THE MOTOR DRIVE [1]
By Asst. Naval Constructor Richard D. Gatewood, U. S. Navy.
1. Both in this country and abroad the various phases, and there are many, of this subject have been taken up in papers published by the different engineering societies and journals, and in nearly all cases the writer of such papers, though considering only a particular division of the subject, begins by lamenting the lack of time and space to enable him to do full justice to his article. What then can be said when an attempt is made to treat the entire subject in a comprehensive way in one small paper?
2. In order to keep the discussion within reasonable bounds, every effort toward brevity will be made, as it is intended for those officers who are desirous of knowing where the subject stands to-day, enabling them to decide accurately and correctly the various questions that arise in the course of their work.
3. The applications of the electric motor that come up for the naval officer’s consideration are (1) its use in shops, and (2) its use on ships.
Application in Shops.
4. At the present time to discuss the advisability of introducing the electric motor as a “ sound and conservative method of furnishing power to shops ” would be to display an ignorance due only to woeful lack of reading and observation on the part of any one who is supposed to be conversant with the subject. There are, of course, many other methods of power distribution—steam, water, air, rope, wire, and belts and shafting all have their proper places. The advantage of the electric drive over any one of these is somewhat involved and depends, of course, upon the nature of the work to be performed, but that the motor is better adapted for many of the forms of shop work than any of those above is now universally recognized. It is practically impossible to give exact figures on this subject, as it is so intimately bound up with improvements in tool steel, with increased modern shop-efficiency methods in general, and with changes in the shop incident to change of drive, etc., but the following facts and estimates are believed to be entirely conservative, and will perhaps serve to give a concrete understanding of this matter.
5. In navy yards the power plant is installed within the yard limits and little question arises over the transmission of power for any considerable distance. In the case, then, of a localized power plant, the general uses of electric motive power are four in number: (1) to distribute the power from the above plant more economically than can be done mechanically; (2) to “take advantage of the singular mobility which electric distribution permits ” in getting the full use of the portable machine tools that are now being turned out in such numbers; (3) to introduce electric traction within the yard limits; and (4) to admit of economically centralizing the mechanical plant for supplying light, heat and power.
6. The advantages of electric drive over mechanical transmission are as follows:
(1) Cheaper power.
(2) Increase in output.
(3) Better control and steadiness of power.
(4) Greater speed range.
(5) Greater convenience in arranging machines and materials.
(6) More economical disposition of floor area.
(7) The ability .to shut down departments without interfering with other departments.
(8) Almost instantaneous response for sudden demands of power.
(9) Ease with which additions can be made.
(10) Increased cleanliness.
(11) More effective light and sanitation due to better grouping.
(12) Reduced cost of repairs and maintenance owing to absence of shafts, pulleys, hangers, belts, gears, etc.; also owing to simplicity and fewness of parts of motors.
(13) Reduced cost of building when planned or when reconstructed for motor drive.
(14) Size of shafting can be much smaller in case of small machines because they can be grouped and power applied to shafts near center of their length.
(15) Volts and amperes being more easily measured than torque and speed, the power consumption can be more definitely determined for a tool, and so a more nearly exact and more useful estimate formed of work capable of being turned out.
(16) In a general way the field of work is broadened, leading to developments impossible by former methods.
(17) Elimination of complication and physical effort in handling machines. ,
(18) Full power limit of machines utilized.
Cheaper Power.
7. The mere economy in power is not so potent as some electrical firms endeavor to make it seem. A glance at the accompanying. diagram will convince one of this fact. From this it is seen that the cost of power is only about 2.1 per cent to 2.5 per cent of the cost of the finished article. As a matter of fact, from a comparison of reports on tests of power plants from the different navy yards, it is fair to assume the cost of steam power to be about $55.00 per horse-power per year (3000 hours). With belt and shaft system, it is estimated that the cost of this power would amount to about 2.5 per cent of the value of the output; with motor “group” drive it would amount to about 1.8 per cent of the value of the output; with individual motors on all large tools (above 3 horse-power), it would amount to about 1.2 per cent. Assuming a yearly output of $500,000, the result is:
Drive. Cost of Power. Saving.
