September, 1923
IND UXTRIAL A N D ENGINEERING CHEMISTRY
921
Electrical Motors for Industrial Service By J. L. McK, Yardley WESTINGHOUSE
ELECTRIC & MANUFACTURING CO., EASTPITTSBURGH, P A .
T IS in the nature of a major accomplishment that the preferable or necessary are those in which great speed range, multitudinous industrial operations requiring power may or great starting torque, or frequent reversal is required. practically all be supplied through proper applications Accordingly, practically every industrial plant requires or of electrical motors. Not only this, but it is a fact that employs at least a small amount of direct current equipment very considerable standardization has been achieved, and whether or not the major part is for alternating current. Sixty-cycle power can be generated a t the lowest first that the gamut of applications is economically covered by a number of types and sizes of motors which is comparatively cost. The ordinary inter-works distribution, or short disinsignificant. Perceiving the chaos which would result from tance transmission, of 50 or 60-cycle power is as cheap and unguided development and react upon industrial progress, economical as for 25-cycle power except when very heavy to be carried in comprehensive classes with logical characteristics and ratings currents-2500 amperes and above-are have been determined upon as those to be generally manu- one circuit. It has a flexibility as to permissible normal factured, from which intelligent selection may be made for motor speeds which is sufficient for the great majority of the particular application involved. Preference for alter- industrial plant applications, and is obviously far superior nating current over direct current has become well nigh uni- to 25 cycles. In addition, the higher frequency motor for versal, and 50 and 60 cycles are with increasing rapidity a given horse power and speed is considerably cheaper. becoming the predominating frequencies. Twenty-five cycle power has advantages in applications The present satisfactory situation has come about in a where the fundamental speed is low. The following normal natural and logical way as a result of a number of definite speeds (approximately) are obtainable depending upon the facts, chief among which, perhaps, is that 50 or 60-cycle number of poles. power is the cheapest and most economical power to generate, Motors Obtain50 Cycles 60 Cycles able for 60 Cycles transmit, and distribute, which is a t the same time very Poles 25R.Cycles p. M . R. p. M. R. p. M. Horse Power economical and flexible to apply in diverse industrial opera2880to3000 2 1440 t o 1500 3450to3600 I/a to 50 1 4 4 0 t o 1 5 0 0 720 t o 750 4 1720 to 1800 1/1 to 350 tions. All important industries, with the exception of the 960 t o 1000 480 t o 500 6 1 1 5 0 t o 1200 ’/a t o 500 720 to 750 8 360 to 375 865 t o 900 ‘/z to 1750 electrochemical and electrometallurgical industries, are able 288 t o 300 576 t o 600 10 6 9 0 t o 720 7’/2 to . . . to use alternating current in by far the major part of their 240 t o 250 480 t o 500 12 576 t o 600 7’/2 to . . . 410 t o 428 205 to 214 14 493 t o 514 power-consuming applications, and the electrochemical and 360 to 375 , . to 187.5 16 432 to 450 to . . . t o ... 18 385 t o 400 electrometallurgical industries use alternating current motors to . . . t o ... 20 345 t o 360 in preference to direct current wherever possible, owing In this article the discussion of electrical motors for to there being less surface of copper exposed to chemical actioii. Here- and there an industrial service will be conindustrial plant or a whole fined to the 60-cycle, 3corporation such as the Ford phase motor, a t standard Motor Company has settled voltages of 220, 440, and upon direct current owing to 560 volts. For other motors its extreme flexibility, and the same general principles uses nothing but direct curapply. Large alternating rent motors. These are current motors in all indususually cases where the tries are usually a t 440, 550, power is generated as direct or 2200 volts, or may even current within the plant and be a t higher voltage. The there are no extra transtextile industry, employing mission or conversion losses. vast quantities of small There are, moreover, a num550-volt motors, may be cited ber of power applications, as perhaps the most conspicsome occurring in nearly uous example of the applicaevery industrial plant, for tion of relatively high voltwhich direct current is esage in small motors. It is sential or generally far prefalso the principal exception erable. Those which may to the very general adoption be mentioned include fans of either 440 or 220 volts and blowers, elevators, for industtrial motors for cranes, rubber calenders, FIG. l-cOMPI,ETE