Voltmeter Caliberation

 The main objective of this practical is to calibrate the volt meter. By that we could see some errors exist in the volt meter. We used a potentiometer to get different values of meter readings. This potentiometer had to be standardized using a standard cell. This is done by letting this potentiometer to read directly from the standard cell and correcting it to room temperature.

For measuring voltage, current, watts each instrument movement has three basic components:

            1. Some mechanism for producing torque which depends on the quantity to be 

                        measured.

            2. A restoring spring which produces an opposing torque, dependent on the amount of

                        deflection.

            3. A damping system which prevents excessive overshoot and swinging of the pointer.

                        Instruments with too much damping require a long time for the pointer to reach a

                        new reading after a change in the measured quantity.

Let’s consider some systems which are used in moving coil instruments.

Moving System

            Mostly, rectangular coil is used. The ratio of length to breadth of the former is kept between 1.3 and 1.5. The thickness of the coil is usually kept narrow since it will be easy to get it into place over the iron without serious distortion. Formers are universally made of aluminium or copper. But aluminium is better than the other in order to reduce inertia and weight. Formers should be pressed up from a sheet metal without joints and the long sides should be curved to a radius to make the conductors concentric with the core and hence to make the gap minimum and uniform. The former is lightly insulated and the coil is wound over this as evenly as possible.

            The moving system is pivoted on pivoted on jewelled bearings. In sensitive and delicate instruments a very sharp spindle supported in a small jewelled cup. In portable instruments the spindle is made flatter on the end and jewelled cup is arranged, then the contact area is large to absorb shock. In some instruments circular coil with spherical core of soft iron is used instead of a rectangular coil. The pivot is located just above the centre of gravity. The current enters through the single control spring and passes out by alignment at the bottom of the coil.

Magnetic System:

            Special alloys of steel are used for the construction of instrument magnet. Tungsten steel was famous for a long time in past. Sometime, cobalt steel has been used to reduce the weight and space. The final adjustment of the calibration of an instrument is usually done by adjusting the series resistance, but sometimes a magnetic shunt is used for this purpose.

Control System:

            In the permanent magnet moving coil instrument the controlling torque is mostly supplied by two phosphor-bronze hair springs either helical or spiral .As the springs also function as current carrying capacity imposes a limitation on the current passing through the coil.

Damping System:

            The damping used in this type of instrument is mostly of eddy current type. The eddy current is induced in the metal former of the coil. Generally a resistance is connected across the coil to provide the necessary damping. The bearing friction also provides damping effect. The relative damping used for dc instruments is not less than 0.2 to 0.3.

There are basically three types of moving coil meters.

                                    1. Magnetic-vane attraction type

                                    2. Dynamometer type

                                    3. Permanent magnet type

Magnetic-vane attraction type:

                This instrument uses a soft iron plunger projection part away into a stationary field coil. Current in the field coil produces a magnetic force which pulls the plunger further into the coil. The instantaneous force is proportional to the square of the current the coil, hence the average torque turning the movement is proportional to the average or mean of the squares of the coil current, i.e. the r.m.s value. This force is independent of current direction, allowing the instrument to be used for either D.C or A.C measurements.

                In this simple solenoid and plunger instrument, the attraction force is least when the plunger is just entering the coil and increases rapidly as more of the soft iron vane enters the coil. This result in crowding of numbers at the lower end of the scale and a great expansion of the upper range. This moment is used in competitive types of low-cost meters.

Dynamometer type:

                In dynamometer type instruments two fixed coils replace the permanent magnet. These coils carry the current to be measured and they are either series or parallel with the moving coil. Coils in this type of meters are air cored. The use of iron is avoided because iron introduces hysterics, eddy currents and other errors when this instrument is used in an AC environment. Therefore unlike the previous type this type of meter can be used to measure both AC and DC quantities. If it was used as an ammeter it will measure the mean square value or usually calibrated to read the r.m.s value.

                The torque of the instrument is dependent upon the strengths of the magnetic fields of both fixed and moving coils. Two hairsprings are used for the control and as leads to the moving coil. Damping is often by air piston or enclosed vane although in some cases eddy current damping buy an aluminum disc rotating in a permanent magnet field is used.

