b. Molecular magnetic friction and eddy currents in iron, copper, and other metallic parts. These losses should be determined at open circuit of the machine at the rated speed and at the rated voltage, + Ir in a synchronous generator,-Ir in a synchronous motor, where I = current in armature, armature resistance. It is undesirable to compute these losses from observations made at other speeds or voltages. = These losses may be determined either by driving the machine by a motor, or by running it, as a synchronous motor, and adjusting its fields so as to get minimum current input and measuring the input by wattmeter. The former is the preferable method, and in polyphase machines the latter method is liable to give erroneous results in consequence of unequal distribution of currents in the different circuits caused by inequalities of the impedance of connecting leads, etc. c. Armature-resistance loss, which may be expressed by I2r; where r resistance of one armature circuit or branch, /= the current in such armature circuit or branch, and the number of armature circuits or branches. d. Load losses as defined in Section 7. While these losses cannot well be determined individually, they may be considerable and, therefore, their joint influence should be determined by observation. This can be done by operating the machine on short circuit and at full-load current, that is, by determining what may be called the "short-circuit core loss." With the low field intensity and great lag of current existing in this case, the load losses are usually greatly exaggerated. One-third of the short-circuit core loss may, as an approximation, and in the absence of more accurate information, be assumed as the load loss. e. Collector-ring friction and contact resistance. These are generally negligible, except in machines of extremely low voltage. f. Field excitation. In separately-excited machines, the 12 r of the field coils proper should be used. In self-exciting machines, however, the loss in the field rheostat should be included. (See Section 6f.) III. Synchronous Commutating Machines.— 12. In synchronous converters, the power of the alternating-current side is to be measured with the current in phase with the terminal E.M.F., unless otherwise specified. 13. In double-current generators, the efficiency of the machine should be determined as a direct-current generator in accordance with Section 6, and as an alternating-current generator in accordance with Section 11. The two values of efficiency may be different, and should be clearly distinguished. 14. In synchronous converters the losses should be determined when driving the machine by a motor. These losses are: a. Bearing friction and windage. (See Section 4.) b. Molecular magnetic friction and eddy currents in iron, copper, and metallic parts. These losses should be determined at open circuit and at the rated terminal voltage, no allowance being made for the armature resistance, since the alternating and the direct currents flow in opposite directions. c. Armature resistance. The loss in the armature is q 12 r, where I = direct current in armature, r = armature resistance, and q a factor which is equal to 1.37 in single-phasers, 0.56 in three-phasers, 0.37 in quarter-phasers and 0.26 in six-phasers. d. Load losses. The load losses should be determined in the same manner as described in Section 11 d., with reference to the direct-current side. e and f. Losses in commutator and collector friction and brush-contact resistance. (See Sections 6 and 11.) g. Field excitation. In separately-excited fields, the I2r loss in the field coils proper should be taken, while in shunt and series fields the rheostat loss should be included, except where fields and rheostats are intentionally modified to produce effects outside of the conversion of electric power, as for producing phase displacement for voltage control. In this case 25 per cent of the 12 loss in the field proper at non-inductive alternating circuit should be added as proper estimated allowance for normal rheostat losses. (See Section 6 f.) 15. Where two similar synchronous machines are available, their efficiency can be determined by operating one machine as a converter from direct to alternating, and the other as a converter from alternating to direct, connecting the alternating sides together, and measuring the difference between the directcurrent input and the direct-current output. This process may be modified by returning the output of the second machine through two boosters into the first machine and measuring the losses. Another modification might be to supply the losses by an alternator between the two machines, using potential regulators. IV. Rectifying Machines or Pulsating-Current Generators. 