PMMC Instruments :
They are also known as d'Arsonval instruments.i instrument works on the electromagnetic effect of current. A permanent magnet is used to produce magnetic flux and a coil that carries the current to be measured moves in this field. A current carying conductor when placed in a magnetic field will expenence a force. So this coil experiences a force andi As the pointer is connected to the coil it also gets deflected in rotates. proportion with the current.
Working principle:
~ A current carying conductor placed in magnetic field experiences a force. It is given by the expression.
F =BIL
where
Force in Newton, Flux density in Tesla Current in ampere
Length of conductor in meter
The current I which is to be measured is passed through the moving coil, and experiences a force which is directly proportional to this current. i Due to this force the coil moves (rotates) and the pointer
attached to it will also move. i
The angle through which the pointer moves is proportional to current I. Hence we can calibrate the scale of the instrument in terms of current.
Construction of PMMC Instrument:
~ A light rectangular coil wound on a metal frame (usually aluminium) is pivoted within the air gaps between the two i poles of a permanent magnet and a cylindrical soft iron core.
~ This coil carries the current to be measured. Soft iron corei provides formation of uniform magnetic field. The aluminium frame supports the coil as well as provides eddy current damping.
~ Two Phosphor-bronze springs coiled in opposite directions serve as leads for the current in the coil. These springs also provide controlling torque. The moving system is balanced by three balance weights.
~ The moving spindle is pivoted in jewelled bearings. The construction of instrument is shown in Fig. 4.5.1 and Fig. 4.5.2.
4.5.4
Advantages of PMMC Instruments :
1. It has uniform scale. The scale may be very long, over about 2509.
2. With a powerful permanent magnet its torque to weight ratioi is very high.
3. The sensitivity is high. The sensitivity is defined as the pointer deflection in degrees per unit current.
4. The eddy currents induced in the metallic frame over which the moving coil is wound, provide effective damping. Additional damping device may not be necessary.
5. The instrument consumes low power.
6. It has very high accuracy.
7. Operating field of such instrument being very strong, external fields present in the surrounding, called stray fields do not affect the instrument.
8. The instrument is free from hysteresis loss.
9. Extension of instrument range is possible with the help of shunts and series resistances.
Disadvantages of PMMC Instruments :
1. The cost of the instrument is comparatively high. This is duei to its delicate construction and the necessity of accuratei machining.
2. Suitable for d.c. measurements only. Additional rectifier Circuit is necessary if it is to be used for a.c. measurements.
3. Aging of permanent magnets and control springs introduces i errors.
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