Calibration of Energy Meter - Theory, Procedure & Precautions

Energy meter calibration is a process required to determine and reduce the error when the energy is measured. The errors in the energy meter can be caused by different sources like voltage transformers, current transformers errors due to phase angle, crystal oscillators, etc.

Theory for Energy Meter Calibration :

Energy meters have specified characteristic constants, which give information about the number of revolutions of the disc and the measured energy in joules. These characteristic constants are given by the manufacturer. The experimental setup of the energy meter calibration is shown in the figure below.

Before carrying out the calibration process, different adjustments such as load adjustments, lag adjustments, creep adjustments, etc., are done on the energy meters.

Generally, the number of revolutions is usually high which cannot be taken into measurement in the laboratories. Thus consider the number of revolutions (m) in the disc as 10 for the energy of n joules by using the characteristic constant. The energy (E) can be computed from 'm' is given as,

The meter possesses no error if the energy calibrated for 10 revolutions should be equal to the energy taken by the load for the same amount of revolutions and time. Energy taken by the load or true energy is denoted as E_T. The loads applied to the energy meter are varied and the time taken for the 10 revolutions is measured using a stopwatch. The parameters such as voltage, current are observed from the circuit and are tabulated as given below,

S.No.	Voltage (V)	Current (A)	Power (p) watts	Energy E1 = PTJ	% Error

Readings observed from the test

For the specified revolutions (i.e., 10) the energy E is constant whereas energy consumed by load E_T gets varied and is determined theoretically. Therefore, under various load, the percentage error is calculated as,

Calibration Curve :

Calibration of energy meter can be obtained graphically by plotting the percentage error against load current I. This graphical representation of the calibration process is called a calibration error. At the initial stage, the load current I is zero, and hence no % error because the energies E and E_T are zero. The calibration curve is shown in the figure below.

The % error obtained can be positive and negative. The limits of the load current error are easily determined by observing the calibration curve. If the limit is not under desired range then the error can be made to a minimal value by applying different adjustments such as lag, friction, creep adjustments.

Procedure for Energy Meter Calibration :

• Make connections as per the circuit diagram.
• Supply rated voltage to the meter which to initially at no-load.
• Check for the connections such that the current coil is in series with the load and the pressure coil is shunted across the voltage supply.
• Note down the voltage, current, and time for the specified revolution of the disc.
• Calibrate the % error theoretically by using the formula.

Precautions for Energy Meter Calibration :

Precautions to be taken while calibrating the energy meter are,

• The connections made to the meter should not be loose.
• There should not be any physical contact between the circuit terminals and the observer.
• The meter should be connected to the load at least for 15 minutes before taking the readings of the meter. This eliminates the friction and temperature error.
• The readings should be noted down carefully.
• For accurate and precise errors, the average or mean error should be calibrated by taking more readings.