Error in Induction Type Energy meters:
Energy meters should give correct readings over a period of several years under normal use conditions. Some of the common errors in energy meter and their remedial measures are discussed below.
1. Phase Error: It is necessary that the energy meter should give correct reading on all power factors, which is only possible when the field setup by shunt magnet lags behind the applied voltage by 90 degree. But the flux due to shunt magnet does not lag behind the applied voltage exactly by 90 degree because of winding resistance and iron losses.
Adjustment: The flux in the shunt magnet can be made to lag behind the supply voltage by exactly 90 degree by adjusting the position of shading band (or shading ring or shading coil) placed round the lower part of the control limb of the shunt magnet.
This adjustment is known as lag adjustment or power factor adjustment (or power factor compensator).
2. Speed Error: Sometimes the speed of the meter is either fast or slow, resulting in the wrong recording of energy consumption.
Adjustment: An error in the speed of the meter when tested on non-inductive load can be eliminated by correctly adjusting the position of the brake magnet.
Movement of the brake magnet in the direction of the spindle will reduce the braking torque and vice-versa.
3. Friction Compensation (or) Friction Error: Frictional forces at the rotor bearings and in the counting (or register) mechanism cause noticeable error especially at light loads. At light loads, the torque due to friction adds considerably to the braking torque on the disc rotor. Since, friction torque is not proportional to the speed but is roughly constant it can cause considerable error in meter reading.
Adjustment: This error can be reduced to an unimportant level by making the ration of the shunt magnet flux Φ2 and series magnet flux Φ1 large with the help of two shading rings (or shading bonds). These bonds embrace the flux contained in the two outer limb of the shunt magnet and thus eddy currents are induced in them which cause a phase displacement between the enclosed flux and the main gap flux. As a result, a small driving torque is exerted on the disc rotor, this torque being adjusted by variation of the positions of these bands to compensate for friction in the instrument. Correctness of friction compensation is achieved by running the meter at high load of about 8 to 10% of full load when the disc should rotate correctly. Over compensation leads to creep. This adjustment is known as light load adjustment.
4. Creeping: Sometimes the disc of the energy meter makes slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load. This is called creeping. This error may be caused due to over compensation for friction, excessive supply voltage, vibrations, stray magnetic fields etc.
Adjustment: In order to prevent this creeping on no load, two holes or slots are drilled in the disc on opposite sides of the spindle. This causes sufficient distortion of the field. The result is that the disc tends to remain stationary when one of the holes comes under one of the shunt magnet.
5. Temperature Error: The error due to variation in temperature are very small, because the various effects produced tend to neutralize one another.
The resistance of the disc of the potential coil and characteristics of magnetic circuit and the strength of break magnet are affected by the changes in temperature. Therefore, great care is exercised in the design of the meter to eliminate the errors due to temperature variations.
6. Frequency Variations: The meter is designed to give minimum error at a particular frequency (generally 50 Hz). If the supply frequency changes, the reactance of the coils also changes, resulting in a small error. Fortunately, this is not of much significance because commercial frequencies are held within close limits.
7.Voltage Variations: The error due to variation voltage is very small (usually 0.2% to 0.3%). This can be eliminated by the proper design of the magnetic circuit of the shunt magnet.
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