PMMC Instrument : Working, Construction, Application, Advantages & Disadvantages

These instruments are used either as ammeters or voltmeters and are suitable for d.c. work only. This type of instrument is based on the principle that when a current carrying conductor is placed in a magnetic field, mechanical force acts on the conductor. The coil placed in the magnetic field and carrying the operating current is attached to the moving system. With the movement of the coil, the pointer moves over the scale to indicate the electrical quantity being measured. This type of movement is known as D'Arsonval movement.

Construction :

Fig. 16.11 shows the various parts of a permanent-magnet moving coil instrument. It consists of a light rectangular coil of many turns of fine wire wound on an aluminium former inside which is an iron core as shown in Fig. 16.11 (i). The coil is delicately pivoted upon jewel bearings and is mounted between the poles of a permanent horse-shoe magnet. Attached to these poles are two soft-iron pole pieces which concentrate the magnetic field. The current is led into and out of the coil by means of two control
hair-springs, one above and the other below the coil, as shown in Fig. 16.11 (ii). These springs also provide the controlling torque. The damping torque is provided by eddy currents induced in the aluminium former as the coil moves from one position to another.

Working:

When the instrument is connected in the circuit to measure current or voltage, the operating current flows through the coil. Since the coil is carrying current and is placed in the magnetic field of the permanent magnet, a mechanical torque acts on it. As a result, the pointer attached to the moving system moves in a clockwise direction over the graduated scale to indicate the value of current or voltage being measured. If the current in the coil is reversed, the deflecting torque will also be reversed since the direction of the field of the permanent magnet is the same. Consequently, the pointer will try to deflect below zero. Deflection in this direction (i.e. reverse direction) is prevented by a spring "stop". Since the deflecting torque reverses with the reversal of current in the coil, such instruments can be used to measure direct currents and voltages only.

Deflecting Torque : 

The magnetic field in the air gap is radial due to the presence of soft-iron core. This means that conductors of the coil will always move at right angles to the field. When current is passed through the coil, forces act on its both sides which produce the deflecting torque.

Advantages : 

(1) Uniform scale i.e., evenly divided scale

(2) Very effective eddy current damping because the aluminium former moves in an intense magnetic field of the permanent magnet.

(3) High efficiency as it requires very little power for its operation.

(4) No hysteresis loss as the magnetic flux is practically constant.

(5) External stray fields have little effect on the readings as the operating magnetic field is very strong

(6) Very accurate and reliable

Disadvantages: 

(1) Such instruments cannot be used for ac measurements.

(2) Some errors are caused due to variations (with time or temperature) either in the strength of permanent magnet or in the control springs.

(3) About 50% more expensive than moving-iron instruments because of their accurate design.

Applications : 

Permanent-magnet moving coil instruments are acknowledged to be the best type for all d.c. measurements. They are very sensitive and maintain a high degree of accuracy over long periods. The chief applications of such instruments are : 

(1) In the measurement of direct currents and voltages.

(2) In d.c. galvanometers to detect small currents.

( 3) In ballistic galvanometers used mainly for measuring changes of magnetic flux linkages.