Permanent magnet moving coil instrument is most accurate type for D.C. measurements because its establish  stationary magnetic field. these are very sensitive and accurate. these instruments can be used on d.c as voltmeter and ammeter.

The Basic principle of Permanent magnet type moving coil instrument is that when a current carrying conductor placed in a magnetic field a force is exerted on the conductor. the current carrying conductor produce a magnetic field  which tries to come in line with main field which produces deflection torque. The pointer attached to the spindle in deflection over the calibrated scale. If the direction of current in coil is reverse then the direction of torque will be reversed Because the filed produce by permanent magnet does not change so this will give wrong direction of rotation. therefore this instrument can not used in a.c., it can use only d.c.  

Construction of PMMC Instrument. 

1. Stationary Part or Magnet System: 
In the present time we use magnets of high field intensities, high coercive force instead of using U shaped permanent magnet having soft iron pole pieces. The magnets which we are using nowadays are made up of materials like alcomax and alnico which provide high field strength.

2.Moving Coil: 
The moving coil can freely moves between the two permanent magnets as shown in the figure given below. The coil is wound with many turns of copper wire and is placed on rectangular aluminium which is pivoted on jeweled bearings.

3.Control System: 
The spring generally acts as control system for PMMC instruments. The spring also serves another important function by providing the path to lead current in and out of the coil.

4.Damping System: 
The damping force hence torque is provided by movement of aluminium former in the magnetic field created by the permanent magnets.

5. Pointer and Scale: 
The Pointer is carried by spindle and moves over a graduated scale. the pointer is a light has the section over the scale twisted to form a fine blade ts help to reduce parallax the reading of scale. 

Torque Equation: 
Deflection of the torque depends upon the force acting on coil sides.
       B = Flux Density 
       l  = Effective lenght of coil sides in meter.
      N = number of turns of the coil.
       d = distance between the center of coil and force.
        I = Current flowing through the coil in amperes.

                                     Deflecting torque   Td = NB l dI  = GI
   Where                                                      G = a Constant = NB ld

  The spring control provides a restoring ( Controlling) torque Tc = Kθ
                                                                    K = spring constant
                                                                   θ  = Angle of deflection
             For final steady deflection             Tc =Td
                    final steady deflection             θ  = (G/K) I
            or                                       Current I   =  (K/G) θ
                                                                   I  ∝  θ
Deflection  is proportional to the operating current flowing through coil , therefore the scale of such an instrument is uniform.

     1. These Instruments have uniform scale.
     2. Very effective and reliable eddy current damping.
     3. No hystersis loss as the former is of aluminium. 
     4. Low power consumption because deriving power is small.
     5. No effect stray magnetic field as working field provided by the permanent magnets is                      very strong.  
     6. High torque/weight ratio, there for such instruments require small operating current.
     7. very accurate and reliable.

     1. These Instruments can not be used for a.c. measurement. 
     2. These are costlier in comparison of moving iron Instruments.
     3. Friction and temperature  might introduce some errors.
     4. Some error are also caused due to ageing of control springs and permanent magnets

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