RUBBERCALENDER INSTALLATION general purposes. Since this application usually requires capability of adjustment through paper driers and calenders, a 4 to 1 speed range--that is, say, from 283 to 1130r. p. m. for a 50 h. p. applireversing rolling mills, ad- cation TEMPERATURE RATING t o from 175 or 200 to 700 or 800 r. p. m. for a 250 or 200 h. p. applicajustable speed mills, such tion, direct current is found more suitable than alternating current; and, In order that industry may as cold rolls, larry cars, in- in the larger applications, operation from double voltage circuits is found dustrial and mining loco- more suitable, in order t o reduce expense in equipment and somewhat better secure the benefits of uniregulation with varying loads. T h e a-wire, 115-230-volt circuit formity and standardization motives, and electrolytic speed for this purpose is usually provided by a 250-volt industrial type synchronous in motor manufacture, it has cells. Applications for which converter. become “recommended pracdirect current is legitimately Motors are rated at 50° C . on a constant torque basis.
I
,
922
FIG. 2-Tw0
INDUSTRIAL AND ENGIWEERIXG CHEMISTRY
“cs”
Vol. 15, No. 9
75-H. P . , TYPE MOTORS TO T H E L E F T , GEAREDT O ARE DRIVENV A R I O U S MACHINES.T O T H E RIGHTARE TWO50-H. P . , TYPE“ C W ’ MOTORS BELTED TO AMXONIA C O M PRESSORS IN THE SAMEROOM
F I G . 3-15-H. P , TYPE“CS” MOTOR TO THE L E F T , T O W A T E R CIRCULATING P U M P , A N D 5-H. P. “CS”
tice” that an open type, general purpose motor shall be capable of carrying full rated load continuously with a temperature rise not exceeding 40” C., and that it shall be so designed as to be capable of carrying sustained overload (in no event exceeding 50 per cent overload) within a temperature of 90” C. measured by thermometer. Many special application motors, and particularly synchronous motors and large induction motors, will be manufactured in accordance with generally accepted practice for such apparatus so as to be capable of carrying full rated load continuously with a temperature rise not exceeding 50” C., and will not be so designed as to be capable of carrying sustained overloads appreciably above full rated load without exceeding desirable temperature. This method of rating is a common sense method, being based upon the fact that insulation of the type universally employed in industrial motors begins to deteriorate if operated at temperatures above 105’ C., and that it has been found in practice that a differential, called LLconventioiial allowance” by A. I. E. E. standards, of 15” C . is likely to exist between the temperature of some point at the surface of the conductor and the highest obtainable thermometer measurement. Furthermore, it pays to analyze power requirements more thoroughly in installing a large motor than a small one, so as not to have excess capacity which will never be used. Power requirements are likely to vary more from day to day with a small general purpose motor than with a large motor. This makes sustained capacity in excess of normal rating more essential in the general purpose motor. Hence, the logical outcome has been to rate general purpose motors at 40” C., with some greater inherent overload capacity than the larger and special motors which are rated at 50” C. ADVANTAGES OF MOTOR DRIVE While the advantages of driving mechanical applications by electrical motors are so universally recognized that with hardly an exception all new industrial construction is completely electrified at the outset, there are very many old mills or factories now completely equipped with steam drive which, when the necessity for renewal, extension, or increased production in the same space arises, must view electrification with utmost practicality and adopt i t only if analysis indicates that tangible benefits from electrical drive will warrant writing off corresponding items in the plant account, representing capital already expended. It is a fact that analysis shows electrical drive t,o be truly
economical in most of such cases, even when process steam in considerable quantity is required. Motor drive eliminates the complicated system of line shafts, belts, clutches, and pulleys. This system, necessary with steam drive, is awkward to instal and expensive to maintain; i t requires constant attention to keep it in working order; it wastes a large amount of power through friction; and it nukes difficult the protection of operators against unnecessary risks of injury. With motor drive, each machine may have its own motor, forming a compact unit that can easily be safeguarded. Production is increased by motor drive because each machine can be maintained a t its proper speed. The time lost on account of shutdown is materially reduced, as