Limitations for Permanent Magnet Instruments

Permanent magnet type instruments cannot be used for measuring alternating current (A.C.) quantities for the following reasons:

• Rapid Current Reversal: A.C. current rapidly reverses its direction multiple times per second (e.g., 50 or 60 times per second for standard power frequencies).
• Reversing Torque: When A.C. flows through the coil of a permanent magnet instrument, the direction of the magnetic field produced by the coil also reverses rapidly. This causes the deflecting torque to reverse direction equally rapidly.
• Inertia of Moving System: Due to the mechanical inertia of the moving coil and pointer system, it cannot respond to such rapid reversals of torque. Instead of showing a steady reading, the pointer would simply vibrate about the zero position or remain almost stationary, indicating no meaningful deflection. The average deflecting torque over one complete cycle of A.C. would be zero.
• Rectifier Requirement: To measure A.C. with a PMMC instrument, an external rectifier circuit must first be used to convert the A.C. into pulsating D.C. This D.C. can then be measured by the instrument. However, the instrument itself, without the rectifier, cannot directly measure A.C.

Instrument Applications Summary

Therefore, while permanent magnet type instruments are highly accurate and widely used for D.C. voltage and current measurements in various applications, their inherent design makes them unsuitable for direct A.C. measurements or impulse quantities without additional circuitry.