Belts and shafts.................... $12,500
“Group” drive ........................... 9,000....... $3,500
Individual motors ..................... 6,000......... 6,500
These figures are rather conservative, but serve at least to give an idea of what is meant by a decreased cost of power. In connection with this yearly saving of $6500 must be considered the increased first cost of the plant equipped with motor drive; it will be found that the interest on the increased, cost exceeds the saving of $6500.
8. In this subject of cost is always to be considered the question of whether a new shop is to be installed or whether the old shop is to be fitted with motor drive. Of course, in the case of installing a new shop (which we will suppose to have the same output as an old one with belt and pulley drive), it must be remembered that if a great saving of power can be effected, the first cost of the power-producing plant can be materially reduced, which will help to counterbalance the increased cost of the machines in the shop itself.
9. On this question of a saving of power, it would be well to have definite figures for general use, but, as before stated, it is exceedingly difficult to obtain any reliable authoritative data on this matter. A close examination of the engineering press for the past few years will result in finding a number of articles nearly all of which give some presumably reliable figures on the subject; but when the methods of obtaining these figures are analyzed, it is found that in nearly all cases the premises are false and the hypotheses untenable. As an example of this may be mentioned one method which to a number of investigators appears very attractive, largely, no doubt, for the reason that it is apparently so simple and logical. The amount of power required .to run the particular shops under investigation was determined either by electrical instruments or by steam indicators, when running with and without load. The difference of the two readings was presumably the loss. The wrong assumptions are (1) that the frictional load of the bearings does not change with the amount of load; it does; (2) that the pull of the belts does not set up strains in the shafting which cause losses, probably as great as the losses in the bearings. The following figures, while not scientifically exact, are believed to be at least conservative; and while they cannot, perhaps, be applied to all cases that arise, they will serve to give something definite on this interesting phase of the problem.
10. Suppose a certain maximum H. P. is required by all of the tools of a shop (say 100 horse-power), then it is required to have at least 80 horse-power to transmit this effective power by belts and pulleys and shafting. This loss of 80 horse-power remains constant whether or not the effective horse-power is reduced; thus there must be generated 180 horse-power. Now should these same tools be driven by individual motors instead of by shafting, only about 50 horse-power is required for transmission, even if every tool be running. But in the average shop every tool is not running at the same time, some being shut down and others operating on less than maximum horse-power. In the average manufacturing establishment it has been determined that the mean effective power used is but 60 per cent of the average effective power required when all of the tools are running at a maximum load. This percentage is called the load factor. In the case, then, of a belt drive, we have 80 horse-power losses for 100 horse-power effective load, hence 55.6 per cent efficiency. With 60 per cent load factor there is but 60 horse-power to use, and as it requires 80 horse-power to transmit it, the efficiency is 60/140, or 43 per cent. With the individual motor the loss varies with the load. Taking the same load factor as before, and assuming 60 per cent to be the loss incurred by operating with 60 horse-power effective load, there is but 60 per cent of 50 horsepower loss against 80 horse-power with belt drive. Thus with individual drive there is generated 90 horse-power of which 60 horse-power is effective, giving an efficiency of 60/90, or 67 per cent, as against 43 per cent with belt drive.
11. The effects of variable and constant speed motors must also be considered. Assume that 60 per cent of the motors are to be variable speed motors; these will have a rated capacity somewhat higher than they will be called upon to supply. Assume that the other 40 per cent have rated capacities the same as they are to supply; as their speed is constant this assumption is justified. Of the total rated capacities of all of the motors, assume that 56 per cent represents the mean power that all the motors are required to turn out, if all are running at the same time. Applying same load factor, 60 per cent, we find that the average power to be supplied to the motors is 33 per cent of the total rated capacities of the motors. This 33 per cent cannot be assumed to represent the capacity of the generating plant, but the figures will give an idea at least as to what is meant by cheaper power. It may be added that in the installation of motor drive in a shop where all the tools to be installed can be foreseen at the outset, and where no further provision need be considered for the addition of such tools in the future, it has been found good practice to install one generator horse-power for every four motor horse-power.
