Page 306 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 3 & 4

Errors and corrrection in energy meter measurement

Objectives: At the end of this lesson you shall be able to:
• explain the errors caused by the driving system and the braking system in energy meters
• explain the different adjustments provided for correcting the errors in energy meters.

Errors caused by the driving system
Incorrect magnitude of fluxes: This may be due to abnormal values of current or voltage. The shunt magnet
flux may be in error due to changes in resistance of coil or due to abnormal frequencies.
Incorrect phase angles: There may not be a proper relationship between the various phasors. This may be due
to improper lag adjustment, abnormal frequencies, change in resistance with temperature etc.

Lack of symmetry in magnetic circuit: If the magnetic circuit is not symmetrical, a driving torque is produced
which makes the meter creep.
Error caused by the braking system

They are:
• changes in the strength of the brake magnet
• changes in the disc resistance
• self-braking effect of series magnet flux

• abnormal friction of the moving parts.
Adjustments are provided for correcting the errors in the energy meters so that they read correctly and their errors
are within acceptable limits.
Preliminary light load adjustment: The rated voltage is applied to the potential coil with no current through
the current coil and the light load device is adjusted until the disc just fails to start. The electromagnet is slightly
adjusted to make the holes in the disc to take a position in between the poles of the electromagnets.

Full load unity power factor adjustment: The pressure coil is connected across the rated supply voltage and
the rated full load current at unity power factor is passed through the current coils. The position of the brake
magnet is adjusted to vary the braking torque so that the meter revolves at the correct speed within the required
limits of error.

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53