What are the factors considered in understanding the operating principles of a watt-hour meter?
BASIC SINGLE-PHASE METER. A single-phase watt-hour meter is essentially an induction motor whose speed is directly proportional to the voltage applied and the amount of current flowing through it. The phase displacement of the current, as well as the magnitude of the current, is automatically taken into account by the meter. In other words, the power factor influences the speed, and the moving element (disk) rotates with a speed proportional to true power. The register is simply a means of registering revolutions, and by proper gearing is arranged to read directly in kilowatt-hours. (Note: In some cases, the meter reading must be multiplied by a factor called the "register constant" or "meter multiplier" to obtain total kilowatt-hours.
DISK DRIVING TORQUE. As stated above, the meter operates similarly to an induction motor. The aluminum disk acts as a squirrel-cage rotor, torque being produced as a result of eddy currents induced in it by the potential (voltage) and current coils on the electromagnet. In order that registration be correct, the torque (and speed) must be greatest at a power factor of 1.0. To have maximum speed at unity power factor, it is necessary that the current in the potential coil lag exactly 90 degrees behind that in the current coil, or in other words 90 degrees behind the voltage applied to the potential coil. This is also necessary if the meter is to register correctly at power factors less than unity. To get this exact 90 degrees displacement, a short- circuited (lag) coil is placed on the voltage coil pole. The resistor in the circuit of this coil may constitute the "lag" or power-factor adjustment of the meter, but in many meters this adjustment is obtained by movement of a "lag plate," and the resistor should not be disturbed.
Do not confuse this 90degrees lag within the meter at unity power factor, which is necessary for proper functioning of the meter, with the current and voltage supplied to the meter by the instrument transformers. These are in phase (0 degrees displacement) at unit (1.0) power factor.
FRICTION. To compensate for friction, additional torque must be introduced. This usually is accomplished by placing a movable short-circuited turn of large cross section in part of the field of the voltage (potential) coil. This also serves as a "light-load" adjustment.
BRAKING MAGNETS. Normally there is very little friction present in meters, and if no additional retarding force other than friction were placed in the meter, the rotating element would travel at a relatively high speed. The necessary retarding action is provided by a magnetic brake consisting of a permanent magnet operating on the aluminum disk. This retarding action is adjustable and is known as the "full load" meter adjustment. Two methods of varying the braking effect of the magnet are in common use. The first is to adjust the position of the magnet; moving it outward radially toward the edge of the disk increases the braking effect and decreases speed and registration. In the second method, the magnet is fixed, and the braking effect is adjusted by a magnet shunt which bypasses part of the magnet flux of the permanent magnet.
GENERAL. For the sake of simplicity, the discussion in this section will be mainly based on a single-element (singlephase) meter. Two- and three-element meters are simply two or three single elements having a common shaft and register, which serve to totalize the energy measured by each element.
BASIC SINGLE-PHASE METER. A single-phase watt-hour meter is essentially an induction motor whose speed is directly proportional to the voltage applied and the amount of current flowing through it. The phase displacement of the current, as well as the magnitude of the current, is automatically taken into account by the meter. In other words, the power factor influences the speed, and the moving element (disk) rotates with a speed proportional to true power. The register is simply a means of registering revolutions, and by proper gearing is arranged to read directly in kilowatt-hours. (Note: In some cases, the meter reading must be multiplied by a factor called the "register constant" or "meter multiplier" to obtain total kilowatt-hours.
DISK DRIVING TORQUE. As stated above, the meter operates similarly to an induction motor. The aluminum disk acts as a squirrel-cage rotor, torque being produced as a result of eddy currents induced in it by the potential (voltage) and current coils on the electromagnet. In order that registration be correct, the torque (and speed) must be greatest at a power factor of 1.0. To have maximum speed at unity power factor, it is necessary that the current in the potential coil lag exactly 90 degrees behind that in the current coil, or in other words 90 degrees behind the voltage applied to the potential coil. This is also necessary if the meter is to register correctly at power factors less than unity. To get this exact 90 degrees displacement, a short- circuited (lag) coil is placed on the voltage coil pole. The resistor in the circuit of this coil may constitute the "lag" or power-factor adjustment of the meter, but in many meters this adjustment is obtained by movement of a "lag plate," and the resistor should not be disturbed.
Do not confuse this 90degrees lag within the meter at unity power factor, which is necessary for proper functioning of the meter, with the current and voltage supplied to the meter by the instrument transformers. These are in phase (0 degrees displacement) at unit (1.0) power factor.
FRICTION. To compensate for friction, additional torque must be introduced. This usually is accomplished by placing a movable short-circuited turn of large cross section in part of the field of the voltage (potential) coil. This also serves as a "light-load" adjustment.
BRAKING MAGNETS. Normally there is very little friction present in meters, and if no additional retarding force other than friction were placed in the meter, the rotating element would travel at a relatively high speed. The necessary retarding action is provided by a magnetic brake consisting of a permanent magnet operating on the aluminum disk. This retarding action is adjustable and is known as the "full load" meter adjustment. Two methods of varying the braking effect of the magnet are in common use. The first is to adjust the position of the magnet; moving it outward radially toward the edge of the disk increases the braking effect and decreases speed and registration. In the second method, the magnet is fixed, and the braking effect is adjusted by a magnet shunt which bypasses part of the magnet flux of the permanent magnet.
BEARINGS. The moving element consists of an aluminum disk on a shaft. The bottom bearing may be either of two types two cupped jewels with a steel ball between or a cone- shaped pivot on the shaft which rotates in a cupped jewel bearing. The top bearing is usually a hardened steel needle-like pin fitting loosely inside the hollow shaft.
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