trouble with line shafts and belts is practically eliminated. Furthermore, as a majority of machines have individual motor drive, the operation as a whole is usually not materially affected by the shutting down of any one machine. A motor-driven plant is thoroughly Aexible, since machines can be arranged in any desired location, according to the needs of production, and additions can be made conveniently and readily, so as to conform with the original layout. With engine drive, suitable location and arrangement of the line shafts largely govern the mill or factory layout and often place undesirable limits on it, especially where extensions are to be considered. Electric power, being readily transmitted long distances to widely separated buildings and process operations, may be generated at a point removed from the dust of plant and located convenient to condensing water and fuel supply. Power charges can be accurately assigned to each department, thus improving the cost system and encouraging increased efficiency. With individual motor drive an ammeter can be used with each machine. This meter shows the current taken by the motor, and since the current consumption is constant or varies over a normal range under proper operation, irregular meter readings indicate a t once that something is wrong and the fault in the operating conditions should be corrected. In this way trouble which might otherwise become serious before discovered can often be avoided. In addition, the current consumption will vary with the adjustment of the operating part of the machinery being driven, so that the meter provides an accurate indication as to the character of these adjustments and thus greatly aids in the production of the desired quality of material.
L I N E S H A F T S FROM W H I C H
BELTEDTO PREHEATER PUMPIN
A
CHLORINE AND
DIRECTLY CONNECTED MOTORTO T H E R I G H T , CAUSTIC SODA PLANT
INDUSTRIAL A N D ElVGINEERING CHEiWISTRY
September, 1923
923
F I G . 5-7.5-H. P . , 4 4 0 - V O L T , 3-PHASJ3, 6 0 - C Y C L E MOTOR DRIVING POTASH DRIER. THE S P E E D OF MOTOR, 685 R . P . M . , Is C H A N G E D THROUGH R E FIG.4 - E N G I N E TYPESYNCHRONOUS MOTORDRIVING AIR C O M P R E S S O R . T O 50 R . P . M. C O N T R O L S W I T C H I s MOUNTED ON C O N C R E T E MOTOR LOCATED BEYOND DANGER FROM CORROSIVE FUMESOF CHEMICAL DUCTION FEAR WAL& BELOW PLANTS
Electrification includes the advantages offered by the use of central station power. In most cases the cost of purchased power is less than the cost of power generated in a private plant, when all the factors of expense are taken into consideration, and, furthermore, the incidental advantages are of much value. The use of power purchased from a central station results in a much lower first cost of plant, while extensions can be readily made and the time and attention devoted to supervision of the power plant can all be given directly to production. Electrification of existing mills and factories and the substitution of electrical motor drive for steam drive is probably held back more by insufficient capital than by any other cause. The supply of capital is, of course, limited. Old mills wear out in time. Many are difficult to modify economically. Executives often see better labor conditions developing in a new locality. So long as the old mill is able to continue in operation at a profit, despite its inefficiencies, better business judgment often decides that available capital can be more profitably invested in an entirely new property than in rejuvenating an old one. I n connection with this whole matter, it is well to appreciate the progress which has been made in the electrical motor itself since the day when it was still general practice to build steam-driven plants. It should be realized that this progress has been much greater than the corresponding progress in steam drive, so that any decisions made years ago may not be considered final as between systems nor detrimental to universal installation of electric motor drives. This progress is well illustrated by the following tabulation for the 10 h. p. induction motor: Effi-
H. P . 10
10
__
Poles "vue 6 C 6
Year 1902
CS May, 1922
Price $350 (equivalent to Bl000 in May, 1922) $234
Weight ciency Bearings Lbs. % In. 1100 83'5 1 3 / r X 4 5 / 8
270
88 5
13/a
x 43/&
In May, 1922, prices on 60-cycle, squirrel-cage motors were only 18 per cent above 1914 prices, although labor was 80 per cent higher and many materials were more costly. With weight and comparative cost reduced to a fraction and efficiency increased very appreciably in the past twenty years, the electric motor itself has become much more attractive economically; and since its reliability has also become established, it is the chief consideration in electrification.
