Pmmc instrument resulting deflection of the coil is indicated by a pointer that moves over a calibrated scale. Different instruments are used onboard for measuring several electrical parameters to analyze and keep these machines in proper running condition. Mcq on measuring instruments 1 how engineering works. In this article all major differences between moving iron and moving coil instruments are discussed. The weakening of spring can be reduced by careful use of material and preaging during manufacturing process.
Reproduce the series type ohmmeter using pmmc of ifsd 30. There are two types of moving coil instruments namely, permanent magnet moving coil. Dc ammeter are mainly pmmc instruments, mi can measure both ac and dc currents, also electrodynamometer type thermal instrument can measure dc and ac, induction meters are not generally used for ammeter construction due to their higher cost, inaccuracy in measurement description of different types of ammeters pmmc ammeter. They are very sensitive and maintain a high degree of accuracy over long periods. Moving coil instruments there are two types of moving coil instruments namely, permanent magnet moving coil type which can only be used for direct current, voltage measurements and the dynamometer type which can be used on either direct or alternating current, voltage measurements. Permanent magnet moving coil instrument or pmmc instrument. The electrical measurements notes pdf em notes pdf book starts with the topics covering control and damping torques, design considerations type of p.
Permanent magnet moving coil instrument or pmmc instrument the current i is shown which breaks into two components at the point the two components are is and before i. Chapter 3 deflection type measuring instruments modern academy. Ammeter working principle and types of ammeter electrical4u. The magnetic field strength and spring tension decrease with increase in temperature. Permanentmagnet movingcoil instruments can be used as ammeters with the help of a low resistance shunt or as voltmeters with the help of a high series resistance. In pmmc meter or darsonval meter or galvanometer all are the same instrument, a. The permanent magnet, polarized instruments are used for the measurement of dc voltagecurrent while the dynamometer type instruments are used for the measurement of power in both dc and ac circuits. Previous years gate problems besed on pmmc instrument part 1.
Both are the examples or the permanent magnet moving coil pmmc galvanometer, and use the same fundamental principle as the dc motor. The instruments which use the permanent magnet for creating the stationary magnetic field between which the coil moves is known as the permanent magnet moving coil or pmmc instrument. Depending on the constructing principle, there are many types of ammeter we get, they are mainly. Permanent magnet moving coil type pmmc instrument working principle the basic principle of operation is that when a current carrying conductor is brought in a magnetic field they should not be parallel to each other a torque on the conductor is produced. The coils move freely in the field of a permanent magnet. This instrument can be used as an ammeter or as voltmeter but is generally used as a wattmeter. An ammeter has a pmmc instrument with a coil resistance of r m. Large current can be measured using low current rated ammeter by using a shunt. The calibration of the electrodynamometer type of instrument is the same for ac and dc. The permanent magnet moving coil pmmc instrument is the most common used as deflection type instruments. And the pointer moves according to the interaction of.
The working principle of pmmc instruments are same as that of darsonaval type of galvanometer, difference being that a direct reading instrument is provided with pointer and scale. Working principle of pmmc instrument is the same as that of the darsonval type of galvanometers, the difference being that direct reading instrument is provided with a pointer and a scale. The other popular nomenclature of this instrument is dalvanometer and. In movingiron instruments the movable system consists of one or more pieces of speciallyshaped soft iron, which are so pivoted as to be acted upon by. Pmmc instruments construction and working principle. Permanent magnet moving coil instrument pmmc working. Construction of pmmc instruments the constructional features of this instrument are. One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it. Measurements and instruments viva short questions and. Pmmc instrument working principle permanent magnet moving coil instrument the permanent magnet moving coil pmmc instrument is an electromechanical meter that can be connected with additional components to function as an ammeter, a voltmeter, or an ohmmeter. Working principle and construction of pmmc polytechnic hub. The action of these instruments is based on the motoring principle.
A major advantage of this core magnet type of construction is that the moving coil is shielded from external. Moving iron instruments principle, operation and type, theory of. The deflection of the coil is reduced by the aluminium piston which is attached to the moving coil. The permanent magnet moving coil instruments are most accurate type for direct current measurements. Meter of these instruments consists of light weight pointer to have free movement and scale. The permanent magnet moving coil instrument or pmmc type. The pmmc instrument has light weight so that it can deflect rapidly. A permanent magnet moving coil pmmc is one such instrument which is popularly used onboard and has several applications. Pmmc type moving iron type dynamometer type hot wire type, electrostatic type, induction type. Moving iron instruments principle and operation repulsion type. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a. The permanent magnet moving coil instrument is the most accurate type for dc measurements. Click here to download the pdf of this topic moving iron instruments.
