Pmmc

1. PMMC
SRUTHI J M

2. Principle
Whenever a current carrying conductor placed in a magnetic field, it
experiences a force.
Faraday’s law of Electromagnetic induction
Direction of force – Fleming’s Left Hand Rule

3. Construction
Permanent magnet : Alnico
Moving coil : Copper
Damping system : Aluminium former
Control system (springs) : Phosphor bronze

4. Working
When a current flow through the coil,
it generates a magnetic field
proportional to the current.
Electromagnetic force generates
throughout the fixed magnetic field
generates a deflection force within the
coil.
Spring generates the force to resist
additional deflection, it helps to
balance the pointer.

5. Working
Damping force can be generated within the
system through the aluminium core
movement in the magnetic field.
Once it attains controlling and deflecting
torque to provide accuracy in
measurement

6. Torque equations
Deflection torque
𝑇𝑑 = 𝑁𝐼𝐵𝐴
𝑇𝑑𝛼𝐼
N – no.of turns in the coil
I – Flow of current in the coil (A)
B – density of flux within the air gap(Wb/m²)
A – Area of coil (m²)
𝑇𝑑 - deflecting torque (N-m)

7. Controllingtorque
𝑇𝑐∝ 𝜙
𝑇𝑐 = 𝑘𝜙 𝑇𝑐 -controlling torque
K – sping constant
𝜙 – angular deflection

8. Dampingtorque
Eddy current damping due to the presence of magnetic field.
𝑇𝐷= 𝐷
𝑑𝜙
𝑑𝑡 𝑇𝐷 - damping torque
D – damping constant
𝜙 – angular deflection

9. Pointstoremember
Measure only DC
Scale is linear
Controlling torque provided by springs
Dampingtorque provided by eddy current damping
More torque to weight ratio – accuracy & sensitivityis high
Without any external device it measures upto a current of 50mA or a voltage of 100mV