PMMC Instruments – Construction and Working Principle

Permanent Magnet Moving Coil or PMMC Instruments are the most accurate type for the measurement of DC current or voltage. The working principle of PMMC instruments are same as that of d’Arsonaval type of Galvanometer, difference being that a direct reading instrument is provided with pointer and scale.

Construction of PMMC Instruments

Permanent Magnet Moving Coil or PMMC Instruments consists of following components:

1. Moving Coil
2. Magnet System
3. Control Spring
4. Damping
5. Pointer and Scale

Moving Coil:

The moving coil made up of copper is wound with many turns on the rectangular Aluminum former. This Aluminum former is pivoted on the jewelled bearing. The coil can move freely in the magnetic field produced by the Permanent Magnet System. In case of PMMC voltmeters, coil is generally wound on metallic frame to have the required electromagnetic damping. But in case of Ammeter, coils are wound on non magnetic former because coil turns are effectively shorted by Ammeter Shunt. Thus we observe that in PMMC instruments, coil itself provides the electromagnetic damping.

Magnet System:

Simple U shaped permanent magnet made of Alcomax or Alnico is widely use in PMMC instruments. Theses magnets have high coercive force and can produce field of the order of 0.1 to 1 Wb/m². A soft iron end cylinder is bored in between the poles to make the field radial and uniform. In addition, this soft iron also reduced the reluctance of the air path between the poles and hence increases the magnetic field strength.

The controlling torque in PMMC Instruments is provided by two control spring mounted on the jewel bearing. Theses control springs are phosphor bronze hair spring either helical or spiral, coiled in opposite direction. Control Spring also serves to lead current in and out of the moving coil.

Damping:

Damping torque in PMMC instruments are produced by the movement of Aluminum former in the magnetic field of Permanent Magnet. Due to movement of Aluminum former an eddy current is developed on the former which produces a damping torque.

Pointer and Scale:

The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light weight construction and has a section over the scale twisted to form a fine blade. This helps in reducing the Parallax Error in reading the scale. In many instruments this parallax error can be further reduced by careful alignment of pointer blade and its reflection in the mirror adjacent to scale.

Torque Equation for PMMC Instruments:

The deflecting torque equation for Permanent Magnet Moving Coil or PMMC Instruments is given as

Deflecting Torque Ƭ_d = NBLdI = GI

Where G = a constant = NBLd

Where N = Number of turns in the moving coil

B = magnetic flux density between the magnetic poles

L = Length of moving coil

d = Breadth of moving coil

As the controlling torque is provided by the spring, therefore

 Ƭ_c = KƟ

Where K = Spring constant

Ɵ = Angular movement of coil

At steady state condition, deflecting and controlling torque shall be equal,

Ƭ_d = Ƭ_c

⇒ GI = KƟ

⇒ Ɵ = (G / K)I  ……………………(1)

Thus from the above equation (1), we observe that deflection in Permanent Magnet Moving Coil or PMMC Instruments is directly proportional to the current flowing in the moving coil. Because of this the meter scale of such instrument for the measurement of current / voltage is linear.