The Electrification of Factories

THE ELECTRIFICATION OF FACTORIES Advantages of Electrification.—In no situation have the benefits accruing from industrial electrification been more im portant than in the factories, where the amelioration of the work ing conditions has been able to affect the lives of so great a portion of the population. Yet through lack of a sufficiently disseminated understanding as to the advantages thus held out, the improve ment is far from being as complete as is possible, either as re gards economy of working or convenience. There is much to gain, therefore, from a consideration of the available benefits, and the means whereby these may be fully secured.

If the old method of single-unit drive for an entire factory by means of a steam or internal-combustion engine and lineshafting is contrasted with that employing electrical means, two changes in principle are found to take place. First, it is not compul sory to lay down an engine-room at all, and the capital so locked up may be devoted to an extension of the factory proper. Sec ondly, the transmission of power about the works is effected electrically, and thus invisibly, safely and economically, instead of by the mass of shafts and belting which is so conspicuous a fea ture in the old order of things. At once, without going into details, several important advantages are evident.

First, the saving in running cost is great. The average expense of supplying a works with motive power by means of a steam engine and the usual belts and shafting, assumed to be in good condition, may be taken as 2% of the value of the output. This figure may be greatly reduced if the machines be driven in groups by motors of, say, io to 20 h.p.; while if all the belts and shafts are dispensed with, and a motor installed for every machine, the cost may undergo an even greater decrease.

Secondly, the full consequences of a breakdown in the mo tive power or in an important link in the transmission are greatly reduced. Thirdly, the power supply is now much more direct, and is therefore subject to less fluctuation. Fourthly, the interior of the factory is transformed and improved in every detail by the absence of the mechanical transmission; while all the additional advantages of electrical working are provided. The individual gains comprehended in the last category are worth enumerating in some detail:-- (I) The factory is made more fit to work in; it has gained greatly in cleanliness, wholesomeness, safety, illumination and consequently in the standard of work that can be turned out in it. (2) Belt troubles are done away with. (3) The factory structure may be more lightly and cheaply constructed, as it does not have to withstand the stresses of belt drive. (4) Fac tory transport is very much facilitated by the possibility of installing overhead cranes wherever required. (5) The whole of a shop need not be run during overtime; while with individual drive only those machines actually required are actuated. (6) Fly-wheel operation at variable speed may be employed, with out special arrangements, for fluctuating loads. (7) A greater amount of power is available at individual machines, giving a greater overload capacity. (8) With individual drive the closest and the most constant speed regulation is possible for every machine, while variable speed may be obtained in any number of steps by field adjustment, with greater ease and exactness than is possible by mechanical means. (9) The difficulties of installing line shafting in concrete buildings are avoided.

Motors may be installed on machines at the factory of the ma chine builder, thus insuring the correct motor for the application. The initial investment in power-driving equipment is higher in the case of a factory using individual drive than in one using the older method of line-shaft drive. The advantages of the former, however, so unquestionably overbalance the latter that there is today practically no justification for the older method.

In designing the electrified factory, the best results will be achieved by bearing these possibilities in mind, and taking pains to secure them to the full ; and at the same time avoiding the introduction of any fresh drawbacks not possessed by the old scheme. The latter category might include, if the installation were carelessly effected, electrical dangers to the building or workpeople, and risk of breakdown through damp, dirt, mechan ical weakness or inexpert handling. Details of design are con sidered below.

Size of Groups.

As the advantages that have been enumer ated are the more fully experienced the more nearly the ideal of individual drive is reached, it is obvious that the size of the groups should be made as small as practicable. All authorities are agreed that the larger equipment, such as punching and shear ing machines, large planers, borers, lathes, etc., should be individ ually driven ; but many still consider that the high cost of numerous small motors as compared with a single large one is not justified. On the other hand, many others, who are more willing to break away from the old traditions, maintain that individual drive is preferable in every case, even with the smallest machines; and the author inclines to the latter view. However, the present section will deal with those cases of group and individual drive in which special control gear is not required, and the more advanced cases of automatic individual control will be considered under MACHINE-TOOLS.

Motors.

