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Single-Phase Energy Meters

Single-Phase Energy Meters

The single-phase energy meters are used to measure electric energy in ac. single-phase circuit.

Operating principle Of Single-Phase Energy Meters 

In the single-phase energy meter, the rotation of the disk is produced by the eddy currents due to the combined fluxes of two electromagnets. The fluxes of these two magnets are kept 90° apart to make the rotating field.

Construction Of Single-Phase Energy Meters 

Figure 1.55 shows the different parts of the single-phase energy meter. It consists of a pin-type top bearing. The side is arranged to revolve in the air gap between the poles supply and the other with a current proportional to the main current of the circuit. The light aluminum disc mounted on a steel spindle pivoted on a jeweled bottom bearing and of two electromagnets, one supplied with a current proportional to the voltage of the braking torque is produced by one or two permanent magnets inducing eddy currents in the disc, as shown in figure 1.55.

Single-phase energy Meter Diagram

In order that the matter registers watt-hours, the torque on the disk must be maximum at unity power factor and zero at zero power factor. These conditions are established when flux produced by the voltage (or shunt) electromagnet is made to lag by 90 behind the flux due to the current (or series) electromagnet. 

This is achieved by suitable shaping of the pressure coil magnetic circuit so that the reluctance of the path of the cross flux in the electromagnet is low compared to the reluctance of the path of the flux across the disc gap. The cross flux has the effect or reducing the shunt Current by its reactive or choking action and keeping it as nearly as possible in quadrature with the voltage. 

To increase the angle of the lag exactly to 90°, a closed coil or loop is placed on the care of the shunt electromagnet to embrace the path of the flux which passes across the disc Thus loop may be in the form of a short-circuited band capable of being adjusted in space in the flux path or a fixed loop with its ends terminating in the adjustable resistance.

Errors and Their Compensation in Induction-Type Energy Meters

Phase Error

Normally, the flux due to the exhaust magnet does not lag behind the supply voltage by exactly 90° due to the fact that the coil has some resistance Therefore, the Torque is not zero at zero power factor. This is known as "phase error" and is compensated by means of an adjustable copper band placed over the central limb of the shunt magnet. Due to this reason, the shading ring is known as the power factor compensator.

Speed Error

The speed error of the meter when tested on a non-inductive load can be eliminated by correctly adjustment of the position of the braking magnet. Movement of the poles of the braking magnet towards the center of the disc reduces the braking torque and vice-versa.

Friction Error

Frictional forces at the rotor bearings and in the register mechanism give rise to an unwanted braking torque on the disc rotor. This can be reduced by making the ratio of
the shunt magnet flux and series magnet flux large with the help of two shading bands. Correct compensation is achieved when the rotor does not run on no load with only supply voltage connected.

Creeping errors

The slow but continuous rotation of the disc when only the pressure coils are excited but no current is flowing in the circuit is called "creeping". It may be caused by various factors like incorrect friction compensation, vibration, stray magnetic fields or the voltage supply being in excess of the normal.

To overcome this creeping effect on no load, two holes are drilled in the disc on a diameter, i.e. on opposite sides of the spindle. This causes sufficient distortion of the field to prevent rotation when one of the holes comes under one of the poles of the shunt magnet.

Temperature errors

The error due to temperature variations of the instruments are usually small because of the various effects produced tend to neutralize one another.

Energy Meter Adjustments

Some adjustments are necessary for energy meters so that they give the correct reading, and their errors are within the permissible limits. These adjustments are given below.

Preliminary light load adjustment

the disc of the energy meter is so positioned that the holes are not under the electromagnets. Rated voltage is applied to the voltage coil with no current through the current coil. The light load device is adjusted until the disc just fails to start.

Full load unity power factor adjustment

For this adjustment, the voltage coil is connected across the rated supply voltage, and the rated full load current at unity power factor is passed through the position of the braking magnet is adjusted to vary the braking torque so that the meter revolves at the correct speed within the required limits of errors.

Lag adjustment

For this adjustment, the voltage coil is connected across the rated supply voltage, and the rated full load current is passed to the current coil at 0.5 (half) power factor lagging. Now the lag device is adjusted by altering the position of the short-circuiting coil until the meter runs at the correct speed.

Light-load adjustment

This adjustment is used to compensate for friction and is in the form of a small iron vane fixed to the middle core of the pressure coil electromagnet, just above the disc (figure 1.55), or a closed conducting loop placed unsymmetrically with respect to the shunt magnet. The purpose of this is to distort slightly the field below the pressure coil magnet (or shunt magnet) and thus set up, in the direction of rotation, an extra torque sufficient to overcome the frictional torque of the meter.

Testing of Energy Meters

The testing of the energy meter means including the checking of the actual registration of the meter, as well as the adjustments, are done to bring the errors of the meter
prescribed limit.

Meters should be tested for the following conditions.
(i) At 5% of marked current with unity power factor.
(ii) At 100% or 125% of marked current with unity power factor
(iii) At one intermediate load with a unity power factor
(iv) At marked current and 0.5 lagging power factor

Meters should be subjected to the following tests.

1. Creep test

With an applied voltage of 110% of its market value and with the current circuit open, the meter should not revolve through more than one revolution, i.e. it should not creep.

2. Starting test

At 0.5% of the market value of current and with normal voltage, the meter should start and run Accuracy is not checked at this current.

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