Moving Coil vs. Moving Iron Instruments: A Detailed Comparison

This article delves into the differences between moving coil (MC) and moving iron (MI) instruments, two fundamental types of devices used for measuring electrical quantities. Let’s explore their principles of operation and compare their key characteristics.

Moving Coil (MC) Instruments

Moving coil instruments utilize the interaction between a magnetic field and a current-carrying conductor to produce a deflection proportional to the current. They come in two main variations:

1. Permanent Magnet Moving Coil (PMMC) Instruments

• These instruments are primarily designed for DC measurements only.
• Principle: When a current-carrying conductor is placed within a magnetic field, it experiences a force. The direction of this force is dictated by Fleming’s left-hand rule.
• Mechanism: A coil, carrying the operating current, is positioned within a magnetic field. The coil is attached to a moving system. As the coil moves under the influence of the magnetic force, a pointer connected to the moving system deflects across a scale, indicating the current value.

2. Dynamometer Type Moving Coil Instruments

• Unlike PMMC instruments, these can be used for both AC and DC measurements.
• Principle: These instruments operate on the principle that a mechanical force exists between current-carrying conductors.
• Mechanism: They consist of a fixed coil and a moving coil. The moving coil is pivoted between the fixed coils. All coils can be excited separately or together if connected in series. The moving coil is attached to a moving system which moves the pointer across a calibrated scale, driven by the deflecting torque.

Moving Iron (MI) Instruments

Moving iron instruments rely on the attraction or repulsion forces produced by the magnetization of soft iron components within a magnetic field. They come in two main types:

1. Attraction Type Moving Iron Instruments

• These instruments can measure both AC and DC.
• Principle: When current passes through a coil, it creates a magnetic field. This magnetic field attracts a soft iron cam placed nearby.
• Mechanism: The soft iron cam is mounted on a spindle, which in turn rotates the pointer across the scale.
• AC/DC Operation: Regardless of the current direction, the iron cam will always be pulled inwards.
• Deflection: The deflection (θ) is proportional to the square of the current (I²), hence θ ∝ I². For AC measurements, θ ∝ I²rms
• Scale: The scale in a moving iron instrument is typically non-linear.

2. Repulsion Type Moving Iron Instruments

• These instruments can measure both AC and DC.
• Principle: When two soft iron rods or strips are magnetized in the same direction by a current-carrying coil, they experience a repulsive force.
• Mechanism: One soft iron strip (F) is fixed, while the other (M) is movable and is connected to the spindle. When current passes through the coil, the repulsion between the two strips causes the movable strip to rotate and move the pointer on the scale.

Key Differences: Moving Coil vs. Moving Iron

The following table summarizes the core differences between moving coil and moving iron instruments:

In conclusion, both moving coil and moving iron instruments have their own sets of advantages and limitations. The choice of instrument often depends on the specific application, considering factors such as accuracy requirements, cost constraints, and the type of electrical quantity to be measured.