PMMC instrument- Permanent Magnet Moving Coil Instrument-Working, Construction

The permanent magnet moving coil instruments (PMMC instrument) are suitable for DC quantity measurements only. PMMC instruments working principle based on Faraday's law of electromagnetic induction state that when the current carrying coil is placed in the magnetic field it experiences force and torque act on the coil and it starts moving. 

Construction of PMMC Instrument

 PMMC instrument diagram :
Construction-working-PMMC Instrument

M=Permanent magnet 
PP =Soft iron pole pieces 
A =Soft iron cylinder (central core) 
C =Rectangular coil
B =Spiral springs 
D=pointer
 
Moving Coil
PMMC instruments consist of a light rectangular shape coil having many turns of fine wire that wound on an aluminium former inside which an iron core placed . 

The coil is placed upon a jewel bearing and mounted between the pole of the permanent magnet. 

Maganet system: 
Two permanent magnets made of the soft iron cylinder are used.

Control Spring: 
The current led into and out of the moving coil using two control springs which are mounted one above and one below the coil these control springs provide the controlling torque.
 
Damping Torque: 
The damping torque in PMMC instruments is provided by eddy current induced in aluminium former. the aluminium former is responsible for producing damping torque. 

Pointer and Scale arrangement: 
When the PMMC instrument is connected in circuit to measure current or voltage, the operating current start flowing through the coil, since the coil is carrying current and is kept in the magnetic field of the permanent magnet. It experiences force and starts moving. 

Hence the pointer attached to the moving system moves clockwise direction over the scale to indicate the value of current or voltage to be measured

Working of permanent magnet moving coil (PMMC) instrument

The working principle of the  PMMC instrument is based on Faraday's law of electromagnetic induction. When the current carrying conductor is kept in the magnetic field it experiences force hence the force exerted on the moving coil due to the magnetic field is directly proportional to the current flowing through the conductor or coil. 

PMMC instruments are used only for DC measurements and are more accurate and reliable. 

The magnetic field in the air gap is radial due to the soft iron core. when measuring current flows through the coil, a constant defecting torque Td act on the coil.

Td=BINA N/m

Where, 

B= Flux density in wb/m2
I= Current through the  coil
N= number of turns in coil
A= Area of coil
Since B, N and A are constant

Hence Td is directly proportional to I

PMMC instruments  are spring controlled when Tc Ø  the pointer will come to rest where Tc=Td , Ø I

Deflection is directly proportional to current

 Advantages of PMMC Instrument
  • Low power consumption 
  • Uniform scale
  • No Hysteresis loss
  • High torque to weight ratio

Disadvantage of PMMC Instruments 

  • Costlier as compared to moving iron instruments
  • Can not use to measure AC quantity
  • Friction temperature affect accuracy
  • Ageing of the control spring can introduce errors in measurement

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