Permanent magnet moving coil instruments pmmc the permanent magnet moving coil instruments are most accurate type for direct current measurements. Ferromagnetic materials magnetize in the direction of a magnetic field, therefore, if they are brought near one end of a current carrying coil. Permanent magnet moving coil or pmmc instrument definition. This instrument is one of the most primitive forms of measuring and relay instrument. Moving iron instruments attraction type and repulsion type complete concept duration. Moving coil instruments permanent magnet moving coil mechanism. In addition to above instruments, we have wattmeters, power factor meters, frequency meters etc. Another type of instrument that is usually classed with the attractive types of instrument is shown in fig. The difference between the pmmc and dynamometer type instrument is that the permanent magnet is replaced by an electromagnet. Both moving coil and moving iron instruments are used measure the voltage and current of circuits. Pdf classification of moving iron instruments atul.
Attraction type moving iron instruments are commonly used and therefore, we will focus on it. These instruments are based on the principle that when an unmagnetised soft iron piece is placed in the magnetic field of a coil, then the piece is attracted towards the coil. It is clear that whatever may be the direction of the current in the coil of the instrument, the iron vanes are so magnetized that there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments. Class 3 for bimetallic meter movements and class 1. The moving system of the instrument is attached to the soft iron piece and the operating current is passed through a coil placed near it. These instruments use soft iron which is a ferromagnetic material. It operates on the principle that the torque is exerted on the moving coil placed in the field of the permanent magnet. Nulltype and deflectiontype instruments deflection type.
Attraction or single iron type figure 2 the brief description of different components of a moving iron instrument is given below. The force of this attraction depends upon the strength said magnetic. Rotation is opposed by a hairspring that produces the restoring torque. The action of these instruments is based on the motoring principle.
The movement is light, quick acting, but extremely robust. Construction of attraction type moving iron instruments points. Moving iron type instruments are of mainly two types. But in actual instrument rate of change of inductance with angular deflection i. The deflecting torque causes the moving system to move from its zero position. The control torque is provided by two hairsprings s1 and s2 in the same way as for a pmmc instrument, but in such instruments, springs are not used to carry any current gravity control can also be used for vertically mounted panel type mi meters. In pmmc meter or darsonval meter or galvanometer all are the same instrument, a coil of fine wire is suspended in a magnetic field produced by permanent. To understand the basic construction of a permanent magnet moving coil. Due to square low response, the scale of the moving iron instrument is nonuniform. The most widely used movingiron instruments are switchboard instruments of classes 1. Electrostatic voltmeters electrometer type and attracted disc type extension of range of e. Repulsion type moving iron instrument about circuit. Here are a few fantastic ebooks to get important maritime information in the next couple of minutes.
I instrument the most common ammeter and voltmeter for laboratory or switch board use at power frequency are the moving iron instruments. 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 the magnetic field produced by the current in coil. The moving coil type has the pen attached to a coil carrying current and so behaves like a magnet whose strength is proportional to the current. The attraction and the repulsion are the types of the moving iron instruments. The damping torque is provided by the movement of a thin vane v in a closed sectorshaped box b, or. Moving iron instruments principle and operation repulsion type. Analog instruments classification of analog instruments. Dc pmmc instruments, extension of range by shunts and multipliers.
A permanent magnet is used to produce magnetic flux and a coil that carries the current to be measured moves in this field. 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. Coaxial vanes type in this type instrument the fixed and moving vanes are sections. A moving iron ammeter may be compensated for frequency errors by a. Moving iron instruments are generally used to measure alternating voltages and currents. To this iron a pointer is attached so that it may deflect along with the moving iron over a graduated scale. The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. Advantages and disadvantages of movingiron instruments. Which of the following instruments can be used for full scale deflection of 300o. Moving iron ammeters and voltmeters, dynamometer type watt meters, induction type energy meter. The movingiron meter was invented by austrian engineer friedrich drexler in 1884.
