The permanent magnet moving coil (PMMC) instrument is the most accurate type for dc measurements. The general constructional features of this instrument are shown in Figure.

Fig: Permanent Magnet Moving Coil Instrument

Moving coil: The moving coil is wound with many turns of enamelled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils move freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping. Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt. The coil itself, therefore, provides electro-magnetic damping.

Magnet systems: Owing to development of materials like Alcomax and Alnico, which have a high coercive force, it is possible to use smaller magnet lengths and high field intensities. The flux densities used in PMMC instruments vary from 0.1 Wb/sq-m to 1 Wb/sq-m. Thus, in small instruments it is possible to use a small coil having small number of turns and hence a reduction in volume is achieved. Alternatively in instruments having a large-scale length it is possible to increase the air gap length to accommodate large number of turns.

Control: When the coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs. These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension. This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

Damping: Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade. This helps to reduce parallax errors in the reading of the scale. In many instruments such errors may be reduced further by careful alignment of the pointer blade and its reflection in the mirror adjacent to scale.

Torque equation:

The torque for a moving coil instrument is

Td = NBI dI = GI                                   where G = a constant.

The spring control provides a restoring (controlling) torque

Tc = Kθ                                                where K – spring constant.

For final steady deflection, Tc =Td or GI = Kθ

Final steady deflection, θ = (G/K) I

As the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.

 

Advantages and Disadvantages of PMMC Instruments

The main advantages of PMMC instruments are:

• The scale is uniformly divided.
• The power consumption is very low as 25 μW to 200 μ
• The torque-weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full-scale deflection.
• A single instrument may be used for many different current and voltage ranges by using different values for shunts and multipliers.
• Since the operating forces are large on account of large flux densities which may be as high as 0.5 Wb/sq-m the errors due to stray magnetic fields are small.
• Self-shielding magnets make the core magnet mechanism particularly useful in aircraft and aerospace applications, where a multiplicity of instruments must be mounted in close proximity to each other.

The chief disadvantages are:

• These instruments are useful only for dc. The torque reverses if the current reverses. If the instrument is connected to ac, the pointer cannot follow the rapid reversals and the deflection corresponds to mean torque, which is zero. Hence these instruments cannot be used for ac.
• The cost of these instruments is higher than that of moving iron instruments.