The selection of the proper motor to employ under service conditions peculiar to any particular installation is determined by several conditions. The characteristics of the motor should be such that it is best adapted for the service required. The torque required at starting, the maximum torque, speed control and speed constancy, as well as the character of the energy available, are all important factors in the selection.
In many instances, such as isolated power plants, the satisfactory operation of the motor load determines the nature of the energy to be supplied as there are some special services for which only alternating-current or only direct-current motors may be used. Where a wide range of speed is required and where the speed at any one setting should remain practically constant from no load to full load. d-c. motors are used, as no satisfactory adjustable speed a-c. motor has as yet been marketed for this purpose.
The characteristics and application of d-c. motors depend largely on the type of field windings used. Shunt motors used without a starting rheostat will take several times the full-load current at starting and will develop to three times full-load torque. Shunt motors above ½ hp. are usually supplied with starting rheostats which limit the starting current to about two times full-load value. The operating speed of shunt motors is practically constant at all loads. This very desirable characteristic makes them applicable to many conditions unless the starting or overload conditions are too severe.
Compound-wound d-c. motors will develop higher starting and maximum torques than will shunt-wound motors having the same current input, but the operating speed is subject to greater variation. They should be applied where high starting torque is desired and where moderate speed variations with change of load are not objectionable, or where starting is frequent.
Series-wound d-c. motors develop higher starting and maximum torque than either the shunt or compound types, but while operating the speed varies considerably with the load, increasing as the load is decreased. Because of this fact they should be used only where they may be geared or directly connected to the load, as the speed becomes dangerously high at very light loads and the motor may race to destruction if entirely disconnected from its load.
Alternating-current motors are now of greater importance than direct-current motors because of the more general use of alternating current in distribution systems.
Synchronous motors are strictly constant-speed motors at all loads up to the "pull-out" point or maximum load possible to carry. They operate in synchronism with the line frequency and have no speed change if the frequency remains constant. Ordinarily with motors of this type the starting torque will vary from 20 to 30 per cent of full-load torque. The "pull-in" torque, that is, the torque the motor will develop during the time it is passing from the speed at which it would operate as an induction motor to synchronous speed is about 10 to 15 per cent of full-load torque. Synchronous motors are therefore not adapted to applications where it is required to start and accelerate large loads. Their use is also limited by their requirement of direct-current field excitation.
Induction motors are very widely used for many conditions of service. They are generally classified according to their secondary windings as squirrel-cage or wound-rotor, or according to the number of phases as single-phase or poly-phase.
Single-phase motors are not generally made in large sizes but are very extensively used in fractional-horsepower sizes for various appliances.
Repulsion-induction motors are also used largely for single-phase service where the starting torque required is high and the starting current should be low.
Poly-phase induction motors in both the squirrel-cage and wound-rotor type are made in numerous standard sizes from fractions of a horsepower to several hundred horsepower. Both squirrel-cage and wound-rotor motors may be considered as either constant speed or varying speed according to the amount of resistance which is put into the secondary or rotor winding.
The drop in speed of an induction motor from no load to full load is known as the "slip." It is proportional to the amount of resistance in the rotor winding. If a motor has a small slip, that is, a low-resistance rotor winding, it is considered a constant-speed motor. The speed and torque characteristics of such motors compare with those of a d-c. shunt motor. If the motor has a high resistance in the rotor winding its slip is comparatively high, and the motor is therefore suited to varying-speed work, such as would be obtained with a heavily compounded or series d-c. motor.
The choice between squirrel-cage and wound-rotor types depends upon the effect of starting conditions on the line or generator and also upon the speed control desired. A low-slip squirrel-cage motor should not be connected to a line where the motor capacity exceeds about 35 per cent of the capacity of the generator supplying power if the starting conditions are severe. This is because a motor of this type develops a lower torque with a higher current and at a lower power-factor than a wound-rotor motor.
A wound-rotor motor will start any load not exceeding its maximum torque and will not draw from the line over 1¼ times full-load current, if the proper resistance is inserted between the slip rings. By varying this external resistance the operation of the motor may be governed as desired and the speed-torque characteristics varied by the control equipment.
Motors are not always classified according to the principles of operation or construction, but sometimes according to the special purposes they are to be used for. Accordingly, the motors described and listed below include these uses, such as railway, mill type, crane and hoist, sewing machine, etc.
