The permanent magnet moving coil instruments (PMMC instruments) are suitable for DC quantity measurements only.
PMMC instruments working principle based on Faraday’s law of electromagnetic induction states that when the current carrying coil is placed in the magnetic field it experiences force and torque acting on the coil and it starts moving.
Construction of PMMC Instrument
PMMC instrument diagram :
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 aluminum former inside which an iron core placed.
The coil is placed upon a jewel bearing and mounted between the pole of the permanent magnet.
Magnet 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 aluminum former. the aluminum former is responsible for producing damping torque.
Pointer and Scale arrangement:
When the PMMC instrument is connected to the circuit to measure current or voltage, the operating current starts 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 the current or voltage to be measured
Working of 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
