Electric railways

by American School of Correspondence; James Raley Cravath

CAR EQUIPMENT.

MOTORS.

The voltage most commonly employed by electric railways is 500 to 600; and the motors are 500-volt direct-current series-wound motors, designed especially for railway service. The electric railway motor must be dustproof and waterproof because of the position it occupies under the car. For this reason electric railway motors are made in the form of a steel case ([Fig. 2]), which entirely surrounds the field-magnet poles and takes the place of the yokes or frames that support the fields on stationary motors. Cast steel is the material now usually employed for railway motor cases and fields, on account of its mechanical strength and its high magnetic permeability. The four poles project inwardly from the case, as seen in the open motor case, [Fig. 3], which is that of a Westinghouse No. 69 motor.

Fig. 3. Railway Motor. Upper Field Raised.

Railway motors have usually four poles because this permits of a symmetrical and economical arrangement of material around the armature, and hence permits the motor to be placed in the small space available on the car truck. Two-pole motors have been used in the past, but they were not as compact as the four-pole type.

Characteristics of Railway Motors. The curve sheet, [Fig. 4], for the Westinghouse No. 69 motor represents in general the characteristics of all direct-current railway motors.

The figures for each curve are found with names corresponding to the curve to which they apply, at each side represented by vertical distance on the sheet. The amperes, represented by the horizontal distance, are marked at the bottom, and apply in common to all the curves.

The tractive effort at different current consumption is represented by a line curving upwards somewhat. This shows that the tractive effort increases, in a proportion greater than directly, as the current increases.

The torque required in starting may be many times greater than that necessary to maintain the car at full speed. The series-wound motor, therefore, furnishes this great starting torque more economically than a shunt-wound motor the torque of which is proportioned to the current. This feature of the series-wound motor makes it especially adapted to street railway work.

WESTINGHOUSE
No. 69 RAILWAY MOTOR
500 VOLTS
GEAR RATIO, 14 TO 68. WHEELS, 33″
CONTINUOUS CAPACITY, 25 AMPERES AT 300 VOLTS,
OR 23 AMPERES AT 400 VOLTS.

Fig. 4. Characteristic Curves of Railway Motor.

The efficiency curve shows the motor to have an efficiency of about 83 per cent with gears. Much other information may be obtained by a proper study of the curves. The fields are worked near the point of magnetic saturation. This economizes metal and space and is also an advantage because of the fact that when so worked the armature reactions have very little effect on the fields. The neutral points between fields are consequently shifted very little and it is therefore not necessary to shift the brushes when the motor is reversed.

Fig. 5. Armature Winding.

Armature Winding. The armature winding is what is commonly known as the series or wave winding, shown developed in the paper on Direct-Current Dynamos. This winding is shown in [Fig. 5], which is an end view of an armature and commutator. In the figure, however, the armature is shown with a much smaller number of slots than a railway armature should have in practice. One reason for the employment of the wave or series winding on railway motor armatures, is that with this winding no cross-connections are necessary when only two brushes are used, and these two brushes may be placed 90° apart in a convenient and accessible position. Another reason is that the current, in flowing from one brush on the commutator to another, must always pass through the magnetic field of all four of the motor poles. This makes it impossible for any unbalancing of the magnetic circuit to cause more current to flow through one portion of the armature than is flowing through another portion. In a railway motor it has been found quite possible to have one pole or pair of poles exerting a greater magnetic attraction on the armature than another pair, owing to differences in the iron and differences in the clearance between the armature and pole pieces, which differences cause more magnetic lines of force to flow from some pole pieces than from others. With the lap-armature or the ring-armature winding, since the various portions of the armature under different poles are in parallel with one another, any difference in the magnetic flux between different poles will cause a different amount of current to flow in the various paths through the armature.

General Data on Street Railway Motors.