The importance of motors from the manufacturing and merchandising standpoint is shown by the fact that over $120,000,000 worth of motors were manufactured in 1920, exclusive of fractional-horsepower motors which were made In large numbers but the number and value are difficult to determine because many appliance manufacturers make the motors and include them in the value of the separate appliances. The value of motors manufactured In 1920 in sizes from 1 hp. to 100 hp. is placed at nearly $100,000,000. Of this amount alternating-current motors represent the largest percentage.
As early as 1914 the a-c. motors manufactured represented about 60 per cent of the total value of motors and this percentage has been steadily increasing. Recent years have seen many small d-c. generating stations closed down and the use of a-c. motors has increased rapidly. They are more simple to operate as a rule, the squirrel cage induction motors, which are the most common, requiring only occasional oiling that can be done by almost any one. There are also many cases where the sparking at commutators or slip rings would cause explosion or other serious trouble in gaseous mines, powder mills, etc., and where d-c. or wound-rotor motors can therefore not be used.
MOTORS, A. C., BRUSH-SHIFTING
These are a-c. commutator type motors with the brush rigging so arranged that the brushes may be shifted around the commutator within certain limits. By shifting the brushes the speed may be varied from 50 to 150 per cent of synchronous speed. The characteristics of this motor are similar to the d-c. series motor, as an increase in load will cause a decrease in speed and vice versa, and are also similar to the slip-ring induction poly-phase motor with external resistance in the secondary. In general, brush shifting a-c. motors are suitable for any application where the slip-ring varying speed induction motor can be used. The brush-shifting motor consists of a rotor with commutator and brushes, and a transformer connecting the rotor with the stator. The stator has a distributed winding similar to the ordinary induction motor. The rotor is practically like that of a d-c. motor or generator. The chief function of the rotor transformer is to reduce the voltage impressed on the commutator; it is simply a series transformer with the primary in series with the stator of the motor and the secondary connected to the rotor through the brushes and commutator.
These motors are made single phase, two phase and three phase. The starting torque may be made anything within the range of the motor merely by having the brushes in different positions. As soon as the motor begins to rotate the speed can be adjusted to the desired value by shifting the brushes. Shifting the brushes in the direction of the rotation lowers the speed and against the rotation increases the speed.
MOTORS, A. C., FRACTIONAL HORSEPOWER
Fractional horsepower a-c. motors are those having a continuous rating of less than 1 hp. at a speed of 1700-1750 r.p.m. In many cases the term is taken to mean motors of ¼ or ½ hp. or less, as the larger sizes are often identical in general design with those exceeding 1 hp. The fractional-horsepower machines are usually single-phase induction motors but sometimes single-phase series motors and very rarely poly-phase. They are usually provided with some special starting device, such as a commutator which is used with a centrifugal starting switch for starting only.
Commutator motors are also made in which the commutator is also used for running, giving both constant and varying speeds. A split-phase auxiliary winding is also largely used for starting. Some of the larger sizes of this type are equipped with a small automatic clutch, which permits starting before throwing on heavy loads. Fractional horsepower motors are widely used for a large number of small household and industrial appliances, such as washing machines, dishwashers, vacuum cleaners, fans, pumps, food choppers, coffee grinders, air compressors, portable drills and grinders, etc.
MOTORS, A. C., INDUCTION, POLYPHASE, SQUIRREL-CAGE
Squirrel cage induction motors are those in which the secondary or rotor consists of a series of bars equally spaced in slots around the core and short-circuited at each end by rings of conducting material. (See Motor, induction.) Squirrel-cage motors are used much more than any other type of induction motor, nearly 90% of all induction motors made in this country being of this type. They are very extensively used in all branches of industry where a-c. energy is utilized, and where adjustable speed is not required. With such wide applications they are made in an exceedingly large range of sizes, ranging up to 10,000 hp.: ratings up to 200 hp. are standard and above that special. Some of the common usages are for nearly all forms of group or individual motor-driven machine tools, such as lathes, drills, boring machines, bulldozers, milling machines, grinders, planers, saws, shapers, etc.; where such machines require adjustable speed, cone or step pulleys, or change-speed gears are used. These motors are also used in steel mills, cement mills, mines, power plants, etc., wherever a-c. power is used.
