Saturday 17 January 2015

Errors in Ammeter, Voltmeter and Their Remedies

Errors in Ammeter, Voltmeter and Their Remedies Points : Errors in Ammeter, Voltmeter and Their Remedies
Errors in Ammeter, Voltmeter and Their Remedies
Type of Instrument
Type of measurements
1.Moving IronDirect and alternating measurements of current and voltage
2.Moving Coil
(i) Permanent Magnet type
(ii) Dynamometer type
D.C. measurements (current & voltage only)
D.C. or A.C. measurements (current voltage and power)
3.Thermal
(i) Hot wire type
(ii) Thermo-couple type
D.C. or A.C. measurements (current voltage and power)
D.C. or A.C. measurements (current & voltage)
4.Electrostatic typeD.C. or A.C. (Voltage only)
5.Induction typeA.C. measurements’ (current, voltage, power and energy) only.
6.Rectifier typeD.C. or A.C. (current & voltage)
It is obvious from the list that there are so many types of instruments used as ammeter and voltmeter. There are certain errors which occur in most types of instruments, while other errors occur only in those of a particular type.
Here we will discuss, the errors, which are common to all types.
Error Common to all Types The common errors are on account of:
(i) Friction
(ii) Change in temperature
(iii) Mechanical unbalance and variation of the electrical and mechanical properties of the material employed, with time.
These are also called the sources of errors.
(i) Friction Error: The friction of the pivot in the jewel produces a frictional torque, which opposes the deflection of moving element and as a result the pointer comes to rest a short distance from Its correct position and so there is an error in the instrument readings. This error is small for a bearing in good condition, but may become very large in case of badly worn or damaged piyot. Friction error is most serious for sensitive instruments designed for low operating torque. Remedies Such errors may be minimized by adopting a moving system of a light-construction and deflecting torque large (large torque-weight ratio and mounting the moving system on a vertical, rather than a horizontal spindle for preference between jewelled bearings). The user can minimize frictional effects by tapping gently on the case with his finger tip before taking an obsevation.
Vigorous tapping will defeat the purpose and may even injure the bearing.
(ii) Temperature Error Apart from the room temperature, this type of error is mainly due to heating of the working coils and other resistance coils connected internally in the instrument casing by the operating current. As a result there is a change of the resistance of the working coils and other resistance coils. Such a change in resistance is not so serious in case of an ammeter but it is very important in case of a voltmeter which is of right resistance and also where the resistance is desired to remain as constant as possible. Remedies For minimizing the error due to change in temperature the power loss in a voltmeter should be kept very small and arrangement for ventilation and cooling should be provided. Sometimes the error is minimized by winding the series resistance coil of material of very small temperature—coefficient of resistance and having the working coil, which is wound with copper wire of very small resistance in comparison with the series coil. Consequently the overall percentage change of the two, together connected in series will be much reduced below that of the working coil alone. Series resistance and shunt housed internally in the instrument cases should be well separated from working elements and well ventilated. (iii) Observational Error Such errors are due to miss-reading of the scale, parallax in the readings and errors of estimation. Precision of readings becomes difficult if the pointer has a broad tip. Interpolations errors may enter, particularly if the scale leadings are too large or too small. Remedies Parallax error is caused by the observer not having his line of sight on the pointer exactly at the right angle to the plane of the scale. Such an error is eliminated by providing a mirror beneath the scale and a knife-edged pointer.

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