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current, at the rated terminal voltage, through the insulation. Where the value found in this way exceeds 1 megohm, 1 megohm is sufficient Dielectric Strength. —
34. The dielectric strength or resistance to rupture should be determined by a continued application of an alternating E.M.F for one minute. The source of alternating E.M.F. should be a transformer of such size that the charging current of the apparatus as a condenser does not exceed 25 per cent of the rated capacity of the transformer.
35. The high-voltage tests should not be applied when the insulation is low, owing to dirt or moisture, and should be applied before the machine is put into commercial service.
36. It should be pointed out that tests at high voltages considerably in excess of the normal voltages are admissible on new machines, to determine whether they fulfill their specifications, but should not be made subsequently at a voltage much exceeding the normal, as the actual insulation of the machine may be weakened by such tests.
37. The test for dielectric strength should be made with the completely assembled apparatus and not with its individual parts; and the voltage should be applied as follows:
ist. Between electric circuits and surrounding conducting material; and,
2d. Between adjacent electric circuits, where such exist, as in transformers.
The tests should be made with a sine wave of E.M.F., or where this is not available, at a voltage giving the same striking distance between needle points in air as a sine wave of the specified E.M.F., except where expressly specified otherwise. As needles, new sewing-needles should be used. It is recommended to shunt the apparatus during the test by a spark gap of needle points set for a voltage exceeding the required voltage by 10 per cent.
A table of approximate sparking distances is given in Appendix V.
38. The following voltages are recommended for apparatus, not including transmission lines or switchboards: Rated Terminal Voltage.'
Capacity Testing Voltage.
....10 K. IV. and over 1500
10 K. W. and over 2000 " 800
* 1200 “ Any . . . . . 3500 -
* 2500 “ . Any . . . . . 5000
| Double the normal
Any . . .': : rated voltages. Synchronous motor fields and fields of converters started
from the alternating current side ....5000 volts.
Alternator field circuits should be tested under a breakdown test voltage corresponding to the rated voltage of the exciter, and referred to an output equal to the output of the alternator ; i.e., the exciter should be rated for this test as having an output equal to that of the machine it excites.
Condensers should be tested at twice their rated voltage and at their rated frequency.
The values in the table above are effective values, or square roots of mean square reduced to a sine wave of E.M.F.
39. In testing insulation between different electric circuits, as between primary and secondary of transformers, the testing voltage must be chosen corresponding to the high-voltage circuit.
40. In transformers of from 10,000 volts to 20,000 volts, it should be considered as sufficient to operate the transformer at twice its rated voltage, by connecting first the one, and then the other terminal of the high-voltage winding to the core and to the low-voltage winding. The test of dielectric resist ance between the low-voltage winding and the core should be in accordance with the recommendation in Section 38 for similar voltages and capacities.
41. When machines or apparatus are to be operated in series, so as to employ the sum of their separate E.M.F's, the voltage should be referred to this sum, except where the frames of the machine are separately insulated both from ground and from each other.
42. The term “regulation " should have the same meaning as the term “inherent regulation,” at present frequently used.
43. The regulation of an apparatus intended for the generation of constant potential, constant current, constant speed, etc., is to be measured by the maximum variation of potential, current, speed, etc., occurring within the range from full load to no load, under such constant conditions of operation as give the required full-load values, the condition of full load being considered in all cases as the normal condition of operation.
44. The regulation of an apparatus intended for the generation of a potential, current, speed, etc., varying in a definite manner between full load and no load, is to be measured by the maximum variation of potential current, speed, etc., from the satisfied condition, under such constant conditions of operation as give the required full-load values.
If the manner in which the variation in potential, current, speed, etc., between full load and no load, is not specified, it should be assumed to be a simple linear relation, i.e., undergoing uniform variation between full load and no load.
The regulation of an apparatus may, therefore, differ according to its qualification for use. Thus the regulation of a compound-wound generator specified as a constant-potential generator will be different from that it possesses when specified as an over-compounded generator.