Increased Output.
12. Now as to the question of increased output: In this are involved many points worthy of consideration. First of all, it is found that the increased output is due to three things: (1) Better control of machines; (2) greater speed range, thus enabling the machines to be used at their full power limit; and (3) eliminating complication and physical effort in handling machines. The “more subtle factors, such as sanitary conditions and light,” must also be considered. The methods of control and the elimination of physical effort will be taken up later in detail (see photographs of installation) but a word or two here will be in order.
13. As shown in a previous diagram, the value of direct labor is some 55 per cent or 60 per cent of the cost of production, and any method of reducing the time spent upon a given piece of work is of course a very important one. The object of machine-shop work, as everyone knows, is to do a given piece of work most efficiently the first time. Now, this “piece of work” in question varies greatly in kind. Heavy cuts and coarse feeds can be used on some work and not on others; some work requires constant speed, some varying- speed. The question of constant and varying speed motors is considered later, but this can be said now; that where work is heavy and speed has to be varied often, or where work is light or of but short duration, it is essential that “ease of handling” be obtained. Also, in heavy work particularly, we naturally want to work the machines up to their full power limits all the time, and to do this we must know the value of several important factors before we can save time. We must know the kind of work, the character of the metal to be machined, the uniformity of tool steel and the size of cut it will pull, and the most severe work that will be required of the machine.
14. Taking a concrete example, let us suppose that certain castings have been received, some of steel, some of wrought iron. A definite amount of metal must be removed and a certain finish given in the shortest possible space of time. How can we do this? The cutting speed for steel is determined by the tensile strength and elongation, while for cast iron the hardness is affected by chemical composition, method of cooling, and size of casting. Knowing these, and we can determine them rather simply, it is possible to determine approximately the speed to be used in cutting. It is then merely a matter of knowing which notch of the controller and which gears to use, and the speed is obtained in a moment. Too close speed regulation is not necessary because estimates, even the best, cannot be much closer than 10 per cent or 20 per cent.
15. In work of varying diameters and requiring varying speeds, especially on large machines, “it is impossible to have the machinist shift his belt as often as he should, as by so doing he would be physically worn out by the end of the day; or if such a condition could be arrived at, it would last only a short time, the workman soon relaxing his efforts until the old conditions exist again. The motor, with its great facility of control, removes all these difficulties.” The above quotation is perhaps exaggerated, but there is unquestionably much truth in it. In addition to the loss of output due to not shifting belts, there is, in a great many cases, an added loss due to improper belts, they being too narrow or not properly tightened, both of which faults can greatly reduce the output. From a careful comparison of all of the data at hand on the subject, it is estimated that machine tools equipped with electrical motors will show an increased output throughout a navy yard of 20 per cent over mechanically driven tools.
Better Disposition of Floor Area, More Light and Sanitation, etc.
16. A glance at the accompanying photographs, Nos. 1,2 and 3, shows the enormous advantage gained in the use of motors on machine tools in obtaining best location of tools, disposition of floor area, and clear head room for crane service. In constructing a plant equipped with line shaft drive it is necessary to supply sufficient additional strength to carry weight of shafting, pulleys, etc., to make higher side walls with greater light area to balance that lost through obstructions of above mentioned pulleys, belts, shafting, etc., thus adding to the first cost of the building, and the structure would have to be larger to accommodate the same number of machines as the motor-driven building. As seen from pictures and from actual calculations, the amount of floor space needed to install a given number of motor-driven machines is only about 60 per cent of that required for successful line shaft operation. It is useless to enlarge upon this well known fact any further.
Cost of Repairs.
