construction and working of PMMC instrument

 what is PMMC instrument?

PMMC instrument is permanent magnet moving coil.

PMMC instrument is a instrument which is used to measure the current and the voltage and this instrument have high accuracy.

construction :

firstly there is a moving coil is the current carrying part of the instrument which is moves freely between the stationary magnetic field. the moving coil is rolled around the former and the former is made up of aluminium. this former creates the radial and uniform magnetic field between the two poles. In the magnetic system of instrument basically uses the permanent magnet 

In the instrument as a controlling torque uses the spring which is made up of phosphorus bronze and the spring is placed between the two jewelled bearings. the main component pointer is attached to the moving coil and moves withe the coil according to magnitude input signals given by the source for moving the pointer freely it is made up of the light material this pointer shows the the measured value on the calibrated scale we use calibrated scale in  this instrument the damping torque is provided in the instrument the main function of the damping torque is to steady deflection of the pointer. 

working :

now let we know how the instrument works when the dc current flows through the circuit the torque is acts on a moving coil because the coil is placed in between the permanent magnets and there is magnetic field produces by the magnets as we kw that whenever a current carrying conductor placed in a magnetic field conductor experiences the force in tangential direction. and force acts on which direction is decided by the Fleming left hand rule. the magnitude of force acts on a magnetic field is directly proportional to the input voltage and current. because it is necessary to the torque acts is according to the input signals. since the pointer is also attached with the coil therefore the pointer is also moves and it indicates the measured current and voltage value on the calibrated scale 

deflection torque in PMMC instrument 

the deflection torque is produces due tot the interaction between a stationary magnetic field created by magnet and the magnetic field produced by following the current in the coil deflecting torque is produces. 

Td  =   BIL  Newton   

where   B = Flux density wb\m2

             I  =  current flowing through he coil

             L  =  length of the coil

force applied on each side of the coil for n number of turns

 Td  =  NBIL  ,Newton 

deflection torque Td  =  force * perpendicular distance

      Td  =   NBIL * b

      Td  =   NBI (l * b)

so,  Td  =  NBIA                                  

                     A  =  Face area of the coil ,                                                                                                         b  =  breadth of the coil

       (if we take B as a constant)

then the defecting torque is directly proportional to the current 

   Td ∝  I

   Td = KI      ...............(1)                         K = constant

     Tc  ∝  θ                       

                Tc  =  controlling torque of a spiral spring , θ    =  angular deflection

     Tc  = Cθ   .................(2)

now for a steady deflection                                                                                                        TC ∝  Td

therefore from equation 1st and 2nd we can write 

      Cθ   =   KI

   so, angular deflection 

                   θ   = K \ C * I

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