Dynamometer type wattmeter working your electrical guide. A moving coil instrument consists basically of a permanent magnet to provide a magnetic field and a small lightweight coil is wound on a rectangular soft iron core. The basic moving coil instrument working principle is that when a current carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor. What are the types of moving coil instrument answers.
When the moving coil instrument is connected in the circuit, the operating current flows through the coil which is mounted on the spindle. Creeping in energy metre implies a slow rotation of rotor with only voltage coil excited. Moving iron instruments principle, operation and type, theory. Permanent magnet moving coil instrument pmmc about circuit. These instruments essentially consist of fine wire moving coil placed in the magnetic field produced by another fixed coil when carrying currents. The frame is carried on a spindle s mounted in jewel bearings j 1, j 2 a pointer pr is attached to the spindle so that it moves over a calibrated scale. While the voltmeters having high ranges use a moving coil together with sufficient series resistance, to limit the instrument current. Its very delicate and sometimes uses ac circuit with a rectifier. The coils are usually air cored to avoid hysteresis, eddy currents and other errors when the instrument is.
B magnetic flux density between the magnetic poles. The sensitivity of moving coil galvanometer is defined as the ratio of the change in deflection of the galvanometer to the change in the current. The strength of the magnetic field induces by the electromagnet is directly proportional to the current passes through it. The spring also serves another important function by providing the path to lead current in and out of the coil. Sep 02, 2018 the permanent magnet moving coil instruments are most accurate type for d. The force of attraction depends on the strength of the magnet field. Jan 15, 2019 let us derive a general expression for torque in permanent magnet moving coil instruments or pmmc instruments. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required.
Electrical instrumentation symbols meters and recorders. A measuring instrument in which current or voltage is determined by the couple on a small coil pivoted between the poles of a magnet with curved poles, giving a radial magnetic field. Permanent magnet moving coil instrument pmmc working. 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. The magnets are generally aged by the heat and vibration treatment. Dynamo meter type moving coil instruments consist of two coils. It is one of the simple and most used instrument on board with sophisticated names. Mar 23, 2018 the deflecting torque equation for permanent magnet moving coil or pmmc instruments is given as. May 12, 2018 permanent magnet moving coil or pmmc instruments are the most accurate type for the measurement of dc current or voltage. Moving coil instrument working principle your electrical guide. This iron tends to move inward that is from weaker magnetic field to stronger magnetic field when current flowing through the coil.
The magnetic field induces by the electromagnet whose strength depends. Moving iron instrument working operation electrical edition. Moving coil instruments permanent magnet moving coil. It works on the principle that the iron place near the magnet attracts towards it. As shown in above diagram, whatever be the direction of current through the coil, the iron piece would always be magnetized in such a way that it is pulled inwards and that is why moving iron instruments can be used for. Pmmc instrument pdf 16 mar pmmc stands for permanent magnet moving coil. Voltmeter working principle, voltage sensitivity, types. The principle, working and theory of pivoted type moving coil galvanometer is the same as suspended type moving coil galvanometer. Apr 20, 2018 in moving iron instruments, a van of soft iron forms the moving element of the system. Ban, where b is the magnetic flux density, a is the area of the.
In these wattmeters, the field produced by the currentcarrying moving coil tries to come in line with the field produced by the currentcarrying fixed coil, and a deflecting torque is exerted on the moving system. Movingcoil instrument article about movingcoil instrument. Moving iron instruments are generally used to measure alternating voltages and currents. The coil rotates in the field of a permanent magnet and turns through an angle determined by the magnetic interaction of this field and the field of the coil, and a restoring force due to the method of. Pmmc instruments pdf in pmmc meter or darsonval meter or galvanometer all are the same. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. It is already seen that the moving coil instruments can carry the maximum current of about 50 ma safely and the potential drop across the moving coil instrument is about 50 mv. Moving coil instrument working principle and operation. Moving iron instruments are rarely used in low power high resistance circuits because of. Pdf electrical measuring instruments and measurements. Moving coil instrument working principle your electrical. Moving iron instruments construction and basic principle operation in moving iron 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. Pdf eec 126 instrumentation theory farouq osuolale.
This attracts soft iron into the coil and the force of attraction depends upon the strength of the coil. It is one of the simple and most used instruments on board with sophisticated names. 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. In this article all major differences between moving iron and moving coil instruments are discussed. Permanent magnet moving coil instruments advantages. In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity. The scale of series ohmmeter is nonlinear with zero at the right and infinity at extreme left. Measurements and instrumentation questions and answers pdf 26. One of the most accurate instruments used for both ac and dc measurement is moving iron instrument.
Moving iron instruments principle, operation and type. Range extension of ammeters voltmeters your electrical. Due to square low response, the scale of the moving iron instrument is nonuniform. The other popular nomenclature of this instrument is dalvanometer and galvanometer. We know that in moving coil instruments the deflecting torque is given by the expression.
Since the coil is placed in the strong field of permanent magnets, a force is exerted on the current carrying conductors of the coil which produces deflecting torque. Working principle and construction of pmmc polytechnic hub. May 19, 2020 pmmc stands for permanent magnet moving coil. A permanent magnet is used to produce magnetic flux and a coil that carries the current to be measured moves in this field. Permanent magnet moving coil instrument pmmc working and.