Permanent magnet type:

                As the name implies in this types of meters there’s a moving coil. This moving part of the meter is a coil wound on an aluminum frame, which is free to rotate around a cylindrical soft iron core. The moving coil is situated in the magnetic field produced by a permanent magnet. The soft iron core ensures that the magnetic field is uniformly distributed. This soft iron core is fixed between the two permanent magnet by a non-magnetic material. The moving coil can be supported either on a spindle which is pivoted in bearings. The current enters the moving coil from the positive end via a spiral hairspring. It is this hairspring, which provides the controlling force for the instrument.  When the current flows in the coil the reaction between each current carrying conductor and the magnetic field produces a mechanical force on the conductor. This is the deflection force of the meter.

                This force causes the pointer to be deflected and as it does so the movement is opposed by the hairspring, which is used to carry current into the meter. The more the pointer deflects the greater the controlling force produce by the hairspring. Unless the moving system is damped the pointer will over shoot the correct position and after a while it would swing back towards the correct position. Without damping the oscillation about the correct position continue for some time. However if the movement is correctly damped the pointer has an initial overshoot of a few percent and then very quickly settles to its correct indication.  Damping is obtained by extracting energy from the moving system. This is done as follows. In the moving coil meter the coil is wound on an aluminum frame and when the frame moves in the magnetic field of the permanent magnet a current known as eddy current is induced in the aluminum frame. This causes power to be consumed in the resistance of the coil frame and energy associated with this damps the movement of the meter. This type of moving coil meter is always used to measure DC circuits and gives the mean value.

                There are several advantages of moving coil meters. They are (a) Uniformity of the scale and the possibility of a very long scale. (b)The possibility of a single instrument being sued with shunts or series resistors to cover a large range of current or voltage. (c) Their power

consumption is very low compared to other type of meters. (d) Perfect damping simply afforded by eddy currents induced in the metal frame of the moving coil.

Errors:

                Errors may appear from erratic pivot friction resulting from worn bearings. Even if the pivot and bearings are in new condition, errors may arise from this source in some instruments if they are used in a physical orientation for which they were not designed. The springs that provide the restoring torque may undergo change with age and use. The shunt of an ammeter or the series resistance of a voltmeter might shift from its correct value because of abuse, such as extreme overload. The magnetic field in the air gap could become permanently changed as a result of having exposed the meter to an intense external magnetic field at sometime in its past. Other magnetic effects may also cause error; such as use of an unwanted instrument designed for steel panel installation. Corrosion may cause ill effects on delicate metal parts such as coil wire and springs.

                A voltmeter should have infinite resistance and an ammeter should have zero resistance. Because of this these meters are connected in circuits in parallel and series respectively. Since the voltmeter is connected across the voltage to be measured therefore it should have a high resistance so it would take only a small current. Ammeter is connected in series therefore it should have zero resistance so it doesn’t alter the current considerably.

                There are several errors, which are common to most type of meters including ammeters and voltmeters. The most of these errors is friction and temperature. To reduce the effect of friction torque and consequently the error produced by it, the weight of the moving system must be made as small as possible compared with the operating forces. The ratio of torque to weight must be large. A vertical spindle generally to be preferred to a horizontal one from the point of view of a small friction torque. The most serious error produced by the heat generated in the instrument or by changes in room temperature is that due to a change in the resistance of the working coil. Such a change of resistance is of little importance in ammeters but in voltmeters in which the working current should be directly proportional to the applied voltage it is essential that the resistance of the instrument shall remain as nearly constant as possible.

                Power loss in the instrument should be small and resistance coils, which are likely to produce appreciable heating, should be mounted if possible in such a position that they are well ventilated. To eliminate temperature errors the working coil is wound with copper wire and is of comparatively low resistance. A high resistance of material whose temperature coefficient is small is connected in series with the coil so that although the resistance of the coil may change considerably the change in total resistance is small. The values obtained are not exactly the same as the meter values. This is because of the errors in the meter. The practical errors also can have a effect on the values such as loose connections, heating of equipments, resistances in the wires etc. Also the standard cell which was used might not have been fully charged and the potentiometer might not be accurate in the room temperature even.

REFERENCES:

                1. Alternating current fundamentals by John .R.Duff

                2. Electrical Measurements Fundamentals by Martin.U. Reissland

                3. Electrical Measurement Analysis by Ernest Frank