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Figure 2.2.1
Permanent Magnet Moving Coil (PMMC) is an instrument that allows you to measure the current through a coil by observing the coil’s angular deflection in a uniform magnetic field. It is a kind of galvanometer that works on the principle of D’Arsonval.
A PMMC meter places a coil of wire in between two permanent magnets in order to create stationary magnetic field. According to Faraday’s Laws of electromagnetic induction, a current carrying conductor placed in a magnetic field will experience a force in the direction determined by Fleming’s left hand rule.
The PMMC instrument working principle is when the torque is applied to the moving coil that is placed within the permanent magnet field, and then it gives a precise result for DC measurement.
Applications of PMMC
Ammeter
Voltmeter
Galvanometer
Ohmmeter
Figure 2.2.2
Moving coil
Can freely moves between the two permanent magnets
The coil is wound with many turns of copper wire and is placed on rectangular aluminium which is pivoted on jewelled bearings.
Permanent Magnet
The PMMC instrument includes two high-intensity magnets otherwise a ‘U’ shaped magnet-based design.
Using magnets of high field intensities, high coercive force instead of using U shaped permanent magnet having soft iron pole pieces.
Cylindrical-shaped soft iron coil
Placed between north and south
Which a coil of fine wire is wound
This fine wire is wound on very light metal frame and mounted in a jewel setting so that it can rotate freely
Damping torque
Can be generated within the PMMC instrument using the aluminium core’s movement within the magnetic field.
Provided by movement of aluminium former in the magnetic field created by the permanent magnets.
Pointer
Attached to the moving coil
Deflects upscale as the moving coil rotates as current flows
Control
These springs are arranged among the two jewel bearings. The spring provides the lane to the lead current to supply in & out of the moving coil.
The torque can be controlled mainly due to the delay of the ribbon.
Diagram of D’Arsonval type galvanometer. As the current flows from + through the coil (the orange part) to -, a magnetic field id generated in the coil. This field is counteracted / responded by the permanent magnet and forces the coil to twist, moving the pointer, in relation to the field’s strength caused by the flow of current.
Figure 2.2.3
Working principles of PMMC
Working principles of PMMC
Working principle is based on ‘motor principle’ where when a current carrying conductor is placed in the magnetic field, it is acted upon by a force which tends to move it to one side and out of the field.
The deflection of the pointer is directly proportional to the amount of current passing through the coil.
When a current flow through the coil, it generates a magnetic field which is proportional to the current in case of an ammeter
The poles of the electromagnet interact with permanent magnet causing the coil to rotate and the pointer deflects up scale
It should emphasized D’Arsonval meter movement is a current scale calibrated, the moving coil responds to the amount of current through its windings
Video of working principle for PMMC:
1. Pointer is used as the deflection instrument to indicate a measured quantity.
2. There are 3 forces/torque operating in the movement.
i- Deflecting force
ii- Controlling force
iii- Damping force
3. When the current flow in the moving coil, the current set up a magnetic field that interact with the field of permanent magnet.
4. A force applied on a current carrying conductor situated in magnetic field
5. Consequently a force applied on the coil turns and causing the coil to rotate on its pivoted
6. The controlling force provided by spiral spring. The spring retain the coil and pointer at zero position when no current. Coil and pointer stop rotating when
Deflecting force = Controlling force
Equations related to PMMC
Torque Equation
The equation involved in the PMCC instrument is the torque equation. The deflecting torque induces due to the coil’s movement and this can be expressed with the equation shown below.
Td = NBLdl
Where,
‘N’ is the no. of turns in the coil
‘B’ is the density of flux within the air gap
‘L’ & ’d’ are vertical as well as horizontal lengths of the surface
‘I’ is the flow of current in the coil
G = NBLd
The restoring torque can be provided to the moving coil can be done with the spring and it can be expressed as
Tc = Kθ (‘K’ is the spring constant)
Final deflection can be done through the equation Tc = Td
Substitute the values of Tc & Td in the above equation, then we can get
Kθ = NBLdl
We know that G = NBLd
Kθ = Gl
θ= Gl/K
I = (K/G) θ
From the above equation, we can conclude that the deflection torque can be directly proportional to the flow of current in the coil.
PMMC Characteristic
v Amount of rotation (angular deflection) is proportional to the amount of current flows through the coil.
v The meter requires low current (~50uA) for full scale deflection, thus consumes very low power (25-200uW).
v Its accuracy is about 2% - 5% of full scale deflection.
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