Below is a diagram of a FM 2S meter type.
Form 2S
240 Volt, 1 Stator, 3-Wire
Self Contained Socket Meter
METER SOCKETS BASIC INFORMATION
The Type-S meter is designed so that its terminals appear as short, rigid, copper contact blades extending outward from the back of the meter. To connect this meter to line and load wires, an auxiliary mounting device is required. This device is the meter socket.
The socket comprises connectors for line and load conductors, contact jaws to receive the meter terminal blades (thus completing connections between conductors and meter coils), and an enclosure for the whole assembly.
Early sockets were round, cast or drawn shallow pans with diameters matching those of the meters. In this type socket, wiring space was limited. This limitation led to the development of the rectangular-shaped trough with a round opening the diameter of the meter.
A sealing ring, which fitted around the meter rim and socket cover rim, secured the meter in place. More recently a ringless type of socket has been developed in which the socket cover opening fits over the meter after the meter has been installed.
The socket cover is then sealed in place to provide protection. In both types the primary functions are to: (1) fix the meter firmly on the socket; (2) close the joint between the meter and socket rim against weather and tampering; and (3) provide means for sealing the meter against unauthorized removal of the meter or cover.
Meter sockets are available in continuous duty current ratings of 20, 80, 100, 120, 150, 160, 200, 320, and 400 amperes and for one, two, or three stator meters. The requirements for indoor and outdoor service differ.
Meter sockets installed on the outside of the house must not only be weatherproof but must be of a material that is highly resistant to corrosion. Under some conditions, such as leakage of pipe joints or cable assemblies, sockets will accumulate varying quantities of water. To guard against water accumulation, sockets are provided with a means for drainage.
Obviously the dimensions of both meters and sockets must be standardized and closely controlled so that meters of any of the major American manufacturers will fit all sockets. The ANSI and NEMA standards have developed standards for meter and socket sizes. The Meter and Service Committees of EEI and AEIC have agreed upon certain basic requirements applicable to meters and the associated mounting devices.
The basic requirements are:
1. Interchangeability of all manufacturers’ meter mounting devices;
2.Mounting devices to be designed for single-meter or multiple-meter mounting either indoor or outdoor;
3. One seal to serve for both meter and mounting device;
4. Terminals to be inaccessible after the meter is sealed in place;
5.Meter base not to be insulated from the mounting device;
6.Mounting device to have an uninsulated terminal for the service neutral.
The material of the socket jaws is important. A tight contact between the meter connection blade and the contact surfaces of the jaw is necessary. This requires the use of an especially high-quality resilient copper alloy, which may be bronze or beryllium.
Even with the best-quality material, it must be remembered that spreading the jaws by pushing screwdriver blades into them may spring the metal beyond its elastic limit and destroy the tight contact with the meter blades.
It is also necessary that the connection between conductors and the line a nd load terminals be secure and of low resistance. The connectors in the lowerrated sockets may be required to accept conductors as small as No. 6 while the 200- and 400-ampere sockets may be required to accept single or multiple conductors that will carry 200 or 400 amperes.
When aluminum conductors are used, the connectors must be designed for this material; that is, they must not cut the comparatively soft strands and they must not encourage cold flow when the wires are under pressure. If aluminum wire is used in the socket that has plating over the copper connectors, use an aluminum electrical joint compound (antioxidant) to prevent metallic reaction between the alumiun and copper connections.
With the growth of domestic loads and the development of self-contained Class 200 meters, the heavy-duty socket also rated at 200 amperes has been introduced. There are two types of such heavy-duty sockets. In one the jaws are made of massive material and sometimes have only one flexible member.
This may be spring loaded but will still depend on jaw resiliency for good contact. In the other type, the jaws are made of a nonflexible heavy material and the jaws are wrench tightened or lever tightened after the meter is in place. Either type of jaw can carry 200 amperes continuously without excessive heating.
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.
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.
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| Basic Electromagnet (for Two-Wire Meter). |
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.
MOVING IRON INSTRUMENT APPLICATIONS IN METERING
Measurement of Current
Since the actuating coil may be wound with a choice of many wire sizes, the instrument may be constructed to measure current from a few milliamperes up to 100 or 200 amperes in self-contained ratings. For measuring currents beyond this range, a 5-ampere instrument may be used with a current transformer.
Current Transformer Field Test Set
A special application of the moving-iron ammeter is the current transformer field test set. The circuit of this instrument is shown below.
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| Circuit of Current Transformer Field Test Set. |
It is essentially a multi-range, moving-iron-type ammeter with a built-in burden which is normally shunted out, but which can be put in series with the meter by the push button.
In the typical instrument illustrated here, ammeter current ranges of 1.25, 2.5, 5, and 10 amperes are obtained from the tapped primary winding of a small internal current transformer, the secondary winding of which is connected to the ammeter which has corresponding multiple scales.
It is thus possible to obtain a reading well up-scale on the ammeter for most load conditions under which the current transformer is operating. The rotary burden switch permits the addition of 0.25, 0.5, 1, 2, or 4 ohms to the secondary circuit as desired.
The imposition of an additional secondary burden on a current transformer having the defects previously mentioned will result in an abnormal decrease in the secondary current. The extent of this decrease and the ohms burden required to effect it depend on the characteristics of the transformer under test.
The check on the current transformer consists of inserting the field test set in series with the current transformer secondary circuit and comparing the ammeter readings under normal operating conditions with the readings after the additional field test set burden is added.
Measurement of Voltage
By the use of an actuating coil of many turns of fine wire in series with a resistor, the moving-iron instrument may be used to measure voltage. Such a voltmeter may have an operating current of around 15 milliamperes with a range up to 750 volts.
External multipliers may be used to extend this range. These voltmeters are used in applications where sensitivities lower than those of the rectifier d’Arsonval instrument are satisfactory.
The moving-iron voltmeter may be used on DC with some loss in accuracy. The best accuracy is obtained by using the average of the readings taken before and after reversal of the leads to the instrument terminals.
This instrument will not indicate the polarity of DC.
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