The advantages that have made their application so extensive are: simple and very rugged construction giving a low first cost and long life, fairly constant speed at all loads, moderate starting torque, ease of starting without any starting equipment for small sizes and simple starting equipment even for quite large sizes. The absence of any brushes, commutator, slip rings or other exposed contacts makes these motors especially serviceable in places where inflammable materials might become ignited by sparking.
Small squirrel-cage motors may be switched directly on the line at starting without drawing an excessive current. In motors of 5 hp. or over the starting current becomes too large if started at full voltage and some means of starting is required. In such cases auto-starters or compensators, which reduce the voltage during the starting period, are widely used or the star-delta method employed. By specially designing the motor to give the "deep slot" effect, larger motors may be started directly from the line with a more moderate rush of current. In obtaining this result, very deep rotor slots and bars are used: this construction gives a high apparent resistance and low reactance at starting and the reverse near synchronism.
MOTORS, A. C., INDUCTION, POLYPHASE, WOUND-ROTOR OR SLIP-RING
Wound-rotor induction motors are those in which the secondary or rotor part has a poly-phase winding similar to that of the primary. See Motor, induction. Wound rotor or slip-ring motors are used in places where speed variation is required. Speed variation may be obtained with squirrel cage motors but the motor must be of very special design which makes it expensive, consequently such motors are practically always constant-speed. With a slip-ring motor the rotor phase windings are brought out to slip rings, and variable external resistances may be connected between these. By properly proportioning the rotor-circuit resistance, the starting torque and starting current may be adjusted to any required value, and by a proper selection of resistance steps any speed-torque curve may be approximated in normal operation.
Motors having wound rotors are used for many of the same purposes as squirrel cage motors, but give the advantage of directly adjustable speed without mechanical belt or gear shifting. Their application to industry for driving machine tools is generally to larger and heavier machines, such as forging machines, bulldozers, hammers, punching machines, bending and straightening rolls, shears, large pumps, blowers, compressors, etc. They are also used very widely for elevators, hoists, cranes, etc., and to a large extent in steel, flour and paper mills.
MOTORS. A. C., INDUCTION, SINGLE PHASE
Induction motors operating from a single-phase supply. Such motors usually have squirrel-cage rotors, though wound rotors are used to a limited extent. A rotating magnetic field is produced, under running conditions, in the single-phase induction motor by the combined action of one magnetizing current in the primary and another in the secondary which is approximately in time and space quadrature with that of the primary. This rotating field produces the torque of the motor as in the poly-phase induction motor.
The starting torque of a single-phase induction motor is zero, for then there is no rotating magnetic field, unless the motor is supplied with a starting device. The most common method of starting is that called the split-phase, which makes use of a starting winding connected in parallel and on the same structure with the main primary winding but disposed from it so as to give a magneto-motive force approximately in space quadrature with that of the primary. The difference in phase between the two currents gives sufficient starting torque for most uses. The starting winding is automatically disconnected as the speed approaches normal value. The repulsion-start method is employed in some motors, making use of the principles of a repulsion motor in which short-circuited brushes bear on a rotor wound like a d-c. armature. The brushes are lifted and the coils automatically short-circuited as the speed approaches normal.
Single-phase induction motors are very common in the fractional-horsepower and other small and medium sizes and are used to drive a large variety of machinery at approximately constant speed. Fractional horsepower, motors are separately listed. The larger ratings of these motors are made in standard sizes of ¾ hp. to 50 hp. for 60 cycles and 1 to 20 hp. for 25 cycles. For ratings exceeding these it is preferable to use the poly-phase induction motor. Central-station companies place a much lower limit on the maximum rating of these motors that may be connected to their lines, in order to prevent serious unbalancing of the phases.
MOTORS, A. C., MISCELLANEOUS
There are many types of a-c. motors manufactured that arc not used quite as extensively as those specially listed herewith. The characteristics and principal features of some of these miscellaneous types are described under Motor, repulsion; Motor, a-c. series: Motor, a-c. commutator; etc. The principal features of the other motors included under this heading are the methods used for speed control. One type is the spinner motor, which by a combination of electrical and mechanical features gives several speeds. It consists of a stator or fixed primary, a rotor, and between them a spinner rotating independently of the rotor and having a short-circuited winding, which is the secondary for the stator and a slip-ring winding, which is the primary for the rotor. The spinner may be clutched to the rotor or to the stator or allowed to rotate freely. The Deri single-phase motor giving speed control by brush shifting is another motor of this type.