17. The question of repairs and renewal of parts has received but scant attention, particularly at the hands of those writers on this subject who are interested from the electrical point of view, and is usually glossed over with the statement that “it is well known that properly installed electrical apparatus needs almost no repairs.” Experience at a number of navy yards shows that the repair bill covering motors is a rather large item, and shows further that direct connected motors give considerably more trouble than motors belted to line shafting. These troubles are principally in the controlling apparatus; the coils in series with the field magnets and the starting coils give trouble. There is also a continual renewal of the contact points of circuit breakers. Then come commutator troubles, necessitating at times complete new commutators. It is believed, however, that the cost of repairs to an electrical installation is considerably lower than the cost of similar repairs on a belt-driven installation.
Best Electrical Equipment Required.
18. In order that the above advantages and figures may hold, great care should be used in the first place to select the best electrical machinery. The accompanying diagrams, taken from an excellent article in the Central Railway Club Journal, illustrate this point very clearly. In Fig. 3, “A” is the efficiency curve of a three horse-power motor of inferior design, compared with “B,” the curve of a well designed motor. Fig. 1 shows the efficiency in percentage of an actual factory operating under various loads, “D” being the curve of electrical efficiency when poor machines are used. Fig. 2 shows the relative coal-pile efficiencies. A glance at these curves, and the story is told of a great many motor-drive advantages.
General Advantages.
19. It would be hard to estimate the advantages due to increasing the field of work of machine tools and to producing developments not yet heard of. One authority says of the belt and drive system, “ It is not a question of does it fulfill present conditions, but will it meet new requirements resulting from the advance in tool steel and our better knowledge of the subject? It certainly will not.” There is also another general statement frequently made which must be given considerable weight. By increasing the production of separate machines, the efficiency of the whole shop is also greatly increased. Other machines are driven faster to keep up with the increased output of those machines performing the prior operation on the same castings; increased handling facilities are introduced to take care of the increased output; modern methods of routing are brought into use, and the entire “tone” of the shop is raised.
Disadvantages.
20. The chief disadvantage is the increased first cost. Actual figures are difficult to give, but it is believed entirely safe to say that in order to install a given number of tools in a building especially built for motor drive, it will cost about 50 per cent more than when built for belt drive. There are other disadvantages of more or less importance such, for example, as that due to disturbance of operation when conduits are flooded and serious grounds are encountered in the mains, or when the navy yard is obtaining a portion of its power from private corporations and the power is interrupted from time to time; also that higher priced men are required to effect electrical repairs than are required in case of belt installation. Considering, however, all the advantages as shown from data collected on the subject of good shop practice, the disadvantages are far outweighed, and no more attempt has been made in the foregoing facts to argue in favor of the motor drive as a reliable and thoroughly trustworthy economical method of transmitting power than would be made in discussing the facts connected with such solid improvements as the card index, loose leaf ledger, modern systems of ventilation, and the like.
Application of Above Facts.
21. We now come to the actual practical application of the above facts. The conditions that arise are two; we have either to design and install the new plant, or we have to make over the old plant. In navy yard work the latter is the more usual. First of all, the kind of work must be known. Then the question of speed enters at once, coupled with cost. It seems to be the practice of the best tool- builders to equip all machines above say 3 horse-power with individual motors. Machines below 3 horse-power are grouped and one motor is used to drive the shaft from which each group takes its power. Just where to drop individual motors and where to take up the group drive is a much disputed question, but the above figures hold in a fairly definite way. Consider now the case of an old shop where machines are to be fitted up with motor drive. To quote from an article by G. A. Damon in the A. I. E. E., 1902, p. 177: “It more often happens that the tool shop equipment is to include a number of tools which have already been used in the old shops and which are poorly adapted to the direct application of individual motors. Take the example of a group of lathes driven by belts from speed cones. To put an individual motor on each lathe would require a multiple voltage or double commutator system of speed control, either of which would involve an investment in motors, controllers, and wiring at least six times as large as the cost of one line shaft motor to drive the entire group, and in addition to this is the expense of special brackets and mechanical connections between the motors and tools. Under these circumstances, it is problematical whether or not the extra investment in electrical equipment is to be justified. The tendency at present seems to be to arrange the old belt-driven tools in groups allowing a 10 to 20 horse-power motor for each group.”
