Single Phase Induction Type Energy Meter Errors

Single Phase Induction Type Energy Meter Errors Points : Single Phase Induction Type Energy Meter Errors 1. Energy Meter Errors In addition to errors due to incorrect settings of the main adjustments, there are errors in meter registrations due to other causes. The more important of these are described briefly below, together with some compensations which are frequently incorporated in meters to minimize the errors. Phase and Speed Errors It is necessary that the energy meter should give correct reading on all power factors, which is only possible when the field set up by the shunt magnet lags behind the applied voltage by exactly 90°. Ordinarily the flux due to shunt magnet does not lag behind the applied voltage by 90° because of winding resistance and iron losses. The flux due to shunt magnet is made to large behind the applied voltage by 90° with the help of copper shading band provided on the central limb. An error due to incorrect adjustment of shading band will be evident when the meter is tested on a load of power factor less than unity. Elimination of Errors An error on the fast side under these conditions can be eliminated by bringing the shading band nearer to the disc and vice-versa. An error in the speed of the meter when tested on non-inductive lead can be eliminate by adjustment of the position of the brake magnet. Movement of the brake magnet in the direction of he spindle will reduce the braking torque and vie-versa. 2. Friction Compensation and Creeping Error Two shading bands embrace the flux contained in the two outer limbs of the shunt electromagnet, 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, this torque being adjusted, by variation of the position of these bands, to compensate for friction torque in the instrument. In some energy metes, the disc continues rotating when the potential coils are excited but with no load current flowing. This defect is known as creeping. Elimination of error This error is prevented by cutting two holes or slots in the disc on opposite sides of the spindle. The disc tends to remain stationary when one of the holes comes under done of the poles of the shunt magnet. In some cases, a small piece of iron wire is attached to the edge of the disc. The force of attraction of the brake magnet upon this wire is sufficient to prevent continuous rotation of the disc under no load conditions. 3. Temperature and Frequency Errors The errors due to variation in temperature are very small. Since the variations effects due to change in temperature tend to neutralize each other on unity power factor. Since the meters are used normal at fixed frequency, they can be adjusted to have a minimum error at declared supply frequency which is normally 50Hz. Poly-phase Meters Similar to the case of wattmeters for measuring power in poly-phase circuits, poly-phase energy metres are also obtained by combining two or more single-phase energy meters in one unit. Two-element energy meter used for measurement of energy in a 3-phase, 3-wire circuit consists of two discs mounted on the common spindle. The moving system drives a single gear train. Each unit is provided will its own shading ring, shading bands and brake magnet for adjustments.

It is necessary that the driving torque of two elements be exactly the same for the same power passing through them. This condition is checked by energizing the coils from a single circuit, the two pressure coils being connected in parallel and the circuit or current coils in series, with one reversed to oppose the other. This arrangement causes the two torques to be in opposition and if the two torques are equal, the disc will not rotate. If there is a slight motion, the magnetic shunt is adjusted till the speed of the disc becomes zero.

The usual adjustments for speed, phase and friction compensation are carried out for each element independently. The final adjustment of the brake magnet position on the two elements is also made separately. Both the shunt magnets are also energized to allow interaction during adjustment. For measurement of energy in 3-phase, 3-wire unbalanced system three element energy meters are used.

Poly-phase meters subject to errors due to interaction between the elements, this way be reduced of suitable positioning and screening of the elements. They also have relatively large friction error due to the greater weight of the rotating parts. They are little cheaper than the corresponding number of single phase metes, and have the clear advantage of giving the total power on a single register.

Internal arrangement and connection diagrams of two element meter and three element meter are shown (a) & (b) respectively. For 3-phase, 3-wire circuits the two element poly-phase meter is usually employed.