Type of Motor.	Horse Power.	Amperes.	Speed Full Load.	Total Field Turns.	Slots.	Conductors per Slot.	Commutator Bars.	Weight complete with Gears.	Armature Complete.	Gears and Casing.	Commutator Bearing. Inches.		Pinion Bearing. Inches.		Diameter Armature.	Length.
GeneralElectric 51	82		640	56	37	12	111	3875	953	338	3	5¾	3¼	8¼	16	10½
52	27		640	155.5	29	24	87	1725	357	265	2½	6⅜	2¾	7¾	11	9
57	52		470	110	33	18	99	2972	704	340	2⅝	6⅜	3¼	8¾	14	12
55	160				47	6	141	5415	1550	490	3¼	7½	3¾	11
67	40			110	37	18	111	2385	595	385	2⅝	6⅛	3	8
54	25						115	1831	395	285	2½	6	2¾	7¾
74	65	113		70.5				3534	845	415	3⅛	6¾	3⅝	8¾
Westinghouse 68	40				55	12	109	2280	505	330	2¾	6¾	3	7¾	14	8
69	30				35		105	1950	385	330	2¾	6	2¾	7	13	6¾
76	75				39		117	3840	505	860	3¼	8	3½	9	16½
56	55				39		117	3000	315	720	3	7½	3¼	8½	14	12
50c	150			144	55	6	115	5550	1500
49	35			114	59		117	1925	438	327	2¾	6	2¾	7½	13⅝	6½

By reference to the winding diagram given in [Fig. 5], it may be noted that a complete circuit through two coils ends at the segment adjacent to the one from which the start was made. It may also be noted in the table of motor data that all of the armatures have an odd number of segments and an odd number of slots. It is absolutely necessary in a wave winding to have an odd number of segments. Otherwise the winding could not be made symmetrical and the circuit through two coils be made to return to a segment adjacent to that from which the start was made. With equal spacing between the top and bottom leads of the two coils, an even number of segments would make the circuit return either on the segment from which the start was made or two segments from it.

The first drum-wound street railway motor armatures had as many slots in the armature as there were coils and segments. The great number of slots necessarily made the teeth very thin and consequently weak. This is very objectionable as sometimes the armature bearings wear away, allowing the face of the armature to drag on the pole pieces and thin teeth are bent out of shape.

Armatures are now almost entirely constructed with either two or three coils to a slot. When two coils are used in each slot with an odd number of slots an even number of coils results. If these were all connected to the commutator an even number of segments would be necessary. As this is not possible with a wave winding, one of the coils is “cut out.” The ends are cut short and taped and it is termed a “dead” coil. This makes the winding somewhat unsymmetrical, all the coils not bearing the same angular relation to the commutator segments to which they are connected. This difference is, however, not great enough to affect the operation of the machine.

The Westinghouse 49 motor is an example of an armature with a dead coil. By reference to the table of motor data it will be seen that this armature has 59 slots. Two coils in each slot would make 118 coils. One of these, however, is cut out, giving 117 segments.

Cutting out a coil can be avoided by putting three coils in each slot.

An odd number of coils results then no matter what the number of slots may be. In the majority of examples given in the table there are three times as many segments as slots. The sides of the slots of modern street railway armatures are straight. The coils are prevented from flying out by bands of wire extending over the tops of the coils around the armature. Steel or silicon bronze wire of about No. 14 gauge is used. Recesses are made in the armature teeth for the reception of these bands so that the wire when wound will come flush with the face of the armature. The bands are usually ¾ to 1½ inches wide. The wires are well soldered together to secure them in place. One trouble experienced with armatures is the slipping off of these bands. The heated armature expands and stretches them. When the armature cools the bands are loose and then often slip off. When they do so the coils fly out by centrifugal force, strike the pole pieces and ground the motor.

Fig. 6. Armature Coil.

Armature Coils. Railway motor armatures are to-day universally constructed with form-wound coils, which are wound on a form of proper shape and carefully insulated before being placed in the armature.

The coils of the smaller motors (those up to 40 or 50 horsepower) are usually wound with round wire. The cotton covering of the wire is depended upon for insulation. To strengthen this, however, the coils after being wound are immersed in an insulating compound and then baked in an oven. The whole coil is usually wrapped with insulating tape (See [Fig. 6]). The armatures of larger motors have coils made of copper bars. Mica is often placed between and around the bars for insulation, though oiled linen cloth tape cut bias is also employed, especially in repair work.

Field Coils. Field coils are so constructed that they may be readily removed should they become grounded or short-circuited. Some makers wind them on a brass shell or form which is slipped over the pole piece. In some motors the field coils are composed of copper ribbon, wound bare, with ribbons of insulating material between the turns. Field coils of wire for the smaller motors, if not wound on shells, are wound on forms and before completion are taped in such a manner that they will hold their shape without being enclosed in a spool. The terminals are brought out where they will be of easy access when the field is in place (See [Fig. 7]).

Fig. 7. Field Coil.

