8.4. Motor-driven Meter-Watt-hour Meter. The motor-driven meter shown in Fig. 29 can be used on direct or alternating current. It contains a small motor and an aluminium retarding disc. The field winding is connected in series with the load, and the field strength is proportional to the load current.
The armature is connected across the source, and the current in the armature is proportional to the source or line voltage. The torque produced in the armature is proportional to the power consumed by the load.
The armature shaft drives a series of counters that are calibrated in watt-hours. The power that is used can be read directly from the dials.
The aluminium disc attached to the armature is used to control the armature speed. The disc turns in a magnetic field produced by the permanent magnets, and the retarding force increases as the rotation increases and stops when the disc’ stops. The retarding force is produced by the
Fig. 29. Motor-driven meter (watt-hour meter) designed to operate on direct or alternating current.
aluminium conductor cutting through the lines of force of the permanent magnets. This is a form of magnetic damping.
The meter must overcome the friction of the bearings and indicators at very light loads. A portion of the field is produced by the armature current (coil in series with the armature winding). This coil is (called as compensating coil) wound to aid the field and is adjusted to the point where it just overcomes the meter friction.