For these reasons this method is used in hot wire, moving coil and induction type instruments. Broadly, the analog instruments and for that matter digital instruments may be classified according to the quantity they measure. Construction of attraction type moving iron instruments the simplest form of attraction type moving iron instrument uses a solenoid and a moving oval shaped soft iron pivoted eccentrically. Movingiron instruments are generally used to measure alternating pdf u3d vulnerability voltages and currents. Working principle and construction of pmmc polytechnic hub. Moving iron instruments having scales that are nonlinear and somewhat crowded in the lower range of calibration. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. The instrument in which the moving iron is used for measuring the flow of current or voltage is known as the moving iron instrument. Before answering your question let me explain the working principle of both moving coil and moving iron instruments that will give you a basic idea.
In repulsion type movingiron instrument consists of two cylindrical soft iron vanes mounted within a fixed currentcarrying coil. One variation of moving iron instrument is repulsion type instrument. Moving iron instruments principle, operation and type, theory of. What is permanent magnet moving coil or pmmc instrument. If there were no saturation, the change of inductance with the angle of deflection i. Moving iron instruments are generally used for measuring ac voltage and currents. A feature of the moving element is that it is fitted with synthetic sapphire jewels. Basic principle operation of movingiron instruments 61. Classification, absolute and secondary instruments, indicating instruments, control, balancing and damping, constructional details, characteristics, errors in measurement, ammeters, voltmeters. Working principle of moving iron instruments electrical.
The current to be measured is passed through the coil, coil produced the magnetic field. 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. Moving iron instruments principle, operation and type. A hollow circular coil is used and one of the iron pieces is attached inside the hollow coil. In both the attraction and repulsion type movingiron instruments it is found that for a given position of the moving system, the value of the deflecting torque is proportional to the square of the current, so long as the iron is working below saturation. We attach a soft iron bar or rod to the shaft so that it can move with the movement of the moving system. Hence moving iron instruments can be used for both a. For example, an instrument meant for measurement of current is classified as an ammeter while an instrument that measures voltage is classified as a voltmeter.
In moving iron instruments, a van of soft iron forms the moving element of the system. The instrument can be constructed to measure current and voltage to an accuracy needed in most engineering works and still be cheaper as compared any other type of a. Measurements and instrumentation questions and answers pdf 26. The instrument having its deflection depends upon average value is. Moving iron instrument type iqe type iqe 72 iqe 96 front flange mm 72 x 72 96 x 96 housing mm 66 x 66 89 x 89 class 1. 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 and voltage measurements and the dynamometer type which can be used on either direct or alternating current and voltage measurements. Moving iron mi instruments generally used to measure ac current or voltage but can measure dc current and voltage without any external circuit pointer is connected to specially designed soft iron that moves according to the intensity of magnetic field acting on it the magnetic field is produced by current carrying coil. Construction of attraction type moving iron instruments. Moving iron instruments attraction and repulsion type. This is achieved by utilizing the various effects of electric current or voltage. The iron van is so situated that it can move in the magnetic field produced by a stationary coil. This method cannot be used in instruments where the introduction of a permanent magnet required for producing eddy currents will distort the existing magnetic field as in. Download practical maritime ebooks with amazing bonuses.
Moving iron instrument this instrument is one of the most primitive forms of measuring and relay instrument. Choosing a backup generator plus 3 legal house connection options transfer switch and more duration. The pmmc stands for permanent magnet moving coil and it is also called as darsonval instruments. What is the difference between movingiron and movingcoil. This type of meter responds to both direct and alternating currents as opposed to the movingcoil ammeter, which works on direct current only.
There are two general types of movingiron instruments namely. Click here to download the pdf of this topic moving iron instruments. The basic dc meter movement is known as the darsonval meter movement. In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity. One important requirement in indicating instruments is the arrangement for producing operating or deflecting torque t d when the instrument is connected in the circuit to measure the given electrical quantity.
Movingiron measuring mechanisms are also used in ratio meters. Home advantages and disadvantages of movingiron instruments. For such pure square response, moving iron instrument scale can easily be graduated uniformly. Moving iron instrument working operation electrical edition. Measuring 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. Principle of generation thermal, hydel and nuclear.
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