Armature Leads. In [Fig. 3] is seen a completed armature in the motor casing of a Westinghouse No. 69 motor. Since the motors are four-pole, the two sides of any one coil occupy slots 90° apart in the armature coil, as indicated in [Fig. 5]. The ends of the coils are connected to commutator bars 180° apart. The relative position of the commutator connections of any armature coil can, of course, be varied so as to bring the brushes in the most convenient position in the motor casing. Brushes are always of carbon, and are placed where they can be easily reached from the opening in the motor casing over the commutator.

Motor Leads. The reversing of the current through the armature, independent of the field current, to secure reversal of direction of rotation of the armature, makes it necessary that four wires enter the motor. The portions of these wires connected permanently to the motor are termed the motor leads because they “lead out” the current. Sometimes an ordinary two-way connector is used in connecting these leads to the wires of the cable, but often a jack-knife connector is employed to facilitate connecting and disconnecting. Considerable difficulty has been experienced by the wearing away of the insulation of the leads where they rest on the motor shell. To avoid this there has recently come into use a lead protected by a spiral metal covering. Brushes. That the motor may operate in either direction equally well, the carbon brushes are placed radially or nearly so. No provision is made for shifting their position relative to the fields. They usually occupy a position equidistant between pole tips. The common types are either ½ or ⅝-inch thick and from 2¼ to 4 inches wide.

Fig. 8. Brush Holder.

Brush Holders. Two methods of securing the brush holders are employed. In [Fig. 3], the brush holders may be seen to be secured in position by being bolted through the end of the motor shell. [Fig. 8] shows the brushes mounted on a yoke which is secured to the motor shell. The yoke is of wood and provides the necessary insulation. Where the holders are fastened directly to the shell a block and washers of vulcabeston or other insulating material intervene to furnish the insulation between the shell and the holder. In practice the greatest difficulty experienced with brush holders is preventing them from becoming grounded by dirt and carbon dust which collects on the insulation.

Opening Cases for Inspection. Accessibility for inspection and repairs is essential in all railway motors. A lid is always provided directly over the commutator to facilitate inspection of the commutator and brushes. To open up the motor casing for more extensive inspection or repairs, three general schemes are employed. One is to have the lower half of the casing swing downward on a hinge as in [Fig. 9], which illustrates the Westinghouse No. 38 B motor. The armature may be placed either in the lower half, as shown in [Fig. 9], or in the upper half. When a motor of this type is to be opened the car is run over a pit, and the repair men work entirely from below.

Often the hinge pins are removed and the lower shell containing the armature is dropped down by means of a jack placed underneath.

Two handholes are usually provided in the bottom shell for observing the clearance between the armature and the pole pieces and also for removing dirt that may collect in the bottom of the shell. Another scheme is to have motors open from the top, either by hinging the upper part of the motor casing, as in [Fig. 3], or by having the top part of the casing lift off. Where this form of motor is used, the car body is hoisted clear of the truck, and the trucks are run out from under the car body before work is done on the motors. In this case, all the work can be done from above without the use of pits.

Fig. 9. Railway Motor. Lower Half of Casing Swung Down.

A third design is the box-frame motor casing, from which the armature can be removed endwise only. Such an arrangement is shown in [Fig. 10], which is a view of a No. 66 motor of the General Electric Company. In this motor a sufficiently large opening is provided in the ends of the motor casing to permit of the armature being removed endwise. A plate or head, which accurately fits into this opening, carries the armature bearing. In removing armatures from motors of this kind, the usual method is to take the motor out of the trucks and stand it on end with the pinion up. The bolts being removed from the end plate, the armature can then be hoisted out of the case by means of a special hook attached to the pinion. Another plan that has been used in removing armatures from such motors, is to place the motor in an apparatus where the armature shaft can be held between centers, as in a large lathe. The motor casing is then moved along in a direction parallel to the armature shaft, until the armature is exposed.

This latter box-frame type of motor is very compact; a stronger casing can be made for a given weight and space than if it were divided horizontally. Moreover, the magnetic circuit cannot be disturbed by imperfect contact between two parts of the casing. Where this type of motor is used, the bearings project inward under the commutator and armature, thus getting long bearings with a short motor, which is important where the room is limited, as, for example, in the case of a large motor mounted on a standard-gauge truck.

Fig. 10. Box-Frame Motor.

