Electrical Engineering ⇒ Topic : Errors in Induction Watthour Meters
Errors in Induction Watthour Meters
The users of electrical energy are charged according to the readings of the energy meters installed in their premises. It is, therefore, very important that construction and design of energy meters should be such as to ensure long-time accuracy i.e., they should give correct readings over a period of several years under normal use conditions. Some of the common errors in energy meters and their remedial measures are discussed below
(1) Phase error. The meter will read correctly only if the shunt magnet flux lags behind the supply voltage by exactly 90°. Since the shunt magnet coil has some resistance and is not completely reactive, the shunt magnet flux does not lag the supply voltage by exactly 90°. The result is that the meter will not read correctly at all power factors.
Adjustment. The flux in the shunt magnet can be made to lag behind the supply voltage by exactly 90° by adjusting the position of the shading coil placed around the lower part of the central limb of the shunt magnet. A current is induced in the shading coil by the shunt magnet flux and causes a further displacement of the flux. By moving the shading coil up or down the limb, the displacement between shunt magnet flux and the supply voltage can be adjusted to 90°. This adjustment is known as lag adjustment or power factor adjustment.
(2) Speed error. Sometimes the speed of the disc of the meter is either fast or slow, resulting in the wrong recording of energy consumption.
Adjustment. The speed of the disc of the energy meter can be adjusted to the desired value by changing the position of the brake magnet. If the brake magnet is moved towards the centre of the spindle, the braking torque is reduced and the disc speed is *increased. Reverse would happen should the brake magnet be moved away from the centre of the spindle.
(3) Frictional error. Frictional forces at the rotor bearings and in the counting mechanism considerably to the braking torque. Since friction torque is not proportional to the speed but is roughly constant, it can cause considerable error in meter reading.
Adjustment. In order to compensate for this error, it is necessary to provide a constant addition to the driving torque that is equal and opposite to the friction torque. This is produced by means of two adjustable short-circuited loops placed in the leakage gaps of the shunt magnet. These loops upset the symmetry of the leakage flux and produce a small torque to oppose the friction torque. This adjustment is known as light-load adjustment. The loops are adjusted so that when no current is passing through the current coil (i.e., exciting coil of the series magnet), the torque produced is just sufficient to overcome the friction in the system, without actually rotating the disc.
(4) Creeping. Sometimes the disc of the meter makes slow but continuous rotation at no load i.e., when potential coil is excited but with no current flowing in the load. This is called creeping.This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields etc.
Adjustment. In order to prevent this creeping, two diametrically opposite holes are drilled in the disc. 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 poles of the shunt magnet.
(5) Temperature error. Since watthour meters are frequently required to operate in outdoor installations and are subject to extreme temperatures, the effects of temperature and their compensation are very important. The resistance of the disc, of the potential coil and characteristics of magnetic circuit and the strength of brake 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 shunt magnet flux will increase with an increase in voltage. The driving torque is proportional to the first power of flux whereas braking torque is proportional to the square of the flux. Therefore, if the supply voltage is higher than the normal value, the braking torque will increase much more than the driving torque and vice-versa. The result is that the meter has the tendency to run slow at higher than normal voltages and fast at reduced voltages. However, the effect is small for most of the meters and is not more than 0.2 % to 0.3 % for a voltage change of 10 % from the rated value. The small error due to voltage variations can be eliminated by the proper design of the magnetic circuit of the shunt magnet
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