1vECHANICAL
TYPESBND KINDSO F DRIVE
While, from an electrical point of view, the motor has been susceptible to standardization, producing a uniformity which tends towards economy in manufacture and in the supply of power to it, from a mechanical point of view the same thing has not been possible, owing to the diversity in the mechanical operations in industrial plants, for which power must be supplied. Accordingly, we have directcoupled, flexibly coupled, geared, chain-driven, belted, backgeared, vertical motors, etc. With regard to methods of drive, the fundamental factors determining which should be employed are practically the same in all industries. Briefly, .group or gang drive is seldom economically justified, except in cases where the group or gang consists of a comparatively large number of intermittently used, smallpowered applications, all located near together, so that the driving motor may be considerably smaller than the total connected load, and the amount of shafting continuously running is not excessive. The small, general purpose machine shop is perhaps the most familiar and best illustration of this. These usual mechanical types in general purpose industrial motors will be discussed in particular reference to some applications which, for mechanical or chemical reasons, exceed the suitability of general mechanical types, and require something special in the electrical design, either of the motor or of the control system. The rubber calender drive already referred to, and shown by Fig. 1, is one of those I requiring special control.
ELECTRICAL MOTORSFOR CHEMICAL PLANTS The smaller mechanical power applications in chemical plants of all kinds frequently require something special in the motor itself, in regard to its insulation. The following substances, frequently found in the atmosphere in mills, factories, and industrial plants of all kinds, and particularly chemical, electrochemical, and electrometallurgical plants where the substances are manufactured or used in quantity, are injurious to insulation: (1) Oil-splashing, dripping, or flooding. ( 2 ) Severe moisture conditions, dripping water, steam, high humidity. ( 3 ) Carbon, iron, graphite, coal, coke, and abrasive dusts. (4) Nitric, hydrochloric, sulfuric acid; ammonia; caustic soda or potash, alkali fumes.
In chemical plants or by-product coke plants, where the acid conditions are found to be so severe that a considerable
924
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
amount of acid will condense and collect on the commutator, it is not advisable to use direct current motors, as the acid will attack the commutator and interfere with successful operation. The critical applications, as indicated, are usually the small ones. Large motors, where required by a large machine, or economically preferable in a group drive, are usually protected to a degree by greater distance from
F I G . 6-5-H. P., TYPE “CS” MOTORM O U N ~ E D O N WALL,AND BELTED AGITATORIN A CAUSTICSHED. XOTEFUMES PROM AGITATOR
TO AN
the injurious substance, by being placed in a separate room (Fig. 2 ) or by being specially ventilated with pure air. Where acid or alkali fumes are densest, the less driving mechanism to be attacked the better, and hence the more direct the drive the better. In some applications, therefore, it is practically essential that the motor be directly connected to, or very near, the source of dangerous fume, and it is not economical to give i t special ventilation. These applications practically all come within the capacity of a 5 , 7.5 or 10-h. p. motor, or thereabouts, and it is essential that motors thus applied have special protective insulation. These motors, and in fact all motors so far as possible employed in the four classes of service indicated above, should have squirrel-cage rotors, for the reason that such rotors can be readily dipped in a protective acid-resisting varnish, whereas slip rings, necessarily exposed, are ever subject to attack. The insulation employed on general purpose industrial motors is cotton fibrous material, impregnated with a substance having good insulation properties, which replaces the air between the fibers, even if it does not completely fill the spaces between the insulated conductors, entirely covers the fibers, renders them adherent to each other and to the conductor, does not produce interstices within itself as a consequence of evaporation of the solvent or through any other cause, does not flow during the operation of the machine at full working load at the temperature limit specified, and does not deteriorate under prolonged action of heat. The impregnation is an insulating varnish of linseed oil or
