Generally, voltage and current are measured using voltmeter and ammeter. There are two classes of these meters. They are:
1.Analog meter (Voltmeters and ammeters)
Ammeters and voltmeters, except electrostatic voltmeters, operate on the same principle. In case of analog or pointer indicating ammeter and voltmeter, there are two classes of instruments depending upon the operations:
The permanent magnet moving coil Instrument or PMMC uses two permanent magnets in order to create the stationary magnetic field. These types of instruments are only used for measuring the dc quantities as if we apply ac current to these type of instruments the direction of current will be reversed during the negative half cycle and hence the direction of torque will also be reversed which gives an average value of torque zero. The pointer will not deflect due to high frequency from its mean position showing zero reading. However, it can measure the direct current very accurately.
let us move towards the constructions of permanent magnet moving coil Instrument. We will see the construction of these types of instruments in five parts and they are described below:
Stationary part or magnet system: In the present time we use magnets of high field intensities, high coercive force instead of using a U-shaped permanent magnet having soft iron pole pieces. The magnets which we are using nowadays are made up of materials like alcomax and alnico which provide high field strength.
There are three main types of errors:
Instead of various advantages, the permanent magnet moving coil instruments or PMMC Instrument possess few disadvantages.
Moving-iron instruments are generally used to measure alternating voltages and currents. In moving iron instruments, the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in the coil.
There are two general types of moving iron instruments namely:
The brief description of different components of a moving-iron instrument is given below:
The deflecting torque in any moving-iron instrument is due to forces on a small piece of magnetically ‘soft’ iron that are magnetized by a coil carrying the operating current.In repulsion type moving–iron instrument consists of two cylindrical soft iron vanes mounted within a fixed current-carrying coil.
One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it the pointer shaft. Two irons lie in the magnetic field produced by the coil that consists of only a few turns if the instrument is an ammeter or of many turns if the instrument is a voltmeter.
Current in the coil induces both vanes to become magnetized and repulsion between the similarly magnetized vanes produces a proportional rotation. The deflecting torque is proportional to the square of the current in the coil, making the instrument reading is a true ‘RMS’ quantity Rotation is opposed by a hairspring that produces the restoring torque. Only the fixed coil carries load current, and it is constructed so as to withstand high transient current.
Moving iron instruments having scales that are nonlinear and somewhat crowded in the lower range of calibration.
1.Douglas Hubbard.(2007).How to Measure Anything.Wiley
3.Stevens, S.S. (1946)On the theory of scales and measurement. Science
The permanent magnet moving coil Instrument or PMMC uses two permanent magnets in order to create a stationary magnetic field to measure the dc voltage and currents.
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