The moving coil is wound with many turns of enameled or silk covered copper wire. The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings. The instruments in which the measured quantity produces physical effects which deflect or displace the moving system of the instruments in known as the deflection type instrument. The function can be selected by the function switch. 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. There are two types of moving coil instruments namely, permanent magnet moving coil. Extension of instrument range is possible with the help of shunt and series resistances. The other popular nomenclature of this instrument is dalvanometer and. As seen, the main distinguishing factor between the two types of instruments is the presence of an electron device in the electronic instruments. Instruments essentials of indicating instruments permanent magnet moving coil pmmc types of instruments. In other words, the instrument in which the deflection provides the basis for measuring the electrical quantity is known as the deflection type instrument. Electrodynamometer type instruments download as powerpoint presentation.
Permanent magnet moving coil pmmc type meter is used in multimeter. In this method precision grade indicating instruments are used as reference standard. 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. Pdf electrical measurements pdf notes em notes 2019 sw. Voltage is display on the scale with the help of pointer. Galvanometer is essentially a pmmc instrument designed to be sensitive to extremely current levels. Pmmc, mi, emmc or edm, esv instruments measurement. Operating torque deflection torque a operating torque is required to move the moving system of the instrument. In the measurement of direct currents and voltage ii. An ammeter has a pmmc instrument with a coil resistance of r m. Torque following are the essentials of indicating instruments. Why eddy current damping is not preferred in electrodynamometer type instruments. Pmmc type or m i type ammeter ii alternating current.
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. Can be used as ammeter or voltmeter of different ranges with the help of shunt and multiplier. The simplest galvanometer is a very sensitive instrument with the type of centerzero scale, therefore the pointer can be deflected to either right or left of the zero position. Show that pmmc type instruments will have a linear scale. 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. The construction and working of a pmmc instrument article716. So you are partially correct, pmmc meter movements cannot measure ac directly, but we can use a bridge rectifier to convert the ac into dc humpy dc ac waveform converted to all positive halves and we can. Permanent magnet moving coil instrument pmmc working. Principle of operation of dc motors, types of dc motors. The input impedance of the permanent magnet moving coil pmmc voltmeter is infinite. Maharashtra state board of technical education autonomous isoiec2700120 certified. Classification, absolute and secondary instruments, indicating instruments, control, balancing and damping, constructional details, characteristics, errors in measurement, ammeters, voltmeters. They are very sensitive and maintain a high degree of accuracy over long periods.
Thus the net effect is to increase the deflection of instrument by 0. Rectifier type instrument construction principle of. A permanent magnet moving coil pmmc is one such instrument which is popularly used onboard and has several applications. Electrodynamic type instruments are similar to the pmmctype elements except that the an electrodynamometer for use as a wattmeter is shown in fig. Technical interview questions for instrumentation engineering. In electrical measurement systems, there are two main principles that are employed for the purpose of instrumentation. The basic pmmc instrument is sensitive to the temperature.
Pmmc type, moving iron type, dynamometer type, hot wire type, electrostatic type induction type. Maharashtra state board of technical education autonomous. A dynamometertype ammeter b dynamometertype wattmeter c movingiron voltmeter d movingiron wattmeter ans. Permanentmagnet moving coil type pmmc instruments home permanentmagnet moving coil type pmmc instruments a permanentmagnet moving coiltype instrument works on the principle that when a currentcarrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field. The action of these instruments is based on the motoring principle. Applications of pmmc permanent magnet moving coil instruments are acknowledged to be the best type of all d. There are some disadvantages of pmmc permanent magnet moving coil are given below, it has comparatively high cost. The electronic instruments always include in their makeup some active electron device such as vacuum tube, semiconductor diode or an integrated circuit etc.
Pmmc instruments, galvanometer, dc voltmeter, dc ammeter. The current and voltages are held constant during the test. What are deflection and null type instruments trivology. Prove that gravity control instruments will have a nonlinear scale. Rectifier type instrument measures the alternating voltage and current with the help of rectifying elements and permanent magnet moving coil type of instruments. Basic electrical engineering pdf notes bee pdf notes. Now one question must arises in our mind why we use rectifier type of instruments. Pmmc temperature compensation instrumentation tools. But the only thing we can measure directly with a pmmc meter movement is a current up to 1ma. Principle of operation of dc machines, types of dc generators. Advantages and disadvantages of pmmc permanent magnet.
Magnetic type moving coil mc instrumentspmmconly dc measurements moving iron mi instrumentsmostly ac measurements, but with accurate design dc measurements too induction type instruments only ac electrodynamic dynamometer type instruments hot wire type instruments more suited for ac rectifier type. M i type, induction type, dynamometer type ammeter 1 mark 1 mark 1 e write any two causes of errors in c. Instruments, types of instruments, permanent magnet moving. Permanent magnet moving coil mechanism pmmc mathematical. The coils move freely in the field of a permanent magnet. The permanent magnet moving coil instruments are most accurate type for direct current measurements. Large current can be measured using low current rated ammeter by using a shunt. Construction of pmmc instruments the constructional features of this instrument are shown in fig. Different instruments are used onboard for measuring several electrical parameters to analyze and keep these machines in proper running condition.
Permanent magnet moving coil instruments pmmc the permanent magnet moving coil instruments are most accurate type for direct current measurements. Maharashtra state board of technical education autonomous isoiec2700120 certified summer 2018 examinations subject code. Instruments essentials of indicating instruments permanent magnet moving coil pmmc types. These types of instruments are only used for measuring the dc quan. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. The pmmc instrument is essentially a lowlevel dc ammeter however, with the. These indicating instruments are connected in the circuit of meter under test. Mean value theorems, theorems of integral calculus, partial derivatives, maxima and minima, multiple integrals, fourier series, vector identities, line, surface and volume. Absolute instruments gives the quantity to be measured in term of physical constant of the instrument.
Scale of pmmc instrument is linear as i for a given deflection, tctd or k blind or i k blnd 8. Losses and torque equation, losses and efficiency calculation in d. However the primary function of rectifier type of instruments work as voltmeter. Galvanometer meter is an instrument which detect very small value current, based on it operating principle it can be of many types like moving iron typegenerally. 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.
Matrix algebra, systems of linear equations, eigen values and eigen. A low shunt resistance connected in parrel with the ammeter to extent the range of current. A moving coil galvanometer is an instrument used for detection and. Pmmc instrument resulting deflection of the coil is indicated by a pointer that moves over a calibrated scale.
Pmmc for concept, refer to measurement knotes, electromechanical instruments point to remember. These are known as the deflection principle and the null principle and consequently the instruments constructed are known as deflection and null type instruments respectively. Pmmc instruments uniform scale, no hysteresis loss, accurate, high efficiency. Why knife edge type pointer is usually preferred in portable instruments. Classification deflecting, control and damping torques ammeters and voltmeters pmmc, moving iron type instruments expression for the deflecting torque and control torque errors and compensations, extension of range using shunts and series resistance. The pmmc instrument has light weight so that it can deflect rapidly. Since at the end of measurement they consume almost negligible power. Pmmcthe permanent magnet moving coil instrument is the most accurate type for d. The magnetic field strength and spring tension decrease with increase in temperature. The pmmc has efficient damping characteristics and is not affected by stray magnetic field.