45. The regulation is given in percentage of the full-load value of potential, current, speed, etc.; and the apparatus should be steadily operated during the test under the same conditions as at full load.
46. The regulation of generators is to be determined at constant speed ; of alternating apparatus at constant impressed frequency.
47. The regulation of a generator-unit, consisting of a generator united with a prime-mover, should be determined at constant conditions of the prime-mover ; i.e., constant steam pressure, head, etc. It would include the inherent speed variations of the prime-mover. For this reason the regulation of a generator-unit is to be distinguished from the regulation of either the prime-mover, or of the generator contained in it, when taken separately.
48. In apparatus generating, transforming, or transmitting alternating cur.
rents, regulation should be understood to refer to non-inductive load ; that is, to a load in which the current is in phase with the E.M.F. at the output side of the apparatus, except where expressly specified otherwise.
49. In alternating apparatus receiving electric power, regulation should refer to a sine wave of E.M.F., except where expressly specified otherwise.
50. In commutating machines, rectifying machines, and synchronous machines, as direct-current generators and motors, alternating-current and polyphase generators, the regulation is to be determined under the following conditions:
a. At constant excitation in separately excited fields;
c. With constant resistances hunting series fields ; i.e., the field adjustment should remain constant, and should be so chosen as to give the required full-load voltage at full-load current.
51. In constant potential machines, the regulation is the ratio of the maximum difference of terminal voltage from the rated full-load value (occurring within the range from full load to open circuit) to the full-load terminal voltage.
52. In constant-current machines, the regulation is the ratio of the maximum difference of current from the rated full-load value (occurring within the range from full load to short circuit) to the full-load current.
53. In constan t-power machines, the regulation is the ratio of maximum difference of power from the rated full-load value (occurring within the range of operation specified) to the rated power.
54. In over-compounded machines, the regulation is the ratio of the maximum difference in voltage from a straight line connecting the no-load and fullload values of terminal voltage as function of the current to the full-load terminal voltage.
55. In constant-speed continuous-current motors, the regulation is the ratio of the maximum variation of speed from its full-load value (occurring within the range from full load to no load) to the full-load speed.
56. In transformers, the regulation is the ratio of the rise of secondary terminal voltage from full load to no load (at constant primary impressed terminal voltage) to the secondary terminal voltage.
57. In induction motors, the regulation is the ratio of the rise of speed from full load to no load (at constant impressed voltage), to the full-load speed.
The regulation of an induction motor is, therefore, not identical with the slip of the motor, which is the ratio of the drop in speed from synchronism to the synchronous speed.
58. In converters, dynamotors, motor generators, and frequency changers, the regulation is the ratio of the maximum difference of terminal voltage at the output side from the rated full-load voltage (at constant impressed voltage and at constant frequency) to the full-load voltage on the output side.
59. In transmission lines, feeders, etc., the regulation is the ratio of maxi. mum voltage difference at the receiving end, between no-load and full noninductive load, to the full-load voltage at the receiving end, with constant voltage impressed upon the sending end.
60. In steam engines, the regulation is the ratio of the maximum variation of speed in passing from full load to no load (at constant steam pressure at the throttle) to the full-load speed.
61. In a turbine or other water motor, the regulation is the ratio of the maximum variation of speed from full load to no load (at constant head of water ; i.e., at constant difference of level between tail race and head race) to the full-load speed. Variation and Pulsation.
62. In prime-movers which do not give an absolutely uniform rate of rotation or speed, as in steam engines, the “ variation" is the maximum angular displacement in position of the revolving member expressed in degrees, from the position it would occupy with uniform rotation, and with one revolution as 360°; and the pulsation is the ratio of the maximum change of speed in an engine cycle to the average speed.
63. In alternators or alternating-current circuits in general, the variation is the maximum difference in phase of the generated wave of E.M.F. from a wave of absolutely constant frequency, expressed in degrees, and is due to the variation of the prime-mover. The pulsation is the ratio of the maximum change of frequency during an engine cycle to the average frequency.
