CHAPTER IV. CONTINUOUS CURRENT ENERGY MOTOR METERS. General Description-Thomson Meter of the British Thomson-Houston Co.-Thomson Meter of the General Electric Co., U.S. A.-Duncan Meter-Vulcan Meter-Eclipse MetersMeter of Mix & Genest, Berlin-Siemens-Schuckert Meters-Elimination of FrictionEvershed Frictionless Motor Meter-Hartmann & Braun Motor Meter. General Description.--Continuous current energy motor meters depend on the well-known electro-dynamometer principle, in which the electro-magnetic action between the currents in a stationary coil and a movable one produces motion in the latter. This motion, in the case of motor meters, is converted into one of continuous rotation by the aid of brushes and a commutator electrically connected to the moving coil and carried on the same spindle. All motor meters consist of three essential parts, which are the motor, the brake system, and the integrating mechanism. The motor, in all the meters included in this chapter, contains no iron in either the armature or its field system. The field or main current coils are stationary, and are connected in series in one of the supply mains for a two-wire meter. The armature is composed of fine wire coils placed as a shunt circuit across the supply leads. The field coils are, therefore, traversed by the whole current taken by the particular installation in which the meter is fixed, whereas the armature is energised by a current proportional to the supply pressure. A high resistance is also used in the pressure circuit in series with the armature, to keep the pressure current low and to reduce the voltage across the brushes. The current in the main or series coils sets up a magnetic field which is proportional to the number of amperes in the main circuit, and the current in the armature produces another field of force, at right angles to that of the series coils, which is proportional to the voltage. These two magnetic fields exert at every instant a driving torque on the armature, which is proportional to the product of both fields, that is, to the product of the P.D. and the main current flowing, or to the actual power supplied in watts. The brake system is to absorb the work done by the motor, and must be such that the retarding torque which it exerts is at every instant proportional to the speed of the armature. This result is obtained by rotating a non-magnetic metal disc, mounted on the armature spindle, between the poles of one or more permanent magnets. The portion of the disc embraced by the poles cuts the lines of force of the brake field at right angles, and at a rate depending on the number of revolutions of the armature spindle, and the Foucault currents generated in the disc will vary in intensity as the speed. The resisting torque is at any moment proportional to the product of the brake field of the permanent magnets and the strength of the induced currents, and, therefore, to the speed. When steady motion has been established, the driving and retarding torques will balance one another, neglecting all frictional resistances, and the speed will vary directly as the power supplied. The number of revolutions made in a given time will then be proportional to the energy consumed in this period. It must be remembered, however, that the assumption is made that the various frictional resistances which occur in a meter are negligible. These frictional resistances are—(1) friction of axle bearings, (2) friction of brushes on the commutator, (3) air friction, and (4) friction of the counting train and gear connecting it to the motor spindle. Generally, friction is considerably reduced, and in a well-designed meter does not influence its indications at high loads, being mainly noticeable at low loads. The matter is complicated still further, as friction is not constant at these low loads, but is a variable quantity. It may be mentioned here that air friction is so slight that it is entirely neglected. To compensate for the frictional resistances, and to enable the meter to readily start with a small current when the magnetic effect of the main current coils is weak, a compound or auxiliary winding is used. This compensating coil consists of a few turns of wire in series with the armature circuit, and is placed relatively to the main current coils, so that the lines of force it produces augment the main field. At these very low loads the magnetic field of the main current coils is so weak that the torque exerted is, unaided, insufficient to overcome the forces of friction. The additional torque is supplied in the manner just explained. It will be seen that this supplemental field is always present, as the pressure circuit is always energised. Moreover, the compound winding becomes relatively of less importance as the load on the meter increases. The compensating coil is usually made adjustable, so that its distance to or from the armature may be varied to suit the immediate conditions of the installation. It is generally so adjusted that the meter starts with a current between 1% and 2% of the maximum capacity of the installation. The registering or integrating mechanism is an ordinary train of wheels and pinions gearing with one another, and the first motion wheel of the train is driven by a worm or pinion on the armature spindle. The staffs of the various wheels carry either hands which travel over graduated circles on the front of the dial, or number discs or wheels, the figures on which appear in line opposite slots in the dial face. The integrating mechanism is invariably arranged so that its indications give the energy consumption direct in either Board of Trade or other convenient units without the use of a multiplier or constant. A motor meter may be regarded as a motor-generator, the generator consisting of a magneto-dynamo with a short-circuited armature. The work the motor does is expended in driving the dynamo, in which the energy is ultimately absorbed in the shape of heat, in driving the counting train and gear, and in overcoming the remaining frictional resistances to motion. Meters of this class are very extensively used, and differ mainly in details of mechanical and electrical design, the relative arrangement of the various component parts, and the methods adopted to overcome the sources of error. In the descriptions which follow, the electrical features of motor meters will be mainly pointed out, and only those meters are included in this chapter which do not contain iron in the armature and field. In Chapter XIII. will be found a few of the more important mechanical details. Thomson Watt-hour Meter.