22. This, then, seems to have been the practice in 1902. There has been much improvement since then in the knowledge of the application of motor drive, and though moderation is still used, and must be used, the practice now seems to be to group only tools under about 3 horse-power and to run others with individual motors, the advantage being not so much a question of efficiency as of convenience and shop output.
Alternating Current, or Direct Current.
contained in the following table:
Alternating Current. Either. Direct Current.
Saws, Constant speed Millers,
Pumps, machines in Boring mills,
Spinning machines, general. Cranes,
Wood work, Slotters,
Constant speed machines Shapers,
in general. Planers,
Screw machines,
Variable machines in general.
23. The next question that arises is whether the shop is to be equipped with A. C. or D. C. motors. The uses of the two are
Alternating Current.
24. Now just where does the use of A. C. stand at the present writing; when is it used; why; how controlled, and what is its efficiency, relatively speaking, compared to D. C.? An attempt to answer definitely the above questions will be given below.
25. In general, the particular layout of the shops, the distance the power has to be transmitted, and the lighting of the yard with its buildings, determines whether A. C. or D. C. shall be used. For the benefit of those not thoroughly conversant with the subject of A. C., let us go to the root of the matter as briefly as possible. Quoting from an excellent article by Mr. Pomeroy in the Central Railway Club:
26. Since the amount of copper varies directly as the resistance of the circuit, which varies inversely as the square of the electromotive force, or to express it in a formula,
Amount Cu ∞R∞ 1/E2
then 100 times as much copper is required for delivering a given current at 100 volts as for delivering it at 1000 volts. If the pressure is increased to 10,000 volts, the size of the wire required is 1 per cent of that necessary for 1000 volts, and is but .0001 part of that demanded by a 100-volt current.
“The ideal system must transmit at high pressure and use at low pressure; it must transmit the 10,000 volts and, for example, transform it to 1000 amperes at 1000 volts.
“There is no apparatus for effectively transforming D. C. from high to low pressure, while the A. C. is readily and simply transformed. It is this characteristic that has given the alternating current its great success.
“The description of the induction motor is negative; no commutator, no collector rings, no brushes, nothing to handle but a switch, nothing to wear but self-oiled bearings, no exposed parts, no danger from fire; the machine composed of rotor and stator.”
27. The characteristics of the induction motor are much the same as those of an ordinary D. C. shunt motor, i. e., its speed tends to remain constant when the impressed voltage and the frequency are constant. Within certain limits, say 10 to 15 per cent, for a given frequency and a given number of poles the speed is practically fixed and independent of all other effects, except the effect of resistance in the secondary circuit. By varying this resistance, however, we may vary the speed down to zero, and this is true for a 2-phase or a 3-phase motor, as their construction is practically identical.
28. There are four ways in which the speed of an A. C. motor may be varied practically:
(1) Varying the potential applied to the primary with suitable resistance in the secondary. In regard to this method, it can be said that the efficiency is low, there is a large heating effect in the motor itself requiring a large motor, and the method is unstable at low speeds.
(2) Varying secondary resistance. This is the simplest method and the easiest to control, giving full range of speed, but it is not efficient.
(3) So changing the primary connections as to change the number of poles.
(4) Changing frequency of E. M. F. Every induction motor
tends to run at synchronous speed—60 X frequency/pairs of poles, so that if we impress a different frequency we get a different speed.
29. These last two are considerably more efficient than the two first, but do not permit of as large a speed range. Number 2 is the method most generally used.
30. An induction motor is cheaper than a D. C. motor, but by the time we have paid for transformers and installation for speed control we lose most of the advantage of the saving in copper and the cheaper motor. The repairs on an induction motor amount to almost nothing, and where a constant speed is desired they are much superior to a D. C. motor. The alternating current motor admits of much greater overload with no appreciable change in speed than a D. C. motor. At the present time, however, where anything like varying speed is desired a D. C. motor is, on the whole, decidedly better.