From what has already been said it is evident that line shafting, requiring as it does a constant speed, should be operated either by shunt or squirrel-cage induction motors, depending on whether D.C. or A.C. is available. If there is a choice of system, the A.C. is preferable, on account of its smaller cost and greater robustness. Care should be taken to locate the motors in positions where they are out of harm's way and do not use up valuable space, such as on the driven machines them selves, or upon the ceiling, walls or columns of the building. Many machine-tool makers provide a seating for a motor as a matter of course, e.g., above the headstock in the case of a lathe ; and it is to be hoped that it will not be long before the practice becomes general.

Control Gear.

The control gear simply consists of a starter, sometimes alone (as with A.C.) and sometimes accompanied by a knife switch and fuses. For individual drive, such as in textile factories, all that is often needed is a simple on-and-off switch, such as a push switch, or one of a small drum ("loom") type, mounted at a place convenient to the operator's hand. The larger and more complicated apparatus may be mounted on the wall or column, or may be at the "pillar" or "pedestal" type. All should be ironclad and as fool-proof as possible, and should be easily accessible to the operator. For starting comparatively large motors driving line shafting, an "inching"' starter mounted in a control pillar has the great advantage of reliability under inexpert handling. As illustrated in the diagram forming fig. 2, the contact arm may with impunity be "backed" in a way that would ruin the studs of an ordinary starter ; for here the loose "follower" arm will lag behind and cause the circuit to be broken effectively at the contactor. Such pillars are usually provided also with overload relays, which at once open the contactor if the arm is moved over too rapidly, or if similar misuse occurs.

Electrical Design.

The great importance of designing an electrically-driven apparatus as a harmonious unit, and not as an aggregate of a machine, a motor and a more or less unsuitable starter connected by wiring but otherwise not co-ordinated in any way, applies with special force to machine tools.

The policy should be rigorously observed of making both motor and control gear as much part of the machine tool as any of its mechanical components. Moreover, the handle controlling the movements of the motor should be located among the other handles and in a position as convenient to the operator as pos sible, and the whole scheme should be so designed that this handle needs no greater care in its operation than any of the purely mechanical levers. The operation of machine tools will be considered under the following headings:— 0) Small-power Tools.—Many tools, such as portable drillers, grinders, etc., as well as small bench drillers and similar ma chines, are driven by small motors that require only a simple switch for setting them in motion. Portable tools in particular are usually equipped with a series-wound "universal" motor suit able for D.C. or A.C. of any periodicity, and having a capacity seldom exceeding i horse-power. Tools having induction motors not exceeding 2 h.p. also come under this category, as do larger induction motors when the supply authority does not insist on star-delta or auto-transformer starting. A robust push switch or simple drum switch is quite appropriate here. A no-volt release is a desirable addition, but is seldom fitted for the smaller examples.

(2) Auxiliary Drives.—A slightly more elaborate installation is required for performing the auxiliary functions of a large machine tool, such as raising and lowering the tool-head, radial arm or cross-bar of large boring mills, planers and radial drillers. Series D.C. or squirrel-cage A.C. motors are usually employed for these purposes, of about 5 to i o horse-power.

The D.C. motor is somewhat the more convenient on account of its self-braking capabilities; and fig. 3 shows a compact drum controller which completely governs such a motor without requir ing a magnetic blow-out, or more than the single small resistor. There are four positions of the drum in all, but these in reality constitute three for each direction of rotation. When the cylinder is moved to the position at one end or to that at the opposite extremity of its movement, the armature is connected across the line in series with the resistance for forward or reverse run ning. When it is moved one step back, the line is disconnected and the motor "drifts" ; but upon a further movement being made, the armature and resistance are connected to the field in the correct direction to drive a powerful current through the circuit, bringing about a quick stop by "dynamic" braking.

(3) Small Lathes, etc.—Many machines of moderate horse power such as small lathes, shapers, drillers, etc., that require infrequent starting and no reversing or braking, and usually no speed control, are driven by shunt or squirrel-cage motors and simple ironclad starters. The latter are usually hand-operated, but automatic starting is preferable, especially in the D.C. case. A simple scheme would consist of start and stop push buttons, or other form of pilot switch, operating a 25 ampere line contactor followed by one or two accelerating contactors of similar capacity.