16. These include: Open-coil arc machines, constant-current rectifiers, constant-potential rectifiers. The losses in open-coil arc machines are essentially the same as in Sections 6 to 9 (closed-coil commutating machines.) In alternating-current rectifiers, however, the output must be measured by wattmeter and not by voltmeter and ammeter, since, owing to the pulsation of current and E.M.F., a considerable discrepancy may exist between watts and volt amperes, amounting to as much as 10 or 15 per cent. 17. In constant-current rectifiers, transforming from constant-potential alternating to constant direct current by means of constant-current transformers and rectifying commutators, the losses in the transformers are to be included in the efficiency, and have to be measured when operating the rectifier, since in this case the losses are generally greater than when feeding an alternating secondary circuit. In constant-current transformers the load losses are usually larger than in constant-potential transformers, and thus should not be neglected. The most satisfactory method of determining the efficiency in rectifiers is to measure electric input and electric output by wattmeter. The input is usually not non-inductive, owing to a considerable phase displacement and to wave distortion. For this reason the apparent efficiency should also be considered, since it is usually much lower than the true efficiency. The power consumed by the synchronous motor or other source driving the rectifier should be included in the electric input. V. Stationary Induction Apparatus. — 18. Since the efficiency of induction apparatus depends upon the wave shape of E.M.F., it should be referred to a sine wave of E.M.F., except expressly specified otherwise. The efficiency should be measured with noninductive load, and at rated frequency, except where expressly specified otherwise. The losses are: a. Molecular magnetic friction and eddy currents measured at open circuit and at rated voltage — Ir, where I = rated current, r = resistance of primary circuit. b. Resistance losses, the sum of the 12 of primary and of secondary in a transformer, or of the two sections of the coil in the compensator or autotransformer, where I = current in the coil or section of coil, r = resistance. c. Load losses, i.e., eddy currents in the iron and especially in the copper conductors, caused by the current. They should be measured by short-circuiting the secondary of the transformer and impressing upon the primary an E.M.F., sufficient to send full-load current through the transformer. The loss in the transformer under these conditions measured by wattmeter gives the load losses + 12r losses in both primary and secondary coils. d. Losses due to the methods of cooling, as power consumed by the blower in air-blast transformers, and power consumed by the motor driving pumps in oil or water cooled transformers. Where the same cooling apparatus supplies a number of transformers, or is installed to supply future additions, allowance should be made therefor. 19. In potential regulators the efficiency should be taken at the maximum voltage for which the apparatus is designed, and with non-inductive load, unless otherwise specified. VI. Rotary Induction Apparatus. — 20. Owing to the existence of load losses and since the magnetic density in the induction motor under load changes in a complex manner, the efficiency should be determined by measuring the electric input by wattmeter and the mechanical output at the pulley, gear, coupling, etc. 21. The efficiency should be determined at the rated frequency and the input measured with sine waves of impressed E.M.F. 22. The efficiency may be calculated from the apparent input, the power factor, and the power output. The same applies to induction generators. Since phase displacement is inherent in induction machines, their apparent efficiency is also important. 23. In frequency changers; i.e., apparatus transforming from a poly phase system to an alternating system of different frequency, with or without a change in the number of phases, and phase converters; i.e., apparatus converting from an alternating system, usually single phase, to another alternating system, usually polyphase, of the same frequency, the efficiency should also be determined by measuring both output and input. VII. Transmission Lines. 24. The efficiency of transmission lines should be measured with noninductive load at the receiving end, with the rated receiving pressure and frequency, also with sinusoidal impressed E.M.F.'s., except where expressly specified otherwise, and with the exclusion of transformers or other apparatus at the ends of the line. General Principles. — RISE OF TEMPERATURE. 25. Under regular service conditions, the temperature of electrical machinery should never be allowed to remain at a point at which permanent deterioration of its insulating material takes place. 26. The rise of temperature should be referred to the standard conditions of a room-temperature of 25° C., a barometric pressure of 760 mm. and normal conditions of ventilation; that is, the apparatus under test should neither be exposed to draught nor inclosed, except where expressly specified. 27. If the room temperature during the test differs from 25° C., the observed rise of temperature should be corrected by per cent for each degree C.* Thus with a room temperature of 35° C., the observed rise of temperature has to be decreased by 5 per cent, and with a room temperature of 15° C., the observed rise of temperature has to be increased by 5 per cent. The thermometer indicating the room temperature should be screened from thermal radiation emitted by heated bodies, or from draughts of air. When it is impracticable to secure normal conditions of ventilation on account of an adjacent engine, or other sources of heat, the thermometer for measuring the air temperature should be placed so as fairly to indicate the temperature which the machine would have if it were idle, in order that the rise of temperature determined shall be that caused by the operation of the machine. 