1701. 15, No. 9
asphaltum base, which, when baked at proper temperature, hardens while it retains some elasticity, or at least does not become brittle. The constituents of this varnish can be varied so as to produce a product little affected by any of the destructive materials which have been mentioned. The cotton fibrous material impregnated in the winding of the general industrial motor is, however, quickly and decidedly affected unless more perfectly coated with protective varnish. Moreover, under any of such conditions, an insulation of higher dielectric strength than the ordinary is desirable. Accordingly, a mica and fish paper wrapper is substituted for the treated cloth wrapper immediately adjacent to the conductors, and additional dippings of impregnating and coating varnish are supplied so as not to leave any little threads of fiber extending through the coating and exposed to oil, water, abrasive, or especially chemical action. When this is done, the motor winding is found to have a relatively long life. This is usually accomplished by giving an increased number of dippings of thinner varnish and bakings, such as experience indicates is necessary to produce absolute impregnation, sealing, and a perfect covering which is hard and yet somewhat elastic. These extra dippings and bakings may be given wholly or in part to the coils alone in the case of open slot motors, or they must be given to the entire stator in the case of the partially closed slot motor. In the former case, coil replacements can be made in the field, while in the latter case, coil and end connections have the same dips and bakes as the rest of the stator. Some industrial plants have a practice for further protecting motor windings under such conditions, which consists of plastering the ends and vents with a mixture of asbestos, plaster of Paris, and shellac. No repairs are made on such motors, but where necessary they are rewound completely. Usually, however, dippings and bakings of coils and motor, if properly and thoroughly done, give all the protection that is necessary, and are economical. A balance must be struck between the cost and reliability of repairs with such treatment as can be given on the field and that of completely rewinding. A practice which is worthy of recommendation as giving motors exposed to these conditions all the protection economically justified or desirable, follows a schedule of treatment in manufacture approximately as follows : The usual standard motor of open slot construction, up t o 25 or 50 h. p. a t 600 volts, being treated as follows: Uninsulated coil given a t least one dip in asphaltum base varnish.
F I G . i--6QrJIRREL
CAGE M O T O R BELTED TO IN
FERTII.IZER PLANT
GRINDER
September, 1923
I S D VXTRIAL Ai’D ENGINEERIATGCHEMISTRY
F I G . 8-100-H. P., 4 4 0 - V O I . T , % P H A S E , 6 0 - C Y C L E h f I L L M O T O R B E L T E D MOTOR ALSO DRIVES TO A KENTLwlLLU S E D FOR C R U S H I N G B A U X I T E . %IS A CONVEYOR
Treated cloth wrapper used. Insulated coils give not less than one dip in linseed oil base varnish, or the partially closed slot construction with untaped end windings being given two or more dips in asphaltum base varnish, the special conditions mentioned above would be met as follows. 1-Open slot, a t least two dips linseed oil base varnish on uninsulated coil, fish paper and mica w-rapper not less than two dips same varnish on insulated coil. Partially closed slot, end windings all taped, at least one dip asphaltum base varnish, followed by at least one dip linseed oil base varnish. 2-Open slot, same as No. 1. Partially closed slot, same as No. 1, except all dips in asphaltum base varnish. 3-Open slot-recommended construction-at least two dips linseed oil base on uninsulated coil, mica wrapper, a t least four dips same varnish on insulated coil, a t least four dips on whole stator. When partially closed slot motors are employed in this service at least four dips asphaltum base varnish for the completely wound stator are recommended. The special purpose t o be accomplished by all this dipping and baking is t o procure a smooth, glossy surface with no breaks or rough spots for abrasive dusts t o act upon, and t o give increased dielectric strength, owing t o materials of class No. 3 being such good conductors. Linseed oil is hard t o dry where not exposed t o air, and when not perfectly dry its insulating