Direct Current.
31. Having decided, then, that we cannot obtain a close, efficient speed control and ease of handling on a constant speed (A. C.) motor, what sort of D. C. type shall we use? Shall we use a standard constant speed, or a special type of motor, and how shall we obtain the speed range?
32. The uses of the three forms of D. C. motors, series, shunt, and compound, are too well known to need further comment. In general, we may say that the series motor is used where variable speed and good speed regulation is desired, the shunt motor where constant speed is required, and the compound motor where a good regulation but not too constant speed is wanted.
33. There are at present four ways of obtaining this speed variation: (1) multi-voltage; (2) three-wire; (3) two-wire; and (4) mechanical variable speed regulators.
34. By means of the multi-voltage there is no doubt but that the most exacting requirements of speed range can be met. In this system the primary voltage impressed upon the motor may be varied into as many steps as the number of wires will give combinations, giving, of course, as many different speeds. These, combined with the pulleys or back-gears in the case of direct drive, will give any speed that might be considered necessary. But such a large speed range and combination of speeds is not needed practically. The multi-voltage system requires balancer sets, as they are called, to take up the difference of voltage between lines, expensive and somewhat complicated interlocked switches, and, in general, more complication and first cost. As now manufactured, variable speed motors fulfill practically all the modern requirements covering range of speed, and the writer does not believe that the multi-voltage system will come into prominence in any future installations.
35. The Edison 3-wire system is too well known to need explanation, and, in the opinion of the writer, is the most economical and satisfactory solution of the problem. The sketch shows the most satisfactory installation for a navy yard. The lights are on a 3-wire circuit as usual, and the motors on only 2-wire; but by means of a small switch at each motor the circuits can be shifted to the neutral wire. By this system, constant speed motors with range of three to one speeds (500-1500 R. P. M.) can be used, and a sufficiently great range of speed obtained for all practical purposes. Much saving in copper and complication in wiring and apparatus is effected, and the system has proved eminently satisfactory and efficient. For a ship the two-wire system is most satisfactory. The reason for this is that the distances are small, and the increased complications due to three wires more than offset any other advantages of such a system.
Combined A. C. and D. C. Installation.
36. A combination of the two currents is often the most satisfactory one for a navy yard. This is particularly the case in such yards as Mare Island, Bremerton, and others located near a large source of supply for A. C. current. Power can usually be purchased from the controlling corporation at a very reasonable cost, and constant speed machines for driving line shafting, for group drives, for elevators, for tools in shipfitters’ shop, and a large number of wood working tools, can thus be driven by induction motors. With such a system the principal difficulty is found in the fact that the navy yard has no control over the supply of current, and in case of a break-down or temporary loss of power, which is quite frequent owing to the long distance over which the current is transmitted, there is a constantly recurring loss of time. In a great many cases the pecuniary loss to the government is practically impossible to determine, and therefore cannot be deducted from the bill of the power company. On the whole, however, it is the opinion of the writer that such a combined installation is distinctly a good one.
Speed Regulators.
37. Of mechanical speed regulators, it only remains to be said that as yet, so far as is known within the experience of the writer, no satisfactory one has been developed. There are many ingenious patents out for this much needed article, but after looking into quite a number of the claims it can be said, with but little fear of contradiction, that an entirely efficient one has not yet been put into actual practice. It is probable that the development of such a device will be perfected along the following lines:
(1) A thoroughly satisfactory mechanical chuck, capable of universal application, should be developed.
(2) An efficient mechanical clutch should be brought out. The steps between such a clutch and a speed regulator would then be considerably shortened.
Application of Motors to Tools.
