PMMC and Moving-Iron Instruments: Complete Beginner's Guide

A PMMC (permanent magnet moving coil) instrument measures current with a small coil that rotates inside the field of a permanent magnet. When current flows through the coil, the magnetic force twists it against a spring, and a pointer shows the deflection. The twisting force is proportional to current, so the scale is uniform and very accurate on DC.

A moving-iron instrument works differently: current through a fixed coil magnetises a small piece of soft iron, which is then attracted or repelled so that a pointer moves. Force depends on current squared, so they respond to the RMS value and work on AC and DC — rugged, cheap panel meters, though less accurate than PMMC.

Key Concepts

• Deflecting torque: the magnetic force that turns the pointer.
• Controlling torque: the hairspring that opposes deflection and returns the pointer to zero when current stops.
• Damping torque: eddy-current or air damping that stops the pointer from oscillating around the reading.
• PMMC responds to the average value — on AC the average of a sine wave is zero, so it cannot be used directly on AC.
• Moving-iron responds to the RMS value because its torque depends on I², so it reads correctly on AC and on DC.
• Scale shape: PMMC scales are linear and uniform; moving-iron scales are cramped at the start because deflection follows a square law.
• Shunt: a low resistance in parallel with a PMMC movement to build an ammeter.
• Multiplier: a high resistance in series with the movement to build a voltmeter.

Formulas

• PMMC: deflecting torque Td = B × A × N × I; steady deflection θ = (B × A × N × I) / k, where k is the spring stiffness — so θ is directly proportional to I.
• Moving iron: Td = ½ × I² × dL/dθ; deflection θ = I² × (dL/dθ) / (2 × k), so θ is proportional to I².
• Ammeter shunt: Rsh = Rm / (m − 1), where m = I_range / I_FSD and Rm is the movement resistance.
• Voltmeter multiplier: Rmul = (V_range / I_FSD) − Rm.

Assumes a linear spring, a uniform radial PMMC field, and constant dL/dθ.

Worked Example

1. A PMMC movement needs 50 µA for full-scale deflection (FSD) and has a coil resistance of 1 kΩ.
2. For a 5 A ammeter: m = 5 / (50 × 10⁻⁶) = 100,000. Rsh = 1000 / 99,999 ≈ 0.010 Ω — a 0.01 Ω shunt carries almost all the current while the movement takes its 50 µA.
3. For a 50 V voltmeter: total resistance needed = 50 / (50 × 10⁻⁶) = 1,000,000 Ω. Multiplier = 1,000,000 − 1,000 = 999,000 Ω, about 999 kΩ in series.
4. Sensitivity = 1 / I_FSD = 20,000 Ω/V.

Common Misconceptions

• "A PMMC meter can measure AC if I just connect it." On AC the coil tries to reverse 50 times a second and the pointer stays at zero; it reads the average, which is zero for a sine wave.
• "Moving-iron meters are just as accurate." Square-law scale and hysteresis make them 1–2% instruments — fine for panels, not labs.
• "Both read the same on DC." They do agree on DC, but PMMC is far more sensitive and accurate there.

Real-World Applications

• Analog panel meters in power distribution boards and generator panels.
• Classic multimeter movements and educational lab instruments.
• Rugged field indicators where digital displays would not survive.

Choose PMMC for accurate, linear DC measurement, and moving-iron for a cheap, tough AC RMS panel meter. The shunt and multiplier formulas let one delicate movement serve as ammeter and voltmeter across dozens of ranges.