9 Nov Permanent Magnet Moving Coil or PMMC Instruments are the most accurate type for the measurement of DC current or voltage. The working. 17 Jul The permanent magnet moving coil instrument or PMMC type instrument uses two permanent magnets in order to create stationary magnetic. 4 Nov It stands for permanent magnet, moving coil. At is core a PMMC meter is a galvanometer that measures current. Adding internal or external components allow it.

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PMMC Instrument Working Principle | Permanent Magnet Moving Coil Instrument

The deflecting torque is produced by the electromagnetic action of the current in the coil and the stationary magnetic field. It is one of the main component of permanent magnet moving coil equipment; and is made up of copper coils wounded to a rectangular block in between the magnetic poles. The eddy currents set up a magnetic flux that opposes the original movement that generates them. In many designs an additional soft iron cylinder is placed in between the magnetic poles to make the field uniform; while reducing instrumet reluctance to increasing field strength.

The coil can move freely in the magnetic field produced by the Permanent Magnet System.

These control springs also serves the purpose of providing lead current path in and out of the system. Due to the movement of coil in the magnetic field; eddy current is produced in the aluminum former.

These magnets have a high coercive force and can produce a field of the order of 0. The weakness of the magnet decreases the deflection of the coil. Leave a Reply Cancel reply You must be logged in to post a comment.


This helps in reducing the Parallax Error in reading the scale.

Subscribe to our mailing list and get interesting stuff and updates to your email inbox. Non-metallic former like that of aluminum are used for current measurement; while metallic former with high electromagnetic damping are used to measure voltage.

In many instruments, this parallax error can be further reduced by careful alignment of pointer blade and its reflection in the mirror adjacent to scale. Springs — The weakness of the spring increases the deflection of moving coil between the permanent magnet. In case of PMMC voltmeters, coil is generally wound on metallic frame to have the required electromagnetic damping.

The pointer is of light weight construction and has a section over the scale twisted inetrument form ommc fine blade. A soft iron core is included between the poles of the magnet so that the coil rotates in the narrow air gap between the poles and the core. These types of instruments are only used for measuring the DC quantities as 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.

Stationary Part or Magnet System: The parts of the PMMC instruments are explained below in details. Errors due to permanent magnets: Ijstrument electromagnetic damping opposes the motion of the coil which is because of the reaction of the eddy current and the magnetic field.

The instruments pm,c use the permanent magnet for creating the stationary magnetic field between which the coil moves is known as the permanent magnet instrumenr coil or PMMC instrument.

Let us consider a circuit lnstrument shown below: The controlling force is usually provided by spiral springs as indicated in figure 2.


One degree rise in temperature reduces the 0. This helps in reducing the Parallax Error in reading the scale. The moving coil itself provides the electromagnetic damping. In the case of the PMMC instruments, the damping force uses eddy currents. Moving Coil — The coil is the current carrying part of the instruments which is freely moved between the stationary field of the permanent magnet.

Circuit Globe All about Electrical and Electronics. One of the most common and obvious questions out of curiosity; its not strange that we ask why its not used for A.

Pointer is a simple construction with light weight design and twisted section to reduce parallax error. Due to movement of Aluminum former an eddy pmmx is instrukent on the former which produces a damping torque.

At steady state condition, deflecting and controlling torque shall be equal. This decrease in magnetic strength reduce the coil deflection affecting the readings. The former increases the radial and uniform magnetic field between the air gap of the poles.

PMMC Instrument; Construction, Working and Application

This happens because the coil is made up of copper wire which pm,c high shunt resistance and the shunt wire made up of Magnin has low resistance. A soft iron end cylinder is bored in between the poles to make the field radial and uniform.

Then damping force is produced in the system by the movement of aluminum core in the magnetic field.

A Parallax error can be further reduced by proper alignment of pointer blades to the initial scale.