MOTOR IN AN ELECTROMECHANICAL SINGLE-STATOR AC METER

The motor is made up of a stator sensing the phase voltage and current with electrical connections, as shown in Figure 7-14, and a rotor, which provides the function of multiplication. The stator is an electromagnet energized by the line voltage and load current.

Basic Electromagnet (for Two-Wire Meter).

The portion of the stator energized by the line voltage is known as the voltage coil and serves the function of voltage sensor. For meters built since 1960, the voltage coil consists of approximately 2,400 turns of No. 29 AWG wire for a 120 volt coil to more than 9,600 turns of No. 35 AWG wire for a 480 volt coil.

These coils are so compensated that the meter can be used within the range of 50 to 120% of nominal voltage. Because of the large number of turns, the voltage coil is highly reactive.

The portion of the stator energized by the load current is known as the current coil and serves the function of current sensor. For a Class 200 meter, the current coil usually consists of two or four turns of wire equivalent to approximately 30,000 circular mils in size. The current coils are wound in reverse directions on the two current poles for correct meter operation.


Dr. Ferraris, in 1884, proved that torque could be produced electromagnetically by two alternating-current fluxes, which have a time displacement and a space displacement in the direction of proposed motion. The voltage coil is highly inductive, as mentioned before, so the current through the voltage coil (and hence the flux from it) lags almost 90° behind the line voltage.

In modern meters, this angle is between 80° and 85°. Although the current coil has very few turns, it is wound on iron, so it is inductive. However, it is not as inductive as the voltage coil. The power factor of a modern meter current coil may be 0.5 to 0.7 or an angle of lag between 60° and 45°.

It is important to remember that the meter current coils have negligible effect on the phase angle of the current flowing through them. This is true because the current coil impedance is extremely small in comparison to the load impedance, which is connected in series. The load voltage and load impedance determine the phase position of the current through the meter.

With a unity-power-factor load, the meter current will be in phase with the meter voltage. Since current through the voltage coil lags behind current through the current coil, flux from the voltage coil reaches the rotor after flux from the current coil and a time displacement of fluxes exists.

The stator is designed so that the current and voltage windings supply fluxes that are displaced in space. These two features combine to give the time and space displacement that Dr. Ferraris showed could be used to produce torque.

In order to understand why torque is produced, certain fundamental laws must be remembered. They are:

1. Around a current-carrying conductor there exists a magnetic field;

2. Like magnetic poles repel each other; unlike poles attract each other;

3. An electromotive force (EMF) is induced in a conductor by electromagnetic action. This EMF is proportional to the rate at which the conductor cuts magnetic lines of force. The induced EMF lags 90° behind the flux that produces it;


4. If a conducting material lies in an alternating-current magnetic field, the constantly changing or alternating magnetic lines of force induce EMFs in this material. Because of these EMFs, eddy currents circulate through the material and produce magnetic fields of their own;

5. When a current is caused to flow through a conductor lying within a magnetic field, a mechanical force is set up which tends to move the current-carrying conductor out of the magnetic field.