Meter of these instruments consists of light weight pointer to have free movement and scale which is linear or uniform and varies with angle. Moving coil instruments moving coil instruments mc. Applications of pmmc permanent magnet moving coil instruments are acknowledged to be the best type of all d. A permanent magnet is used to produce magnetic flux and a coil that carries the current. The instrument in which the moving iron is used for measuring the flow of current. The basic pmmc instrument is sensitive to the temperature. Permanent magnet moving coil mechanism pmmc mathematical. A permanent magnet moving coil pmmc is one such instrument which is. Measurement and instruments objective questions and answers pdf free download. Advantage and disadvantages the permanentmagnet movingcoil pmmc type instruments have the following advantages and disadvantages. Repulsion type instruments the repulsion type instrument has two vanes. Moving coil instruments eee made easy pmmc permanent magnet moving coil instruments can read only dc measurements. In the attraction type moving iron instrument, the deflecting. Pmmc instrument working principle permanent magnet.
Ebooks, materials, notes, previous solved papers, questions and answers with. This principle is used in moving iron attraction type instrument. Permanent magnet moving coil or pmmc instruments are the most accurate type for the measurement of dc current or voltage. The operation of pmmc instrument is based on the principle that. Which of the following instrument is free from hysterisis and eddy current error. Aging of the instrument permanent magnet and control spring may introduce some errors.
The general theory of movingcoil instruments may be dealt with considering a rectangular coil of turns, free to rotate about a vertical axis. Classification, absolute and secondary instruments, indicating instruments, control, balancing and damping, constructional details, characteristics, errors in measurement, ammeters, voltmeters. The coil is pivoted on the jewelled bearing and thus the coil is free to rotate. Construction of pmmc instruments the constructional features of this instrument are shown in fig. In a moving iron meter, the deflecting torque is proportional to a. Permanent magnet moving coil type pmmc instrument working principle. These types of instruments are only used for measuring the dc quan. Previous years gate problems besed on pmmc instrument part 2. Various error in pmmc instrument electrical concepts. The construction and working of a pmmc instrument article716. The pmmc instrument is the most accurate type for dc measurement.
In an electrodynamic instrument the operating field is produced by another fixed coil and not by permanent magnet. The pmmc instrument is essentially a lowlevel dc ammeter. The repulsion type instrument is a nonpolarized instrument, i. The coil is suspended so that it is free to turn about its vertical axis. The moving coil instruments can be of permanent magnet type pmmc or dynamometer type. Measurements and instrumentation questions and answers pdf free download for ece mcqs objective type interview questions lab viva manual gate cat syllabus. Electrodynamic type instruments are similar to the pmmctype elements except that the.
The attraction type instruments use spring, which provided the controlling torque. Moving iron instruments are generally used to measure alternating voltages and currents. The coil resistance increases with increase in the temperature. State the advantages of pmmc instruments uniform scale. Permanent magnet moving coil instruments advantages. Meters, double element and three element dynamometer. Moving iron instruments principle, operation and type. Permanent magnet moving coil instruments pmmc the permanent magnet moving coil instruments are most accurate type for direct current measurements. Thus the deflection is directly proportional to the current passing through the coil in permanent magnet moving coil type pmmc instrument. A permanent magnet moving coil pmmc meter also known as a darsonval meter or galvanometer is an instrument that allows you to measure the current through a coil by observing the coils angular deflection in a uniform magnetic field. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves.
Permanent magnet moving coil instrument the permanent magnet moving coil instrument or pmmc type instrument uses two permanent magnets in order to create stationary magnetic field. Construction of pmmc instruments the constructional features of this instrument. The same situation prevails for thermal instruments. In the measurement of direct currents and voltage ii. A low shunt resistance connected in parrel with the ammeter to extent the range of current. Pmmc torque equation let us derive a general expression for torque in permanent magnet moving coil instruments or pmmc instruments. Electrodynamic or dynamometer instruments electrical. The permanent magnet moving coil instrument or pmmc type instrument uses two permanent magnets in order to create stationary magnetic field. The pmmc stands for permanent magnet moving coil and it is also called as darsonval instruments. It is one of the simple and most used instrument on board with sophisticated names.
The moving coil is wound with many turns of enameled or silk covered copper wire. For measuring several electrical parameters to analyze and keep. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required. Advantages of permanent magnet moving coil type or pmmc instruments a low power consumption b high torqueweightratio. The coil is pivoted on the jeweled bearing and thus the coil is free to rotate. Thus we can say that the value of instrumet s much greater instrumetn the value of i m as pmmc instrument of shunt is low.
Pmmcthe permanent magnet moving coil instrument is the most accurate type for d. Permanent magnet moving coil or pmmc instrument definition. Let us have a comparison between two major type of electrical measuring instruments they are moving coil mc and moving iron mi instruments. When you go for viva or interview most often question on electrical measurements is what is the difference between mi and mc type instruments. The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings. Owing to development of materials like alcomax and alnico, which have a high. Permanent magnet moving coil instrument pmmc working and. Pmmc instruments are the most accurate type for the measurement of dc current or voltage. The working principal of this instrument is same as that of the darsonval type of galvanometers,the difference being that a direct reading instrument is provide with a pointer and a scale. If you cannot read the numbers in the above image, reload the page to generate a new one. A air damping c eddy current damping b fluid damping d magnetic damping using a magnet. What is permanent magnet moving coil or pmmc instrument.
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