Permanent Magnet Moving Coil PDF

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Permanent Magnet Moving Coil (PMMC) Instruments ·

Pmmc is the type of moving coil instrument In moving coil instrument the coil rotates b/w the magnetic field of permanent magnets PMMC instrument is the indicating type electrical instrument which indicates the electrical quantity that is voltage or current. these type of instruments are most accurate for dc measurements. Working principle

The operation of pmmc is based on the principle that when the current carrying conductor that is coil in our case is placed in a magnetic field produced by the permanent magnet the coil experiences a force and moves(as per flemming left hand rule )as the coil is moving and the magnets are permanent the instrument is called as permanent magnet moving instrument. FLHR: 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. The amount of force experienced by the coil is propotional to the current passing through the coil.

PMMC Instruments construction

Magnetic field system :- As suggested by its name , the instrument consists of a (U-shaped) permanent magnet made of Alnico. This material is used becz it has high coercive force so they donot lose their magnetism easily.Between the poles is a rectangular shaped coil wound on a light aluminium frame inside which is a cylindrical soft iron core. The soft iron core makes the magnetic field radial & uniform. Damping system :- The aluminium frame provides damping by the eddy currents induced in it when the coil moves. Controlling system :- Two spiral shaped Phosphor-bronze springs are connected to the two ends of the moving coil , one above and one below. The springs provide not only controlling torque but additionally serve the purpose of leading current in and out of the coil . The two springs are spiralled in opposite sense to compensate for the temperature and aging effects on reading.

PMMC Instruments Working The three important torque involved in this instrument are: Deflecting torque: When the current is passed through the coil, couple of forces acts upon both its sides and produces a deflecting torque. DT is induced becz of the movement of the coil. The force F which will be perpendicular to both the direction of the current flow and the direction of magnetic field as per 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

Deflecting torque , Td=total force*distance= NBIL*b =BINA(N-m) This same , maximum torque , acts on the coil at each of its positions as the magnetic field is radial. This torque move the coil and hence the pointer. The coil movement will produce controlling torque in the springs.

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).

Controlling torque , Tc =K θ At equilibrium, Td = Tc i.e K θ= BINA

... let K be the spring constant

Let Then,

K1= ( BNA/K )

be a new constant

θ∝I

Showing that PMMC instruments have linear scale. PMMC instruments Advantages : Have linear scales therefore, readings can be taken easily.  With a powerful magnet, its torque to weight ratio is very high. So operating current of PMMC is small.  The eddy currents induced in the metallic former over which coil is wound, provide effective damping.  It consumes low power, of the order of 25 W to 200 mW.  It has high accuracy.  Instrument is free from hysteresis error. PMMC instruments Disadvantages : Cannot be used on ac as the average torque over an ac cycle would be zero and pointer always shows zero value.  These are costlier compared to moving iron type instruments.  Ageing of permanent magnet and the control springs introduces the errors. For deflection of scale we require torque if we apply AC current to these type of instruments the direction of current will be reversed during negative half cycle and hence the direction of torque will also be reversed which gives average value of torque zero. The pointer will not deflect showing zero reading. However it can measure the direct current very accurately....