28. The temperature should be measured after a run of sufficient duration to reach practical constancy. This is usually from 6 to 18 hours, according to the size and construction of the apparatus. It is permissible, however, to shorten the time of the test by running a lesser time on an overload in current and voltage, then reducing the load to normal, and maintaining it thus until the temperature has become constant. In apparatus intended for intermittent service, as railway motors, starting rheostats, etc., the rise of temperature should be measured after a shorter time, depending upon the nature of the service, and should be specified. In apparatus which by the nature of their service may be exposed to overload, as railway converters, and in very high voltage circuits, a smaller rise of temperature should be specified than in apparatus not liable to overloads or in low voltage apparatus. In apparatus built for conditions of limited space, as railway motors, a higher rise of temperature must be allowed. 29. In electrical conductors, the rise of temperature should be determined by their increase of resistance. For this purpose the resistance may be measured either by galvanometer test, or by drop-of-potential method. A temperature coefficient of 0.4 per cent per degree C. may be assumed for copper.t Temperature elevations measured in this way are usually in excess of temperature elevations measured by thermometers. 30. It is recommended that the following maximum values of temperature elevation should not be exceeded: * This correction is also intended to compensate, as nearly as is at present practicable, for the error involved in the assumption of a constant temperature coefficient of resistivity; i.e., 0.4 per cent degree C. taken with varying initial temperatures. T By the formula R=R (1+0.004). Where R is the resistance at room temperature, R, the resistance when heated, and the temperature elevation (7-1) in degrees centigrade. Commutating machines, rectifying machines, and synchronous machines. Commutator and collector rings and brushes, by thermometer, 55° C. Rotary induction apparatus: Electric circuits, 50° C., by resistance. Bearings and other parts of the machine, 40° C., by thermometer. In squirrel-cage or short-circuited armatures, 55° C., by thermometer, may be allowed. Transformers for continuous service - electric circuits by resistance, 50° C., other parts by thermometer, 40° C., under conditions of normal ventilation. Reactive coils, induction and magneto regulators -- electric circuits by resistance, 55° C., other parts by thermometer, 45° C. Where a thermometer, applied to a coil or winding, indicates a higher temperature elevation than that shown by resistance measurement, the thermometer indication should be accepted. In using the thermometer, çare should be taken so to protect its bulb as to prevent radiation from it, and, at the same time, not to interfere seriously with the normal radiation from the part to which it is applied. 31. In the case of apparatus intended for intermittent service, the temperature elevation which is attained at the end of the period corresponding to the term of full load should not exceed 50° C., by resistance in electric circuits. In the case of transformers intended for intermittent service, or not operating continuously at full load, but continuously in circuit, as in the ordinary case of lighting transformers, the temperature elevation above the surrounding air-temperature should not exceed 50° C. by resistance in electric circuits, and 40° C. by thermometer in other parts, after the period corresponding to the term of full load. In this instance, the test load should not be applied until the transformer has been in circuit for a sufficient time to attain the temperature elevation due to core loss. With transformers for commercial lighting, the duration of the full-load test may be taken as three hours, unless otherwise specified. In the case of railway, crane, and elevator motors, the conditions of service are necessarily, so varied that no specific period corresponding to the full-load term can be stated. INSULATION. 32. The ohmic resistance of the insulation is of secondary importance only, as compared with the dielectric strength, or resistance to rupture by high voltage. Since the ohmic resistance of the insulation can be very greatly increased by baking, but the dielectric strength is liable to be weakened thereby, it is preferable to specify a high dielectric strength rather than a high insulation resistance. The high-voltage test for dielectric strength should always be applied. Insulation Resistance. 33. Insulation resistance tests should, if possible, be made at the pressure for which the apparatus is designed. The insulation resistance of the complete apparatus must be such that the rated voltage of the apparatus will not send more than 18000s of the full-load |