What is a Permanent Magnet Moving Coil (PMMC) Instrument ?

Permanent Magnet Moving Coil (PMMC)

The operation of a permanent-magnet moving-coil type instrument is be based upon the principle that when a current-carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field.

Construction

As its name indicates, the instrument consists of a permanent magnet and a rectangular coil of many turns wound on a light aluminium or copper former inside which is an iron core as shown in Fig. (a).

two phosphor-bronze hair springs, one above and one below, which additionally serveh the purpose of lending the current in and out of the coil. The two springs are spiralled in opposite directions in order to neutralize the effects of temperature changes.

Deflecting Torque

When current is passed through the coil, force acts upon its both sides which produce a deflecting torque as shown in

Fig. (c) Let 

• B = flux density in Wb/m2
• l = length or depth of the coil in metre
• b = breadth of coil in metre
• N = number of turns in the coil

For N turns, the force on each side of the coil is = NBIl newton

∴ deflecting torque Td 

= force × perpendicular     distance

= NBIl × b = NBI(I × b) = NBIA N-m

where A is the face area of the coil.

It is seen that if B is constant, then Td is proportional to the current passing through the coil i.e. Td ∝I.

Such instruments are invariable spring-controlled so that Tc ∝ deflection θ.

Since at the final deflected position, Td = Tc ∴ θ ∝I

Hence, such instruments have uniform scales. Damping is electromagnetic i.e. by eddy currents induced in the metal frame over which the coil is wound. Since the frame moves in an intense magnetic field, the induced eddy currents are large and damping is very effective.

Advantage and Disadvantages

The permanent-magnet moving-coil (PMMC) type instruments have the following advantages and disadvantages :

Advantages

1. They have low power consumption.
2. Their scales are uniform and can be designed to extend over an arc of 170° or so.
3. They possess high (torque/weight) ratio.
4. They can be modified with the help of shunts and resistances to cover a wide range of currents and voltages.
5. They have no hysteresis loss.
6. They have very effective and efficient eddy-current damping.
7. Since the operating fields of such instruments are very strong, they are not much affected by stray magnetic fields.

Disadvantages

1. Due to delicate construction and the necessary accurate machining and assembly of various parts, such instruments are somewhat costlier as compared to moving-iron instruments.
2. Some errors are set in due to the aging of control springs and the parmanent magnets.

Such instruments are mainly used for d.c. work only, but they have been sometimes used in conjunction with rectifiers or thermo-junctions for a.c. measurements over a wide range or frequencies.

Permanent-magnet moving-coil instruments can be used as ammeters (with the help of a low resistance shunt) or as voltmeters (with the help of a high series resistance).

The principle of permanent-magnet moving-coil type instruments has been utilized in the construction of the following :