925
capacity is reduced Asphaltum base varnish is a good insulator whether hard or not. 4-Open slot-recommended construction-complete perfect coating essential, no fibers protruding; varnish, largely plastic asphalt with some linseed oil and gum-not less than two dips on uninsulated coil, mica wrapper, not less than four dips on insulated coil, not less than six dips on completely wound stator. This varnish has good filling qualities and good dielectric strength. It produces a fairly hard coating of sufficient elasticity and unaffected by these atmospheric impurities. It bakes in 8 hours at 110” C. Where partially closed slot motors are employed in this service, a total of a t least six dips in t h e sameqvarnish for the completely wound stator are recommended. CHEMICALPLANTAPPLICATIONS Liquor Handling. (A) Lifting. (1) Direct-coupled, high-speed, motor-driven, centrifugal pumps (such as Fig. 3)-vertical units being especially desirable for protection of motor. (2) Smaller plunger pumps may be included in group drive along with small vacuum pumps, and also agitating or stirring mechanism. I n such case motors are protected somewhat b y distance. (3) Compressed air for corrosive liquors supplied by electrically driven compressor plant a t distance from danger (Fig. 4). (B) Agitation or stirring: Whether in open tubs or vessels or in closed pots, autoclaves, etc., the power requirements of individual devices are small, about 1 t o 2.5 h. p. The low-speed paddles or blades, or pump-like impellers are usually most satisfactorily driven by bevel gears from a cross shaft supported on the structure of the vessel, and finally belted through fast and loose pulleys t o a common countershaft running a t from 120 t o 160 r. p. m., serving a number of vessels, as well as liquor pumps of plunger type, crushers and elevating devices for handling solids, and possibly vacuum pumps of small size in conjunction with individual closed vessels, or adjacent ovens for dehydrating or drying some of the products, and driven b y constant speed, single reduction, belted motor. Fig. 5 shows similar device direct-motor driven through reduction gear, and Fig. 6 shows another operated by belted drive. Solids Handling. (A) Grinding and mixing: (1) Individual drive, direct coupled and flexibly coupled. (2) Belt drive (Fig. 7). (3) Slow-speed devices, usually b y reduction gearing or countershafts. These devices usually take not over 5 or 6 h. p. each, except at starting, and are generally suitable for group drive. (B) Crushing : Individual drive by belt or gear, usually with suitable flywheel (Fig. 8). (C) Conveying, mixing, screening (Fig. 9). (1) Mixing: ( a ) Worm and stirrer types are often in sizes suitable for individual drive, usually through gear trains. ( b ) Cylinder and ball mills, usually group drive. (2) Screening, small aggregate power : ( a ) Individual drive, on account of room taken up. ( b ) Included as a unit with the grinding equipment and driven by same motor. Separating Materials. (A) Solids: Screening, covered by “Solids Handling C2” above.
(B) Li&ors: (1) Settling tanks, requiring pumps merely t o fill and empty. (2) Centrifugals : ( a ) Geared up from usual standard motor. ( b ) Driven by special high-speed motor. Motor requires No. 4 special impregnation if liquor is corrosive. Elevators and Hoists. As has been indicated, these are usually driven by direct current motors. I n any case, total enclosing of control gear is required, or special ventilation for it, and special pipe ventilation, or special insulation for the motor. Both control and motor may, however, be installed in a separate chamber, F I G . 9-30-H. P. MOTOR DRIVING BLOWER A N D C O N V E Y O R FOR HANDLING and not in the usual space directly above the elevator shaft. These precautions must be considered since the elevator shaft ALUMINATAILINGS.NOTEC L O U D O F ALUMINA D U S T I N W H I C H T H E acts as an excellent chimney and draws through i t vapors reMOTOR O P E R A T E S , A N D T H E DEGREE T O W H I C H D U S T HASBECOME PLASleased below. T E R E D OVER T H E MOTOR