Gearing. In most cases, spur gearing is used to transmit power from the armature shaft to the car axle, although a few motors with armatures mounted directly on the car axle are in use. Various gearings other than the simple spur gear have been tried, such as worm gears, chain and bevel gears. Practically all have been abandoned in favor of the single-reduction spur gearing, which is the most satisfactory from the standpoint of wear and efficiency. This gearing is shown in Figs. [3] and [9]. The gearing is covered with a gear case ([Fig. 9]), which is usually of steel, though gear cases of thin sheet metal and wood are sometimes used. A solid gear is shown in [Fig. 11], and a split gear in [Fig. 12]. The gear ratios in common use vary from 5 to 1 to 2 to 1, the larger ratio being common on the smaller motors. A ratio often used on motors of 30 to 50 horsepower is 4.78 to 1, the gear having 67 teeth, the pinion 14 teeth.

Street car wheels are usually 33 inches in diameter. This makes necessary 612 revolutions per mile. With a gear ratio of 4.78 the armature revolves 2,925 times per mile. At 15 miles per hour, this gives 731 r.p.m.

Fig. 11. Solid Gear.

Fig. 12. Split Gear.

Lubrication. The lubrication of railway motors was for a number of years carried on almost exclusively with grease, which it was customary to place in the gear casing and in grease boxes over the armature and car-axle bearings. Grease becomes most efficient as a lubricant only when the bearing is heated sufficiently to make the grease run like oil. Oil is now being used to a considerable extent, especially for larger motors. It is fed to the bearings by various devices that allow a very slow feed, such as wicks and lubricators adjusted to pass a small amount of oil per hour.

Bearings. Railway motor bearings are usually of Babbitt metal, which metal is cast into a steel shell. This shell fits into receptacles in the motor casing, which can be seen in Figs. [3] and [9]. A steel shell is used so that the worn-out bearings can be easily renewed and the shells taken to a Babbitt melting furnace to have new Babbitt poured into them.

The motor has two sets of bearings, those for the armature and those for the axle upon which the motor is mounted. The axle bearings are always split diametrically to avoid removing a wheel when a bearing is replaced. On the later designs of motors these are of brass, no Babbitt metal being used. The armature bearings are distinguished by the terms “gear end” and “commutator end” bearings. The gear end bearing is usually of larger diameter and of greater length because of the thrust of the gears it must take in addition to the weight of the armature. This bearing is split so that it may be removed and replaced without the removal of the gear. The commutator end bearing is in one piece. Armature bearings are shown in [Fig. 13].

Fig. 13. Armature Bearings.

Motor Suspension. Two methods of suspending motors flexibly on trucks are in common use. That end of the motor which has bearings on the car axle cannot, of course, be flexibly suspended with regard to the axle; but the other end of the motor can be placed on springs, or rest on a bar supported on springs, as shown in [Fig. 14]. This suspension is commonly called nose suspension. Instead of having a special bar and special springs for the nose of the motor, the nose may rest upon some part of the truck that is carried upon springs. Thus, on the M. C. B. type of swivel truck, the nose usually rests on the truck bolster, and thus gets the benefit both of the bolster springs and of the equalizer springs of the truck. Another general plan of suspension is that known in one form as cradle suspension, and in another form as side-bar suspension. A side-bar suspension is shown in [Fig. 15]. Here a larger percentage of the weight of the motor is evidently taken by the springs than in the case of nose suspension. It is desirable to relieve the car axle of as much dead weight as possible. By dead weight is meant weight resting upon it without the intervention of springs.

Fig. 14. Nose Suspension.

Fig. 15. Side-bar Suspension.

Motors of the New York Central Electric Locomotive. These motors are a radical departure from the usual type of railway motors. The locomotive on which they are mounted has four driving axles, upon each of which is mounted an armature, direct, no gears being used, Figs. [16] and [17]. The motors are remarkable for three special features: The method of mounting the armature, the shape of the pole pieces, and the path of the magnetic flux.

Fig. 16. Longitudinal Section of New York Central Locomotive.

The mounting of the armature upon the driving axle and the motor fields on the truck frame makes it necessary to have flat pole pieces in order that the armature may play up and down as the journal box and axle slide in the guides of the truck frame. The shape of the pole pieces may be observed in the drawing [Fig. 16]. When in the central position there is a ¾-inch air gap between the armature and pole pieces. The magnetic flux is continuous through the fields of all four of the motors. It returns through the cast steel side frames of the truck and two bars placed in the path.

The brush holders are so mounted that the brushes occupy a fixed position relative to the armature. The armature is removed by lowering it with the wheels and axle upon which it is mounted. This can be done without disturbing the fields of the motor.

95 TON ELECTRIC LOCOMOTIVE FOR NEW YORK CENTRAL RAILROAD.
General Electric Company.