Still another single-phase type secures speed adjustment by an external vibratory contact device that changes the average time per cycle that the voltage is impressed.
Induction motors with commutators are also used to a limited extent, as they may be arranged to have characteristics similar to the adjustable-speed d-c. shunt motor. This is accomplished in a single-phase induction motor by having a rotor similar to the armature of a d-c. machine, on which two short-circuited sets of brushes hear, that are displaced from each other by 90 electrical degrees. One set gives a magnetic axis through the rotor in line with that of the stator. The object of using the commutator rotor is to provide a means for controlling the power-factor or the speed by introducing e.m.f.'s into one or the other of the brush circuits. A poly-phase motor may also be arranged in a similar manner so that its speed can be adjusted either by shifting brushes or introducing auxiliary voltages between the brushes. This kind of motor is designed to do from an a-c. source of supply, what the d-c. adjustable-speed motor does. It is more expensive and has a limited application.
MOTORS, A. C., REPULSION-INDUCTION
A single-phase a-c. motor combining the characteristics of the repulsion motor with those of the single-phase induction motor. The rotor is wound and provided with a commutator like that of a d-c. machine or a repulsion motor. Two sets of short-circuited brushes make contact on the commutator 90 electrical degrees from each other, one set giving n magnetic axis through the rotor displaced several degrees from the magnetic axis of the stator as in the repulsion motor. The second set of short-circuited brushes combine with the first to form with the rotor winding the secondary of a single-phase induction motor. The repulsion-start principle is used with many induction single-phase motors, but since these are induction motors while running, they are listed under Motors, a-c. induction, single-phase.
The speed-load characteristic of the repulsion-induction motor is similar to that of a cumulative compound d-c. motor, showing the series characteristic of the repulsion motor combined with the shunt characteristic of the single-phase induction motor. It may be designed to have its speed adjusted by shifting the brushes. Repulsion-induction motors are used for a number of purposes, such as blowers, rotary compressors and pumps, etc.
MOTORS, A. C., SYNCHRONOUS
Synchronous motors consist of a field structure and an armature practically identical with those of a synchronous generator. Any synchronous generator may operate is a motor and, vice versa, any synchronous motor may operate as a generator. Machines designed to operate as synchronous motors usually have heavier damping windings and greater synchronous reactance than corresponding machines designed to operate as alternators. The average speed of synchronous motors is dependent only on the number of poles and the frequency of the supply. Momentary changes of speed occur with changes in load, in applied voltage, or in excitation and pulsations of speed, or hunting, may follow these changes or the pulsations in the frequency of the supply, but amortisseur or damping windings usually effectively reduce hunting to a negligible value. In general the synchronous motor gives the greatest constancy of speed of any type of electric motor.
The starting torque of a synchronous motor, as such, is zero. It may be brought up to synchronous speed by mechanically driving it from some outside source, or, In the poly-phase motor, starting it as an induction motor, the amortisseur or damping winding serving as the secondary of an induction motor.
The power-factor of the synchronous motor depends upon its excitation and the mechanical load. It usually takes a lagging current when the excitation is such that the counter e.m.f. is less than the applied voltage and a leading current when it is greater. Easy adjustment of the excitation permits securing unity or leading power-factor. The excitation is provided, like that of alternators, from some d-c. source. Often this is a small d-c. generator built on an extension of the shaft of the motor, or sometimes it is belted to the shaft.
Synchronous motors are naturally adapted to uses where very constant speed and high or adjustable power-factor are desired with infrequent starting at low or moderate starting torque. Such conditions exist with many motor-generator sets, of which frequency converters are a typical class, also with refrigerating machines, compressors, pumps, etc. The ability of the synchronous motor to take leading or lagging current according to its excitation enables it to be used for power-factor correction and voltage control. It can perform this service either with or without carrying a mechanical load. If it carries no load, it is called a synchronous condenser. The ability to raise the power-factor while carrying load makes these motors favored by central-station companies who In some cases give preferential rates to power users employing them.