38. Let us suppose, then, that after due consideration of the problem as a whole, and after an application of the foregoing principles, it has been decided to adopt a certain installation for a new plant, or for equipping an old plant with motors; the power has been carefully proportioned to (1) work requiring constant speed, and (2) work requiring variable speed; and the machines are to be grouped or given individual motors according to the solution of this rather involved problem. In general, it is well to remember that an excellent plan, and one worthy of a considerable amount of sacrifice in smaller matters, is to install standard types and as far as possible to have units all of the same type; i. e., standard types, of the same design and horse-power. Something has already been said of the size of motors to use, but practically,the size is governed by the particular requirements of each machine and by the best practice existing at the time the installation is made. (The naval officer will, of course, use the Bureau’s Specifications for Electrical Apparatus in connection with this part of the problem.)
39. The specifications having been sent out, it now becomes necessary to decide upon how the installation is to be effected, and to know the advantages under the particular conditions of installation of reducing the complication and the amount of auxiliary apparatus, and of installing motors of the simplest type possible.
40. This question is a very important and essentially a practical one, varying with each case and requiring as much engineering common sense as do the other parts of the problem. There are, however, certain general principles to be kept constantly in mind, and the most important of these are the following:
(1) The question must always be looked at from the point of view of the man who is to use the tool. From the operator’s standpoint, ease of handling, simplicity of control, and safety are absolutely essential.
(2) The motor must be easily accessible, yet never in the way of the operator; usually under or at the side of the tool. Some vertical drills have motors at the top of the spindle (see photographs), and this is sometimes unavoidable. This is not a desirable installation, as the effect of vibration on the motor is very noticeable.
(3) Controlling apparatus must be placed so that the work can be watched and controlled at the same time. On a lathe, for example, particularly a large lathe, the controller handle should always be on the apron.
(4) As far as possible, avoid panels for switches and controlling apparatus. They take up useful room and cannot readily be shifted in case need arises to change position of the tool.
(5) Wiring should be carefully protected, usually with circular loom for flexibility.
(6) Controlling apparatus should be well protected.
41. With these ideas in mind, we can almost at once decide the relative worths of the various methods on installation. Some of the most representative of these methods are shown in the accompanying series of photographs collected by the writer.
42. We come now to the use of motors on board ship. For a great many purposes it is not a question of whether we shall use motors or some other form of power transmission, it is simply a fact that an electric motor is the only thing that will do the work and so we cannot consider its advantages and disadvantages compared with other power transmitting devices. On a modern battleship electric motors are substituted wherever possible to do the work previously done by hand or by steam, and nearly every day sees advances made to extend its use still further. Certain it is that the limit of its use is not by any means yet reached.
43. The accompanying table, though not entirely up to date, shows the equipment of some of our battleships, from which at a glance some idea of the amount of power required, and the use to which it is put, may be obtained. In connection with the amount of power required, it is interesting to consider again the subject of alternating or direct current, and determine how this question is to be answered at the present time.
Alternating Current on Shipboard.
44. There are some officers who will be inclined to consider lightly what is intended as a serious consideration of this subject, who will say that A. C. may be all well enough on shore but on shipboard cannot be thought of for a moment. Strange as it may seem, this is not at all the case, and the more the question is looked into the more feasible does it become. At the present time it is not practicable to place A. C. on a ship, but the writer is of the opinion that as soon as a reliable mechanical speed regulator is invented a very large percentage of the difficulty will be removed.
45. Even at the present time it is quite practicable to use A. C. mains for lighting, chain ammunition hoists, blowers (where variable speed is not at all an absolute necessity), and even turret turning apparatus. This last may appear rather startling at first, but it is easily explained as follows:
46. There are three distinct systems of turret turning control now in use in the service: The Leonard system, the Cutler- Hammer system, and the Mechanical Speed Gear system. The Leonard system must have D. C. At the present time the power furnished to the turret is given through a separate generator or motor generator set for each turret, and the current in the mains supplying the motor is D. C., but the current in the case of the motor generator might just as well be A. C., and the only change necessary would be to supply an A. C. motor with the set instead of D. C. as is now done. The Cutler-Hammer system consists of one large and one small variable speed motor for each turret. This system presents greater difficulty than any others to the use of alternating current, but the writer can see no reason why, by the use of a motor generator set, this system could not just as well be run from A. C. mains. The Mechanical Speed Gear system is produced by means of a constantly running constant speed motor geared to the turret through a speed gear. Here, again, it is believed that an A. C. motor could just as well be used as a D. C. motor. But the points where the difficulties arise are in the question of boat cranes, ammunition car hoists, and perhaps also in the workshop where variable speed is required. Of course even here we could get D. C. out of our mains by the use of a motor- generator set for each crane or hoist, but this is not practicable and the thing needed, as above mentioned, is a mechanical speed regulator.