The most convenient method of operating a manual drum starter, or a master switch for contactor gear, when employed for lathe control, is to mount it horizontally at the end of a spline shaft which is rotated by a lever situated upon the tool saddle.

(4) Radial Drillers and Screwing Machines.—Machines of moderate horse-power (e.g., 5 to io h.p.) that need repeated starting and reversing and must be lever-controlled, but do not call for special braking or speed regulation, mostly require auto matic control gear and are represented by a radial driller having a built-in compound motor. The controller may be built into a pocket in the radial arm, or may be mounted out of the way at the back of the arm, and operated from the handle at the tool-head by means of bevel gearing. Great simplicity may be given to the control scheme by the use of a reversing drum switch for making, breaking and reversing the line connections, the first current-limit relay being mechanically interlocked with the shaft of the drum in such a way that its armature is released as soon as the drum contacts have met; and the acceleration then pro ceeds as usual. Upon the drum switch being completely reversed, the motor is brought to rest by "plugging" and immediately accelerated in the opposite direction.

(5) Boring Mills.—Machines of fairly large horse-power, such as boring mills, that require to be started infrequently in one direction only, but must be capable of being inched both forward and back for setting up, require automatic push-button operation. Shunt-wound D.C. or slip-ring induction motors of about 25 h.p. are used for this purpose, a suitable control scheme, designed for a D.C. motor, being shown in fig. 5. This gear is housed in a control pillar located near the motor and is interesting in that it illustrates the application of inching buttons and no-volt protection to a scheme. The push buttons are all of the "spring off" type, making only momentary contact.

Direct current equipments sometimes have speed control by field regulation, and dynamic braking to render inching more effective.

(6) Wheel Lathes.—Large lathes for wheel and tyre turning must have provision for slowing down at "hard spots," or for turning cranks. A push button is usually located on the head stock for effecting the slow-down, and this is accomplished by the cutting out of field resistance with a variable speed motor or else by the cutting in of the armature or rotor-starting resistance. D.C. compound or slip-ring induction motors of about 15 to 40 h.p. are employed, the compounding in the former case being required for steadying the running when operating with a weak field.

(7) Large Planers, Slotters and Shapers.—Reciprocating ma chine tools of fairly large power, such as planers, Blotters, shapers, etc., that require reversal of the motor at the beginning and end of each stroke, demand a special form of motor and the most advanced form of contactor gear. There are three grades of planer which may be taken as typical, viz., plate-edge planers, ordinary planers with a speed range not exceeding 3 :1, and those with a range of about 1o:1. The first cuts in both directions at an even speed, and problems as to reversal and braking are less severe than with the other forms. The latter have a shorter stroke, must reverse very quickly and accurately and therefore need dynamic braking, and nearly always need an accelerating arrangement for saving time when the tool is "cutting air" and for permitting the tool to enter the metal slowly when planing sheets. In general, a special "planer" motor is used, having a low diameter armature to decrease the stored energy and thus secure quick reversal. Owing to ease of dynamic braking and speed control, D.C. compound motors are at an advantage.

Both A.C. and D.C. motors are used for plate edge planers. The control gear is usually operated by a stationary switch opened and closed by an endless cord. For ordinary planers with a 3 :1 speed range, a D.C. compound motor of about 20 h.p. is used, controlled by contactor gear having a single accelerating contactor, push buttons for starting and stopping and inching both ways, double-throw dynamic-braking reversing contactors, two field rheostats for cutting and return strokes respectively, and an auxiliary switch turned "on" and "off" by the platen for bringing about quick speed during a portion of the cutting stroke by temporarily changing over to the return-stroke rheostat.

For all A.C. equipments, and for D.C. circuits when a greater speed range than about 3 : r is required, the Ward-Leonard type of control is used. Instead of the potentiometer regulator be ing employed, two ordinary rheostats are provided, for cutting and return strokes respectively, and a switch actuated by the platen changes over from one to the other.

In a io : I range planer, it is usual to obtain a 2 : i adjustment of the return speed by field regulation of the reversing motor, and a 5 :I adjustment of the cutting speed by means of the generator field rheostat.