-The latest type of the Thomson watt-hour meter, manufactured by the British Thomson-Houston Company, Limited, Rugby, is illustrated in Fig. 28. It represents their standard house-service meter, form A, for two- and three-wire circuits. Two main current coils of rectangular shape are used, and are arranged parallel to one another, and with their planes at right angles to the base of the meter. They are securely held in position by means of clamps which fit in grooves in the meter base, and which are fixed by means of screws. By loosening these screws and disconnecting the two coils they can be readily removed to give access to the armature. The compound winding, which consists of a few turns of fine wire connected in series with the armature circuit, is divided into two sections, one being placed in each series coil. The compensating coils are adjustably mounted, and can be moved relatively to the main current coils, to alter the effect for the light-load adjustment. When the meter is calibrated at the FIG. 28. works, the position of each compounding coil is adjusted until the supplemental torque is just sufficient to overcome the friction of the bearings, when the meter will register correctly on light loads. If, however, a meter should afterwards be found to run slow on light loads owing to the bearings wearing rough, it is only necessary to readjust the position of each coil by moving it nearer to the armature, so as to increase the auxiliary torque to a point where the increased friction is balanced. Symmetrically supported between the series coils is the armature, which is drum-wound and is composed of about 3000 turns of insulated fine copper wire. It is carried on a light steel spindle, which rests on a lower sapphire jewelbearing, flexibly supported on a spring cushion in the jewel screw. The upper pivot of the meter spindle runs in a guide-bearing, which serves solely to keep it central. The armature is wound in sections connected to an eight-part commutator, which is built up of silver segments and is mounted on the lower extremity of the spindle. Three phosphor-bronze wires are used for the brushes, and their ends are fitted with tiny silver sleeves, with which they bear on the commutator. They are kept in position by their own tension and are easily detachable. The commutator, brushes, and the jewel step-bearing can be quickly examined without having to disturb the main cover of the meter. They are protected by a separate dome-shaped cap, attached by a bayonet catch to the main cover, as shown in Fig. 29. This is an important consideration, as the commutator, brushes, and jewel form the vital parts of a meter, and constitute the chief sources of trouble inherent to meters of this class. They should therefore be quickly and easily accessible during the operation of the meter. The brake system is arranged at the top of the instrument, and consists of two permanent magnets of tungsten steel, very carefully aged, and a brake disc of copper, which rotates between their poles. The disc is mounted on the meter spindle which drives through a gunmetal worm the integrating train. The dial registers the energy consumed direct in B.O.T. units, and is carried on the same bracket which supports the high-resistance coil in series with the armature circuit. The terminals are at the bottom of the instrument, and are protected by a separate ebonite cap, which also covers the fixing screws of the meter. The magnets are so mounted that their position relatively to the brake disc can be altered for the purpose of decreasing or increasing the speed of the meter at full load. With the general arrangement adopted, immunity is secured from any demagnetising action of the main coils, or other current-carrying parts, on the brake magnets. This might happen were these parts in close proximity to the brake system, and with excessive overloads or a short-circuit current the effect would become very pronounced. In Fig. 30 is given a diagram of connections for a two- or three-wire meter up to 75 amperes, the dotted line showing the neutral or middle wire in a three-wire system. FIG. 29. The Thomson Meter. The Thomson meters, manufactured by the Compagnie pour la Fabrication des Compteurs, Paris, and by the Danubia Actiengesellschaft, Vienna, are almost identically the same as the one first described. Slight modifications occur in connection with the brushes. These are composed of tiny silver lamellæ, bent round to form half-cylinders, and bear with their two edges on the commutator. maintained by means of springs. The contact pressure is One of the main features of the Thomson meter of the General Electric FIG. 32. Co., Schenectady, New The adjustable shunt field coil used by this company is very clearly shown in the illustration in Fig. 31 of their standard two-wire small capacity meter for currents from 3 to 50 amperes. It is shown more in detail in Fig. 32. |