Construction And Working Of Permanent Magnet Moving Coil Instrument Pdf

construction and working of permanent magnet moving coil instrument pdf

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Electrical Academia. Figure 1 shows the basic construction of a PMMC instrument.

Permanent Magnet Moving Coil or PMMC Instrument

Facebook Twitter. The permanent magnet moving coil instrument is the most accurate type for dC measurements. Working principle of pmmc instrument is the same as that of the d'Arsonval type of galvanometers, the difference being that direct reading instrument is provided with a pointer and a scale.

Construction of pmmc instruments:. The general constructional features of pmmc instrument are shown in below figure:. Moving coil: The moving coil is wound with many turns of enamelled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils move freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electromagnetic damping.

Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt. The coil itself,therefore, provide electromagnetic damping. Magnet systems: There has been considerable development in materials for permanent magnets and therefore, magnet assemblies have undergone a lot of change in the recent past.

Old style magnet system consisted of a relatively long U shaped permanent magnets having soft iron pole pieces. The flux densities used in pmmc instruments vary from 0.

Thus in all small instruments it is possible to use a small coil having small number of turns and hence a reduction in volume is achieved. Alternatively in instruments having a large scale length it is possible to increase the air gap length to accommodate large number of turns. The movement of the coil is restricted in the above design. This is because no actual part of the coil is allowed to reach the extreme positions near the pole tips where, there is fringing field.

Thus the angular span of scale is restricted to 90 degrees. In order to obtain longer moment of the pointer and a longer swing of the coil a concentric magnet construction is used. Since the magnet is concentric type it produces a radial flux pattern which extends over degrees or more.

This type of construction is used for many panel type instruments and some portable instruments. In recent years, with the development of improved magnetic materials like Alnico, it has become feasible to design a magnetic system in which the magnet itself serves as the core as shown in figure below. The moving coil moves over the magnet. The active sides of the moving coil located in the uniform radial field between pole pieces and steel yoke.

This arrangement eliminate the magnetic shunting effect in steel panel construction, where several meters operating side by side may effect each other's readings. The need for magnetic shielding in the form of iron cases, is also eliminated by core magnet construction.

Control torque: When the coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs.

These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown in figure. This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction. Damping torque: Damping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.

Pointer and scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight instruction and apart from those used in some inexpensive instruments has a section over the scale twisted to form a fine blade. This helps to reduce parallax errors in the reading of the scale. In pmmc instruments such errors may be reduced further by carefully alignment of the pointer scale and its reflection in the mirror adjacent to scale.

The weight of the instrument is normally counter balanced by weights situated diametrically opposite and rigidly connected to it.

In micro-ammeters and low range milli-ammeters upto about 20mA, the entire the entire current to be measured is sent through the moving coil. This is because instrument springs serve as current leads to the moving coil. Their current carrying capacity limits the current which can be safely carried to about 20mA.

The scale is uniformly divided. The torque - weight ratio is high which gives a high accuracy. A single instrument may be used for many different current and voltage ranges by using different values for shunts and multipliers.

Since the operating are large on account of large flux densities which may be as high as 0. Self shielding magnets make the core magnet mechanism particularly useful in aircraft and Aerospace applications, where multiplicity of instruments must be mounted in close proximity to each other. These pmmc instruments are useful only for DC.

The torque reverses if the current reverses. If the instrument is connected to a. Hence these instruments cannot be used for AC. The cost of these pmmc instruments is higher than that of moving iron instruments. Tags Electrical measurements.

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PMMC Instruments – Construction and Working Principle

It is one of the simple and most used instruments on board with sophisticated names. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required. It is a simple machine that creates stationary magnetic fields with two powerful permanent magnets. A PMMC Equipment consists of two main parts; moving coil and a permanent magnet along with other parts. These parts are explained below:.

An ammeter from ampere meter is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes A , hence the name. The ammeter is usually connected in series with the circuit in which the current is to be measured. An ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments that relied on the Earth's magnetic field for operation.

Moving Coil Instrument Working Principle

Hi friends, in this article, I am going to discuss about moving coil instrument working principle and other related information. You will appreciate my effort, I hope so. The simple view of construction of moving coil instrument is shown in the figure. It consists of a powerful permanent shoe magnet.

Permanent Magnet Moving Coil Instrument or PMMC Instrument

PMMC Instrument; Construction, Working and Application

The PMMC works on the electromagnetic effect. A permanent magnet is used to produce magnetic flux and a coil that carries the current to be measured moves in this field. As the pointer is connected to the coil it gets deflected in proportion with the current. When a current carrying conductor placed in the magnetic field experiences a force. It is given by the expression is given below,. The basic construction of PMMC is shown in above fig.

Facebook Twitter. The permanent magnet moving coil instrument is the most accurate type for dC measurements. Working principle of pmmc instrument is the same as that of the d'Arsonval type of galvanometers, the difference being that direct reading instrument is provided with a pointer and a scale.

It operates on the principle that the torque is exerted on the moving coil placed in the field of the permanent magnet. The moving coil and permanent magnet are the main part of the PMMC instrument. The parts of the PMMC instruments are explained below in details. Moving Coil — The coil is the current carrying part of the instruments which is freely moved between the stationary field of the permanent magnet. The current passes through the coil deflects it due to which the magnitude of the current or voltage is determined. The coil is mounted on the rectangular former which is made up of aluminium. The former increases the radial and uniform magnetic field between the air gap of the poles.


Construction. A coil of thin wire is mounted on an aluminium frame (spindle) positioned between the poles of a U shaped permanent.


There are different electrical machines are used in the ship so that it can travel from place to another safely as well as capably. But these machines need maintenance while traveling to avoid any type of breakdown. To measure different electrical parameters on the ship, different instruments are used so that we can check the machines to maintain their proper working condition. Similarly, an instrument like PMMC permanent magnet moving coil is frequently used on ships as well as different applications. This article discusses an overview of the PMMC instrument.

Moving Coil:. The moving coil made up of copper is wound with many turns on the rectangular Aluminum former. This Aluminum former is pivoted on the jewelled bearing. The coil can move freely in the magnetic field produced by the Permanent Magnet System.

Several electrical machines and panels are fitted on board 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. Different instruments are used onboard for measuring several electrical parameters to analyze and keep these machines in proper running condition. A permanent magnet moving coil PMMC is one such instrument which is popularly used onboard and has several applications.

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