64. If n = number of poles, the variation of an alternator is "times the
variation of its prime-mover if direct-connected, and p times the variation of the prime-mover if rigidly connected thereto in the velocity ratio p.
65. The pulsation of an alternating-current circuit is the same as the pulsation of the prime-mover of its alternator.
RATING 66. Both electrical and mechanical power should be expressed in kilowatts, except when otherwise specified. Alternating-current apparatus should be rated in kilowatts on the basis of non-inductive condition ; i.e., with the current in phase with the terminal voltage.
67. Thus the electric power generated by an alternating-current apparatus equals its rating only at a non-inductive load ; that is, when the current is in phase with the terminal voltage.
68. Apparent power should be expressed in kilovolt-amperes as distinguished from real power in kilowatts.
69. If a power-factor other than 10 per cent is specified, the rating should be expressed in kilovolt-amperes and power-factor, at full load.
70. The full-load current of an electric generator is that current which with the rated full-load terminal voltage gives the rated kilowatts, but in alternating-current apparatus only at non-inductive load.
71. Thus in machines in which the full-load voltage differs from the noload voltage, the full-load current should refer to the former.
If P=rating of an electric-generator and E= full-load terminal voltage, the full-load current is :
in a continuous-current machine or single-phase alternator.
in a three-phase alternator.
I=57, in a quarter-phase alternator.
72. Constant-current machines, such as series arc-light generators, should be rated in kilowatts based on terminal volts and amperes at full load.
73. The rating of a fuse or circuit breaker should be the current strength at which it will open the circuit, and not the working-current strength.
Classification of Voltages and Frequencies. —
74. In direct-current, low-tension generators, the following average terminal voltages are in general use and are recommended : 125 volts. 250 volts.
550 volts. 75. In direct-current, and alternating-current, low-pressure circuits, the following average terminal voltages are in general use and are recommended : 110 volts.
220 volts. In direct-current power circuits, for railway and other service, 500 volts may be considered as standard.
76. In alternating-current, high-pressure circuits at the receiving end, the following pressures are in general use, and are recommended : 1000 volts. 2000 volts. 3000 volts. 6000 volts.
10000 volts. 15000 volts. 20000 volts. 77. In alternating-current, high-pressure generators or generating systems the following terminal voltages are in general use, and are recommended : 1150 volts. 2300 volts.
3450 volts. These pressures allow of a maximum drop in transmission of 15 per cent of the pressure at the receiving end. If the drop required is greater than 15 per cent, the generator should be considered as special.
78. In alternating-current circuits, the following approximate frequencies are recommended as desirable: 25 ~ or 30 ~
120 ~ (*) These frequencies are already in extensive use, and it is deemed advisable to adhere to them as closely as possible. Overload Capacities. —
79. All guaranties on heating, regulation, sparking, etc., should apply to the rated load, except where expressly specified otherwise, and in alternating. current apparatus to the current in phase with the terminal E.M.F., except where a phase displacement is inherent in the apparatus.
80. All apparatus should be able to carry a reasonable overload without selfdestruction by heating, sparking, mechanical weakness, etc., and with an increase of temperature elevation not exceeding 15° C. above those specified for full loads. (See Sections 25 to 31.)
81. Overload guaranties should refer to normal conditions of operation regarding speed, frequency, voltage, etc., and to non-inductive conditions in alternating apparatus, except where a phase displacement is inherent in the apparatus.
82. The following overload capacities are recommended :
1st. In direct-current generators and alternating-current generators: 25 per cent for one-half hour.
2d. In direct-current motors and synchronous motors: 25 per cent for one-half hour, 50 per cent for one minute ; except in railway motors and other apparatus intended for intermittent service.
* The frequency of 120 ~ may be considered as covering the already existing commercial frequencies between 120 and 140 , and the frequency of 60 as covering the already existing commercial frequencies between 60 ~ and ~ 70.