MOTORS, CRANE AND HOIST¶
Motors of the series or compound type when operating on direct current, and generally of the wound-rotor type of poly-phase induction motors when operating on alternating current, though the a-c. series type is also used at times. Of these types the series wound d-c. motor is best adapted for crane and hoist service and is probably the most common. It has a tendency to slow down under heavy loads and with light loads the speed increases, which is usually desirable. Compound wound d-c. motors are sometimes used with hoists to simplify the control if dynamic braking is required. When direct current is not available and it is not feasible to install a converter or a motor-generator, a-c. wound-rotor motors designed for heavy momentary overload capacity are generally used. As low-speed induction motors are not satisfactory, the motors are usually connected to the drum through single-reduction gearing. d-c. machines in large sizes can be built for such low speeds as 60 to 75 r.p.m. and are sometimes directly connected to the drum. In general, however, it is desirable to use a moderate-speed motor and connect to the drum through helical reduction gearing.
MOTORS, D. C., COMPOUND
Direct current motors having both shunt and series field windings. The series winding may be connected so as to oppose the shunt, giving the differential compound motor, in which case the speed may be more nearly constant than it is with a shunt motor; see Motors, d-c. constant speed. More commonly the series winding acts in the same direction as the shunt around the magnetic circuit giving cumulative compounding. Cumulative compound motors decrease in speed as the load increases and their torque increases somewhat more rapidly than the armature current, depending in part upon the degree of compounding. They combine the characteristics of series and shunt motors. Cumulative compound motors are used where a large starting torque is required, a variable speed is desired, or at least is not objectionable, and a safe light-load speed is necessary.
Such motors are very largely used for elevators and heavy machinery drive of all kinds. They are used extensively for nearly all kinds of mining machinery in mines supplied with direct current and are also applied to some types of variable-speed fans and blowers. Compound motors are sometimes made in small sizes but being used mostly for industrial work where the load is heavy, most of them are large machines. They have a large starting torque and the speed does not drop off as much as with a series motor when the load Is increased. If the load Is entirely thrown off the speed does not reach a dangerous value, as may be the case with a d-c. series motor.
MOTORS, D. C., CONSTANT SPEED
The d-c. shunt motor gives a nearly constant speed throughout its range of load. For most purposes a drop of 2% from no load to full load is permissible, so that the shunt motor serves adequately. In special cases, however, a greater constancy is required. Where the speed must be maintained within less than 1% a differentially compounded d-c. motor is used. By special design and careful selection of the degree of compounding the speed may be kept within a fraction of 1% regardless of load. Also see Motors, d-c. compound.
MOTORS, D. C., FRACTIONAL-HORSEPOWER
Fractional-horsepower motors are defined as those which have a continuous rating less than l hp. at 1700 to 1750 r.p.m. Many manufacturers, however, are inclined to call ¼ or ½ hp. the upper limit for fractional-hp. motors, as motors over that size are often identical in design principles and general proportions to the larger motors. These small motors are generally wound for the standard voltages 32, 115 and 230. For ½ and ¾ hp. motors both shunt and compound windings are standard. Compound windings only are standard for 1/4, 1/6 and 1/8 hp., while shunt windings are used for 1/12 and 1/20 hp. Fractional-hp. motors are used for driving a large variety of small appliances, such as portable drills and grinders, office appliances, fans, dental engines, washing machines, vacuum cleaners, etc.
MOTORS, D. C., SERIES
Direct-current motors in which the field winding and the armature winding are electrically in series. The field current and the armature current being the same, the field is strong at heavy loads and weak at light loads with corresponding slow and high speeds. The speed is usually controlled by adjusting resistance in series with the motor. The speed-load characteristic with a very large starting torque makes this type of motor especially suitable for traction work where it is very extensively used for both light and heavy cars and locomotives. It is also used for cranes, hoists, freight elevators, rolling mills, cement mills, mining machinery, propeller type fans and blowers and many other similar applications. A series motor may race to destruction if entirely disconnected from a load, and for this reason is generally geared or rigidly connected to its load. If a belt drive were used the belt might break or come off and the motor would race.
MOTORS, D. C., SHUNT
Direct-current motors in which the field winding is in shunt with the armature circuit. With constant applied voltage, the speed of a shunt motor is nearly constant for nil loads. The torque is approximately proportional to the armature current. Shunt motors are often designed and built for adjustable-speed operation. The speed is most commonly adjusted by changing the field current by means of the field rheostat, the speed increasing as the field current is decreased. Shunt motors have a very extensive application where d-c. power is available for driving a large variety of machines at constant or adjusted speeds, as may be desired. They are used widely in machine shops and industrial plants In general for driving nearly all kinds of machine tools, except such as require very large starting torque and are frequently started and stopped. They are also used for ventilating systems with either centrifugal, propeller or positive blowers and fans.
