FULL LOAD AND LIGHT LOAD ADJUSTMENT OF WATT HOUR METER

Full-Load Adjustment
The eddy currents in the disk caused by the permanent magnets produce a retarding force on the disk. In order to adjust the rotor speed to the proper number of revolutions per minute at a given (or “rated”) voltage and current at unity power factor, the full-load adjustment is used.

Basically, there are two methods of making the full-load adjustment. One is to change the position of the permanent magnet. When the permanent magnet is moved, two effects result.

As the magnet moves further away from the center of the disk, the “lever arm” becomes longer, which increases the retarding force. The rate at which the disk cuts the lines of flux from the permanent magnet increases and this also increases the retarding force.

The second method of making the full-load adjustment, by varying the amount of flux by means of a shunt, depends on the fact that flux tends to travel through the path of least reluctance. Reluctance in a magnetic circuit is resistance to magnetic lines of force, or flux.

By changing the reluctance of the shunt, it is possible to vary the amount of flux that cuts the disk. One way of doing this is by means of a soft iron yoke used as a flux shunt, in which there is a movable iron screw.

As the screw is moved into the yoke, the reluctance of this path decreases, more lines of flux from the permanent magnet flow through the yoke and less through the disk, so the disk is subject to less retarding force and turns faster.

In either case, the retarding force is varied by the full-load adjustment and, by means of this adjustment, the rotor speed is varied until it is correct. Normally the full-load adjustment is made at unity power factor, at the voltage and test current (TA) shown on the nameplate of the watthour meter, but the effect of adjustment is the same, in terms of percent, at all loads within the class range of the meter.

Light-Load Adjustment
With no current in the current coil, any lack of symmetry in the voltage coil flux could produce a torque that might be either forward or reverse. Because electrical steels are not perfect conductors of magnetic flux, the flux produced by the current coils is not exactly proportional to the current, so that when a meter is carrying a small portion of its rated load it tends to run slower.

A certain amount of friction is caused by the bearings and the register, which also tends to make the disk rotate at a slower speed than it should with small load currents. To compensate for these tendencies, a controlled driving torque, which is dependent upon the voltage, is added to the disk.

This is done by means of a plate (or shading pole loop) mounted close to the voltage pole in the path of the voltage flux. As this plate is moved circumferentially with respect to the disk, the net driving torque is varied and the disk rotation speed changes accordingly.

The plate is so designed that it can be adjusted to provide the necessary additional driving torque to make the disk revolve at the correct speed at 10% of the TA current marked on the nameplate of the meter. This torque is present under all conditions of loading.

Since it is constant as long as applied voltage does not change, a change in the light-load adjustment at 10% of test amperes will also change full-load registration, but will change it only one-tenth as much as light-load registration is changed.