Ans. There being no iron in either field or armature of the motor element, no considerations of saturation are involved. The torque or pull of the armature is dependent upon the product of the field and armature strength. The strength of the field--there being no iron--varies directly with the current in the field. Thus the strength of the field with 10 amperes flowing to the load is exactly twice the strength of the field with 5 amperes flowing to the load. The strength of the armature is dependent on the voltage of the system to which it is connected, the armature element of the meter being practically a voltmeter. There is, therefore, a torque or pull varying directly with the strength of the armature multiplied by the strength of the field, or, in other words, varying directly with the watt load, and except in so far as influenced by friction, the speed of rotation varies directly with the torque or pull. The currents generated in the disc armature consist of eddy currents, which circulate within the mass of the disc.

Installation of Wattmeters.--The various types of wattmeter differ so widely either in mechanical details, or operating principles, that it is customary for manufacturers to furnish detailed instructions for the installation of their meters. Such instructions should be carefully followed in all cases, but the following will be found generally applicable to all types of motor meter:

  1. After unpacking the meter, and before opening the case or cover, clean the latter carefully to remove all adhering particles of dust and excelsior.
  2. The proper location for the meter should be one where there is no vibration. When this location has been selected, nail or screw upon the walls, a board somewhat larger than the dimensions of the back of the meter, and upon this board hang the meter by the top hanger.
  3. After hanging the meter, open or remove the case or cover, and if necessary, put the mechanism in order according to instructions furnished by the manufacturer.
  4. In order to operate satisfactorily, the meter should hang plumb, so that the spindle of the revolving element will be vertical, and the horizontal planes through the armature and retarding disc will be level. Many complaints relative to meters being slow on light loads, are invariably due to the fact that the meters have been installed out of plumb[B].
  5. In making the circuit connections, be very careful that the positive lead or wire is placed in the positive binding post of the meter. This precaution is essential for insuring an accurate and sensitive measurement on small loads.
  6. When a meter of the commutated motor type sparks at the brushes at starting, it is an indication that the commutator is dusty. Clean it with a piece of closely woven cotton tape 1/4-inch in width.
  7. Meters should never be allowed to remain with their covers off, in the testing room, station, or any other place. In order to get the best service, and to give them long life they must be kept clean.

[B] NOTE.--The most practical and accurate method of plumbing a meter is to level it by means of a small brass weight placed upon the retarding disc. Place the weight upon the front or back upper surface of the disc, close to the edge. If the disc and weight rotate toward the right, move the bottom of the meter in the same direction so as to raise the disc on the right. When the disc is level, the weight and disc will remain stationary when the weight is placed on either the front or the back of the disc. Next, place the weight on the disc close to the edge on either side. If the disc rotate towards the front, swing the bottom of the meter away from the wall or board until the disc remains stationary when the weight is placed upon it on either side. If the disc rotate toward the back, raise it up on that side by bringing the top of the meter away from the wall or board. It is possible that the second levelling operation will alter the position of the disc obtained by the first operation, therefore, the first should be repeated, and after that the second also, until the disc remains stationary when the weight is placed at any point upon its surface. This method of levelling is more reliable than any method in which a spirit level is employed.

Fig. 657.--Interior view of Thompson watt hour meter (type CQ). The capacities of this type range from 50 to 400 amperes inclusive, two wire, and 50 to 200 amperes inclusive, three wire, and for voltages of from 100 to 600 volts. These meters are made with either front or back connections. In front connected meters the positions of the leading-in wires and cables are the same as in the type C-6, [fig. 654], so that either type of meter may be installed in the same location.

Fig. 658.--Specimen record from General Electric recording ammeter. The record is made on a band of specially prepared paper four inches wide and sixty feet in length. On this paper are ruled lines corresponding to time, and the instrument calibration. The lines ruled across the paper represent time; those ruled lengthwise represent volts, amperes, or watts, depending upon the instrument construction. This form of paper has the advantage of permitting the use of time divisions of equal length throughout the entire range of the recording pen. The recording pen is attached to the moving element in such a manner that its motion is transmitted in a straight line parallel to the time division on the chart. As the paper is unwound and passed under the recording pen, it is paid into a space at the bottom of the instrument case. To assist in removing paper, the instrument is provided with a stripper, which enables the paper to be torn off evenly and without damage. The paper feeding mechanism is simple. By means of suitable gearing, the clock drives a drum having peg teeth which engage the holes located near the edge of the paper. These teeth not only feed the paper under the recording pen, but also give it a definite and accurate position along the axis of the drum. The feeding drum is driven by a friction clutch.

Fig. 659.--Westinghouse type CW-6 watt hour meter with cover off. This meter is of the commutator type without iron in the magnetic circuit. The spherical armature is closely surrounded by circular field coils which provide the shortest magnetic path and smallest magnetic leakage, thus securing high torque with small consumption of energy. The armature winding is wound on a hollow sphere of prepared paper which is moulded in corrugated form to secure strength. Uniform brush tension is maintained by gravity. Each brush consists of two small round wires placed side by side and held against the commutator by a small counterweight whose distance from the fulcrum is adjustable. The current winding consists of two flat coils of strap copper, one clamped rigidly on either side of the central mounting frame which supports the armature bearings. These coils are connected either in series or parallel, depending on the capacity. In three wire meters one of the coils is connected in series with each side of the line. The retarding element consists of a light aluminum disc rotating between two pairs of permanent magnets. The magnets are prepared by a special aging process to insure permanence. Full load adjustment is made by shifting the position of the permanent magnets. Ample light load adjustment or friction compensation is provided by means of the movable coil, which can be shifted horizontally or radially on loosening one screw. The meter registers directly in kilowatt hours.