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A. GENERAL. Despite the long history of resistor manufacture and use, development of industry-wide standards for resistors is only now receiving serious consideration. The National Electrical Manufacturers Association (NEMA) has developed standards, generally recognized and accepted, though limited in application. These are presented here as the basic standards. The Radio-Electronics-Television Manufacturers Association ( RETMA) is presently engaged in the development of more comprehensive standards involving resistor dimensions, values, tolerances, power ratings, styles, markings, tests, materials and construction. However, these RETMA standards have not as yet been approved by the industry, or put into general use and therefore are not presented here. The importance of U. S. Government standards or specifications for resistors built for use in military and associated equipment should not be overlooked by those applying resistors. The latest government specifications, including up-to-date revisions, should always apply. Due to their broad scope and the fact that such specifications are constantly undergoing slight modification, no attempt is made to review any of the U. S. Government specifications for resistors. However, for the convenience of those readers interested in the application of resistors in military equipment, a partial listing of U. S. Government specifications for resistors is given in the Bibliography and Reference Section, Page 193. B: NEMA STANDARDS. RESISTORS-RATING STANDARDS ICI-13.01 Rating of Resistors--Resistors shall be rated in ohms, amperes and class of service. ICI-13.02 Service Classification of Resistors-- A. Resistors shall be designated by class numbers in accordance with the Tables 1, 2, 3 and 4. ( Table 4 omitted.) B. Starting and intermittent-duty resistors are primarily designed for use with motors which require an initial torque corresponding to the stated per cent of full-load current on the first point and which require an average accelerating current ( rms value) of 125 per cent of full-load current. With a secondary-resistor alternating-current controller, the figures given in the tables for the percent of full-load current on the first point, starting from rest, apply to rotor (secondary) current and to torque. The primary current will, in general, be a higher percentage of the full-load current. C. Starting and intermittent-duty primary resistors which are designed for use with squirrel-cage motors and which meet the test described in ICI-13.12 are included in the tables. D. Continuous-duty resistors shall be capable of carrying continuously the current for which they are designed. E. An adjustable-speed motor having a given horse power rating, when started with full field, generally requires a resistor having large ohmic value than does a constant-speed motor of the same horsepower rating. For this reason, the current on the first point and the capacity of the complete resistor of the same NEMA classification for an adjustable-speed motor may be different from that for a constant-speed motor of the same horsepower. ------------------------- Table I Class Numbers of Resistors For non-reversing service and reversing non-plugging service without armature shunt or dynamic braking When an armature shunt resistor is added, the class number shall include the suffix AS. EXAMPLE--Class !SSAS is a resistor which includes an armature shunt and which will allow an initial inrush of 150 percent with the armature shunt open. When a dynamic braking resistor is added the class number shall include the suffix DB. EXAMPLE-Class !SSDB. -------------------- Table II Class Numbers of Resistors For reversing plugging service without armature shunt or dynamic braking Approx. Per Cent of Full-Load Current o First Point Starting from Rest with all Resistance In circuit. NOTE--The class numbers apply to the complete resistor, but the duty cycles apply to the accelerating resistor only. When an armature shunt resistor is added, the class number shall include the suffix AS. EXAMPLE-Class 153P-AS is a plugging resistor which includes an armature shunt and which will allow an initial inrush of 70 percent with the armature shunt open. When a dynamic braking resistor is added the class number shall include the suffix DB. EXAMPLE-Class 153P-DB. ------------------------ NOTE--A continuous-duty resistor will be so designed that the controller may be operated continuously on any point when the load follows its normal speed-torque curve, except that it must not be operated continuously below the minimum speed specified. Table III Class Numbers of Resistors For dynamic lowering crane and hoist controllers PERFORMANCE STANDARDS ICl-13.10 Temperature of Resistors--When a temperature test is made on a resistor, rheostat or dimmer, at the current values, duty cycle and elapsed time specified, the temperature rise above the ambient temperature and the methods of temperature measurement shall be in accordance with following: 1. For bare resistive conductors, the temperature rise shall not exceed 375 C as measured by a thermo couple in contact with the resistive conductor. 2. For resistor units, rheostats, and wall-mounted rheostatic dimmers, which have an imbedded resistive conductor the temperature rise shall not exceed 300 C as measured by a thermocouple in con tact with the surf ace of the embedding material. 3. For rheostatic dimmers which have embedded resistive conductors and which are arranged for mounting on switchboards or in noncombustible frames, the temperature rise shall not exceed 350 C as measured by a thermocouple in contact with the surface of the embedding material. 4. The temperature rise of the issuing air shall not exceed 175 C as measured by a mercury thermometer at a distance of one inch from the enclosure. ICI-13.11 Temperature Test-Resistors-- A. When a temperature test is made on a starting- or intermittent duty resistor without its motor, the resistor shall be connected to a voltage which will give the initial inrush current specified, the steps shall be cut out at equal intervals of time in the time-on period of the cycle specified, and the current shall he maintained at 125 per cent of the full-load current for those steps through which 125 per cent of full-load current can flow. The specified cycle shall be repeated for one hour. B. When a temperature test is made on a continuous duty resistor without its motor, any tested step shall be subjected to 100 per cent of the current for which the resistor is designed, and this value of current shall be maintained until the maximum temperatures are reached. C. When a temperature test is made on a rheostatic dimmer, it shall be made on a single plate which is operated at rated voltage, and connected in series with lamps totaling the rated lamp load. NOTE--Single plates meeting this test will operate at safe temperatures when assembled into switchboard groups because of the diversity of loading of the various plates. The maximum temperature rise may be approximated by setting the lever so that the dimmer is dissipating maximum wattage. ICI-13.12 Temperature Test-Primary Resistors for Squirrel-Cage Motors--When a temperature test is made on a general-purpose single-step primary starting resistor for a squirrel-cage motor, the resistor shall be tested with 300 percent of normal full-load current of the motor for which the resistor is designed, and the current shall be maintained for a duty cycle as indicated in the resistor classification. This cycle shall be repeated for one hour, after which period the temperature rise shall not exceed the limitations given in ICI-13.10. APPLICATION STANDARDS IC1-13.20 Resistor Application Table--The following table is intended as a guide in specifying or designing resistors. The classifications are those which experience has shown to he correct for the average installation. It is recognized that there will be exceptions. The table applies to resistors composed of wire-wound units or cast grids. It is also applicable to unbreakable resistors provided that the time-on period does not exceed the values given in IC1-13.O2. The table gives the basic class number. Where plugging, dynamic braking, armature shunt or dynamic lowering functions are required, the appropriate suffix letters given in IC1-13.02 should be added to the basic number. INSTALLATION CLASS NUMBER OF RESISTOR ------------ RESISTOR APPLICATION TABLE |
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