Permanent magnet moving coil instruments (PMMC)

Permanent magnet moving coil instruments (PMMC)

The permanent magnet moving coil instrument in PMMC type instrument used for measuring DC quantities only. In pmmc type instruments we use permanent magnets for create a strong stationary magnetic field.
If we are apply a AC current to this instrument its cannot work and pointer and vibrate its initial position because we know that in AC cycle half cycle is positive value and other half cycle is negative value show the direction of torque will be also reversed in the other half cycle so the equivalent to produce will be zero pointer does not show any deflection and its vibrate at zero position.

Now we are discussing about the construction of the permanent magnet moving coil instruments here we are divided this instruments into 5 parts that are described below:--

1. Stationary magnet
2. moving coil 
3. control system 
4. damping system 
5. Meter

Stationary magnet

in the present days we are using U shaped permanent magnet in pmmc instrument in this magnets high field intensities,high coericive force. The material which we are using to  made magnet like alcomax and elnico which provide high field strength.

Moving coil

the moving coil can freely move between the u shaped magnet in BMC instrument recoil is made using many returns of the copper wire and is placed on the rectangular aluminium which is fixed on the jeweled bearings.

Control system

Control system for the controlling of the pointers we are using a Spring in pmmc this spring can provide the controlling torque and as well as the path for the passing of lead current through this spring if the spring is break the current passing will be stop and pointer comes to zero initial position.

Damping System

Damping torque test of ensuring the phone to comes to equilibrium position example at the rest position this girl without oscillation to give the accurate readings but in the case of pmcs the coil moves in the magnetic field some amount of Eddy current is set up in the metal forming or core ( at this core our coil is wounded) or in the circuit of the coil itself which oppose the motion of the coil resulting in the slow swing of the pointer and then comes to rest quickly with very small amount of oscillation.

Meter 

Meter is this type of instruments where  pointer can move freely according to Deflection and give the reading on the scale which is linear for uniform and varies with angle.

So now we drive the expression for the torque in a permanent magnet moving coil instrument (pmmc instruments)
We know that moving coil instrument deflecting torque is given by following expression:-

Where
N= numbers of turn
L= length of the moving coil
D= width of the moving coil
I= electric current
Sin=1

The moving coil instrument deflecting to always should be proportional to current so mathematically we can write this Td=GI
Here G=NBA    and A=l×b
When the both of controlling torque and deflecting torque is equal we can get the steady state position.