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With the help of diagrams, explain the construction and working principle of permanent magnet moving coil instruments. obtain the expression for its deflecting torque.
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Several electrical machines and panels are fitted onboard so that the ship can sail from one port to another, safely and efficiently. The electrical machinery and system require scheduled maintenance and checks to avoid any kind of breakdown during sailing.


Permanent Magnet Moving Coil: Principle of Working: 

When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left hand rule.


Fleming left hand rule:

 If the first and the second finger and the thumb of the left hand are held so that they are at right angle to each other, then the thumb shows the direction of the force on the conductor, the first finger points towards the direction of the magnetic field and the second finger shows the direction of the current in the wire.


Equation involved

The interaction between the induced field and the field produced by the permanent magnet causes a deflecting torque, which results in rotation. The three important torque involved in this instrument are:


Deflecting torque

The force F which will be perpendicular to both the direction of the current flow and the direction of magnetic filed as per the fleming’s left hand rule can be written as

F = NBIL

Where,

N: turns of wire on the coil

B: flux density in the air gap

 I: current in the movable coil

 L: vertical length of the coil

image

Theoretically the torque (here electro-magnetically torque) is equal to the multiplication of force with distance to the point of suspension. Hence Torque on left side of the cylinder
TL = NBIL x W/2 and torque on right side of the cylinder TR = NBIL x W/2
Therefore the total torque will be = TL + TR
T = NBILW or NBIA where A is effective area (A= LxW)
Controlling Torque:
 This torque is produced by the spring action and opposes the deflection torque so as the pointer can come to rest at the point where these two torques are equal (Electromagnetic torque= control spring torque).
 The value of control torque depends on the mechanical design of spiral springs and strip suspensions. The controlling torque is directly proportional to the angle of deflection of the coil.
Control torque Ct =Cθ where, θ = deflection angle in radians and C = spring constant Nm / rad .

Damping torque: This torque ensures the pointer comes to an equilibrium position i.e. at rest in the scale without oscillating to give accurate reading. In PMMC as the coil moves in the magnetic field, eddy current sets up in a metal former or core on which the coil is wound or in the circuit of the coil itself which opposes the motion of the coil resulting in slow swing of pointer and then come to rest quickly with very little oscillation

Construction:
 A coil of thin wire is mounted on an aluminum frame (spindle) positioned between the poles of a U shaped permanent magnet which is made up of magnetic alloys like alnico.
The coil is pivoted on the jeweled bearing and thus the coil is free to rotate.
 The current is fed to the coil through spiral springs which are two in numbers. The coil which carries current, which is to be measured, moves in a strong magnetic field produced by a permanent and a pointer is attached to the spindle which shows the measured value
Working:
 When a current flow through the coil, it generates a magnetic field which is proportional to the current in case of an ammeter. The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field.
 When the torques are balanced the moving coil will stopped and its angular deflection represent the amount of electrical current to be measured against a fixed reference, called a scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear.
 Torque is provided by two phosphorous bronze flat coiled helical springs. These springs serve as a flexible connection to the coil conductors. Damping is caused by the eddy current set up in the aluminum coil which prevents the oscillation of the coil.
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