The basic construction of this instrument is shown in figure 2. Its costly as compared to moving coil iron instruments. One movingcoil movement may be used in a circuit so that by connecting the source of current, voltage or resistance to the. However, in practice, heavy currents and voltages are required to be measured. The coil c has a number of turns of thin insulated wires wound on a rectangular aluminium former f. The internal impedance are 30,000 ohms and 20,000 ohms. The instrument in which the coil rotates between the magnetic field of the permanent magnet for measuring the dc such type of instrument is known as the moving. The range offers iron vane and moving coil ac and dc ammeters and voltmeters. One coil is fixed and the other coil rotates around it. Moving coil meter an overview sciencedirect topics. A moving coil instrument consists basically of a permanent magnet to provide a magnetic field. The instrument consists of two high intensity, high coercive force magnets or a big ushape magnet based on design.
Working principle of moving iron instruments electrical. What are the functions of moving coil instrument answers. The coil is wound with many turns of copper wire and is placed on rectangular aluminium which is pivoted on jeweled bearings. Moving iron instrument multiple choice questions and answers. Mcq on measuring instruments 1 how engineering works. Permanent magnet moving coil type pmmc instrumentworking. The instrument in which the moving iron is used for measuring the flow of current or voltage is known as the moving iron instrument. What is the difference between movingiron and movingcoil. The iron van is so situated that it can move in the magnetic field produced by a stationary coil.
Oct 21, 2016 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. Permanent magnet moving coil mechanism pmmc mathematical. Since the coil winding in pmmc meter is small and light, they can carry only small. In moving iron 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. 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 whole of the moving system is made as light in weight as possible to keep the friction at. When the movement is used in an instrument calibrated to read current, most of the current flows through a. Any instrument in which current is sent through one or more coils suspended or pivoted in a magnetic field, and the motion of the coils is used to measure. If i is the current flowing in the moving coil in the direction shown, forces f, f will act on the two sides of this coil, the direction of force being determined by flemings lefthand rule and is in the direction for this system as shown in fig. A moving iron ammeter may be compensated for frequency errors by a. Moving coil instruments 39018 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. The basic principle can also be stated that when a field resultant magnetic field produced by the movable current carrying coil tries to come in line with the main field. The moving iron instruments use the stationary coil of copper or aluminium wire which acts as an electromagnet when an electric current passes through it. When a current flows through the coil it turns against a return spring. To understand the basic construction of moving iron instrument and its principle operation. 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. Impetus gurukul physics vivek phalke 65,510 views 59.
Seventh lecture moving coil instruments 8 b series ohmmeter. It is a horseshoe shaped permanent magnet with soft iron. 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. To study the advantage and limitation of pmmc instruments. A given movingcoil movement can be used in an instrument designed to indicate either current or voltage. The moving coil can freely moves between the two permanent magnets as shown in the figure given below. Hence moving iron instruments can be used for both a. Electrodynamic or dynamometer instruments electrical. The interaction of the two fields produces a deflecting torque. The general theory of moving coil instruments may be dealt with considering a rectangular coil of turns, free to rotate about a vertical axis. The function of the moving coil is to deflect the instruments needle across a calibrated scale to give the meter operator an accurate reading of what ever he is testing. Both moving coil and moving iron instruments are used measure the voltage and current of circuits. The action of these instruments is based on the motoring principle. The moving coil meter is an analogue data presentation element involving a pointer moving across a fixed scale.
A moving coil instrument consists basically a permanent magnet to provide a magnetic field and a small lightweight coil is wound on a cylindrical soft iron core that is free to rotate around its vertical axis. Hence it is very easy to measure quantities from these instruments. Moving coil instruments electrical technology youtube. Whereas in moving coil instrument the conductor coil is. The permanent magnet moving coil instruments are most accurate type for direct current measurements. When a current is passed through the coil windings, a torque is developed on the coil by the interaction of the magnetic field and the. With no current flowing, the coil generates no magnetic field and the pointer stays at zero. T d nbldi where n is number of turns, b is magnetic flux density in air gap, l is the length of moving coil, d is the width of the moving coil.
C supply as the reversal of current produces a reversal of torque on the coil. A permanent magnet moving coil pmmc is one such instrument which is popularly used onboard and has several applications. Moving coil instruments permanent magnet moving coil mechanism. The moving iron fixed to the spindle is kept near the hollow fixed coil fig. The pmmc stands for permanent magnet moving coil and it is also called as darsonval instruments. Advantages of permanent magnet moving coil instruments 1the scale is uniformly divided as the current is directly proportional to deflection of the pointer. Jan 03, 2020 the moving coil instrument can only be used on d. Rx is the unknown resistor to be measured, r2 is variable adjusted resistance so that the pointer read zero at short circuit test. 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.
Which of the following meter is an integrating type instrument. Whereas in moving coil instrument the conductor coil is used for dc measurement. With the probes unconnected, the meter is like a circuit broken by an open switch. Permanent magnet moving coil instruments pmmc the permanent magnet moving coil instruments are most accurate type for direct current measurements. The spring generally acts as control system for pmmc instruments. The range offers iron vane and moving coil ac and dc ammeters and voltmeters, elapsed time and frequency meters. The action of the permanent magnet moving coil pmmc instrument is based on the motoring principle when a currentcarrying coil is placed in the magnetic field produced by the permanent magnet, the coil experiences a force and moves. Measuring instrument moving coil with rectifier incorporated ionization chamber thermoluminescence detectors scintillator telemetry receiver symbols o description test point resonator cavity meter quotients control panel generic symbol telemetry transmitter electronic symbology o. Let us have a comparison between two major type of electrical measuring instruments they are moving coil mc and moving iron mi instruments.