MOTORS, D. C., SHUNT AND COMMUTATING–POLE¶
Commutating-pole motors, or interpole motors as they are sometimes called, are d-c. motors which have commutating poles between the main field poles. Such a construction is almost necessary in adjustable-speed shunt motors having a wide range in order to secure sparkless commutation at all loads and all speeds. The commutating poles are series wound and provide a commutating field increasing with the load, a condition desired for good commutation. They are used for many of the same purposes as shunt motors, but principally for industrial machine drive involving wide adjustment of speed. Car-dumping equipment is an example of drive where the adjustable speed with frequent starting, stopping and reversing requires the use of commutating poles.
MOTORS, D. C., SPECIAL ADJUSTABLE-SPEED
Adjustable speed can be obtained in several types of d-c. shunt motors; see Motors, d-c. shunt; also Motors, d-c. shunt and commutating pole. These two types permit speed change by adjustment of the shunt-field resistance, the commutatlng-pole construction giving a very wide range of speeds. Another method consists in changing the reluctance of the magnetic circuit of the motor without changing the field current. One way in which this may be done is by mechanically moving the armature axially along or with the shaft. The armature core is slightly tapered so that the effect is to change the length and area of the air gap, thus affecting the magnetic field strength and the speed. This method permits a very gradual change of speed over a wide range and is especially adapted to drives where close speed setting is desirable and where the motor is accessible for the necessary adjustment.
MOTORS, MILL TYPE
Motors distinguished by specially rugged construction. They may be of any of the various types of a-c. or d-c. motors, and are used where severe operating conditions are experienced, such as in steel mills, cement mills, ore-treating and smelting plants, mines, etc. In installations where there are special speed and reversing requirements, d-c. motors are used, but poly-phase induction motors are more common when these special requirements are not met with.
There are certain types of drives, such as reversing blooming mills in a steel plant, which cannot be driven advantageously by a-c. motors. These are either driven directly by d-c. motors, or in case of very large machines a special motor-generator set is used to control the speed and reversal of a d.-c. load motor. Motors for use in steel mills are made in sizes from 5 to 10,000 hp. In cement, textile, flour and pulp mills, refrigerating plants, etc., mill type poly-phase induction motors are widely used; where the speed is required to be adjustable the wound-rotor or slip-ring type induction motor is used.
MOTORS, MINE SERVICE
The mine type of motor was designed to fill the demand for a motor of more rugged mechanical construction than the standard line of motors. The frame is of cast iron of exceptionally heavy section, and is of the box type without openings. An eyebolt is provided on the top of the frame to facilitate handling the machine. Both end shields are split horizontally and held together by large bolts which are placed so as to be as accessible as possible. The shields are enclosed with the exception of a hole for ventilation in each lower half. The windings are all supplied with moisture-resisting insulation and open-slot construction with form-wound coils. The rotor is especially designed to be as rigid as possible to resist the shocks and vibrations of gearing. The rotor spider has an extra long hearing surface on the shaft.
MOTORS, SPECIAL AND MISCELLANEOUS¶
Some two dozen classes of motors are listed herewith together with their manufactures; the basis of classification is chiefly according to the construction of the motor and partly as to important specialized services, which have considerable effect in modifying essential design and construction features. A classification by service alone would give hundreds of classes, since the kinds of machines driven by motors are almost countless. There are numerous machine drives, however, that impose other important changes on the construction, such as special housing or frame construction, exceptionally low or very high speeds, extraordinary sizes and ratings, etc.
For instance, motors for vacuum cleaners are usually built right into the casing of the cleaner; explosion proof motors and submergible motors require special frames, as do back geared and many other kinds of motors. Unusually low and extremely high speeds may be desired sometimes without gearing or other speed-changing devices; these require special design. As to size, there are such extremes as the miniature dental motor that may be held in the dentist's hand and weighs but a few ounces, and the powerful reversing rolling-mill motor with a momentary rating exceeding 15,000 hp. Practically all large rolling-mill motors as well as other high-rating motors are special. In this classification, therefore, are included all special motors and miscellaneous but distinctive types as are not covered by the other score or more classifications of motors.
This information on electric motors is from the
- EMF electrical year book, Volume 1, 1921 pages 484 to 492