47. The advantages in using an A. C. motor on board ship are many. In the first place A. C. apparatus costs less, weighs less, and takes less space. A higher voltage could be used and so less transmission copper. There would be no commutators, no brushes, only a switch to throw for motors below 7½ horse-power, and the motor begins work. Repairs are much fewer and simpler, and motor is more “fool-proof” than D. C. apparatus. For these reasons, then, A. C. would be much better than D. C. on board a ship, but just as in the case of machine tools, what we want in a number of cases is variable speed, and this we cannot now get satisfactorily with an A. C. motor.
48. In regard to blower motors but little can be said. At present the specifications call for a 20 per cent speed variation, but there does not seem to be any very good reason for this, and induction motors with their constant speed and other advantages would then make an ideal blower installation.
Workshop.
49. The remarks made on the application of motors on shore in the beginning of this paper apply equally well, of course, to the workshop of a ship. The machines used are small necessarily, and need not be individually fitted with motors, as is done, however, on some of our very recent ships. The installation would be cheaper, almost as efficient, and weigh less, if one motor were used to run all the tools, as is now found to be the case on most of our ships in commission. Of course the advantages of the individual motor drive would not then be realized, but there is a decided question as to their relative importance on shipboard compared to increased weight, cost, and space required for installation.
Auxiliary Machinery.
50. In the case of pumps and auxiliary machinery in general, motors are now replacing steam in many places, but as yet the “reform” has not extended to the engine room. It is well known that the pumps now in service in the engine rooms are very wasteful in their use of steam, and the efficiency and steam consumption of auxiliary machinery might be much improved by the use of motors. How to install these would be rather a difficult question, but it has been done with success. Boat cranes and deck winches are already motor driven. In the opinion of the writer, anchor engines will soon be in the same class so far as motive power is concerned. The advisability of installing motors for this purpose has already been suggested, one motor to be used for each bower chain, and to be so protected by circuit breakers or other safety devices as to eliminate any risk of their burning out or of disablement due to sudden or sustained overloads when breaking away the anchor. “The chief advantages to be gained by the use of motors are the elimination of the long, heavy, inefficient steam and exhaust leads to the present engine, and the possibility of controlling motor and windlass from .the forecastle where the operator can be under direct control of the officer in charge of the operation.”
Installation of Motors on Shipboard.
51. But little can be said in a definite way as to how the installation on board ship is to be effected, as it is so essentially a practical question, varying with each case. In general, it is well to place motors where there is the least possible vibration and the best possible ventilation, the least danger from dust and sweepings, and where they are accessible for overhauling. If possible, resistance boxes should be placed where they will be well ventilated and out of the way. Wiring should be protected as much as possible, either with piping, or with circular loom in case more flexibility is required than would be possible in the case of piping.
52. The question of protecting the wiring on shipboard is somewhat too large to discuss in this paper, particularly as the corresponding question in the shops has not been touched upon, but it might be mentioned in passing, that in the opinion of the writer, there is too much conduit installed on shipboard, and wiring could be made equally as efficient at a much less cost and at greatly reduced weight.
Note.—It is hoped that in the above paper the most important points affecting the efficiency and economy of the motor drive for use in the service have been brought out, and it is believed that the figures given therein are sufficiently conservative to be entirely reliable for use in practice.
[1] The writer desires to acknowledge his indebtedness to the General Electric Co. for the courtesies extended to him while many of these notes were being made, and particularly to thank the representatives of that company, Mr. Rohrer, and his able assistant, Mr. Burkholder.