primary objective of the design is, of course, to secure a good engine for developing draw-bar pull and a good vehicle. This is relatively simple when operation is limited to low speeds and with little critical curvature. When, however, high speeds or high speeds with critical curvature, in addition to the two-way operation, must be provided for, it becomes less simple, even with the great latitude in design and the possible combinations which the electric locomotive permits. A large motor may drive more than one axle, in which case there is required a side rod drive direct or through gearing, or a number of small motors may each drive a single axle through a direct connection or through gearing. The motive power units under common control, which is, by the way, a proper definition of an electric locomotive for railway service, may have a single wheel base, or an assembly of short or long wheel bases, coupled to gether by cab or by hinges or by draw-bars. Each individual unit may or may not have auxiliary trucks for guiding or for bearing weight. Each unit may be an independent vehicle or it may be a vehicle whose guiding or stability is effected by the preceding or following unit through hinges. The cab structure may be integral with the locomotive frame or it may be independently borne, and attached to two or more wheel bases. Within certain limits the electric system adopted and the service for which the locomotive is intended are the chief factors which will govern the funda mental design of electric locomotives. The electric system chosen together with the service requirements will determine the type of motor, its characteristics and its size and speeds. The type of motor and restriction to driver weights will determine the drive within limits of narrow choice; and the drive will, in turn, to gether with the absence or presence of critical curvature essen tially determine the configuration of the rigid running gear. The necessity for auxiliary trucks will be determined mainly by the service requirements. The d.c. motor is the outgrowth of the street railway motor. The speed curve falls rapidly as the tractive effort increases, so that a maximum tractive effort is available at low speeds, whereas at high speeds the tractive effort falls. With the usual series motor the only practical way of controlling speed (except for the restricted range of field control) is by varying the voltage across the motor armatures. This is accomplished by use of external resistance and by changing motor combinations.
The limitations of design are influenced not only by speed and load requirements but by the need for stability against flash over. This last is also affected by the conditions of current supply and is a material factor in determining the size and weight of motor selected. The a.c. series motor characteristic is some what steeper than that of the d.c. series motor. Inasmuch as the use of alternating current in a motor is always accompanied by induced currents due to transformer action, this, in a series com mutator motor, affects the conditions under the brushes which determine their action, thus restricting within definite limits the torque output per motor pole. The voltage of the a.c. motors is low, their stability high, and they may be run on ungrounded cir cuits so that under proper conditions their operation is comparable to d.c. motors. Control of speed is easily and efficiently secured.
The three phase induction motor, used also with phase con verter, is practically a constant speed motor and has character istics very different from the two described above. Additional speeds are obtainable through varying connections, but in general for freight service for which this type has many advantages, it is seldom necessary to have more than two speeds. Very high start ing torques are obtainable with this type of motor, as well as capacity to carry heavy loads. As constant speed is maintained irrespective of the load, the horse-power input will vary almost directly with the tractive effort.
All three types lend themselves to regeneration. The induction motor is inherently the best as it automatically, without additional apparatus for switching, separate excitation or regulation, be comes a generator whenever the locomotive while descending a grade exceeds the synchronous speed. No question of stability, flashing or expert manipulation enters. Both the direct current series motor and the a.c. series motor have been successfully used for regeneration although a special arrangement is necessary. The d.c. series motor, if furnished a local supply of direct current from a motor generator set on a single phase locomotive, secures the utmost in favourable conditions for the operation of a d.c. series motor.
The advantage of the direct drive with armature mounted upon the axle is the directness by which transfer of torque to the wheels is accomplished with the entire elimination of drive mechanism. This advantage is offset by the high copper losses of the field and armature. The slow motor speed results in a low weight efficiency of motor and the proximity of motor to the roadbed imposes certain difficulties from exposure. The use of this type is neces sarily confined to relatively high speed passenger service owing to the limited torque capacity.
The direct quill drive has the advantage that it permits the use of a motor whose armature as well as its field frame is completely spring supported. It further permits relative economy in design and use of windings. It has, however, the common disadvantage of low speed with relatively low weight efficiency of the motor and is further subject to certain inaccessibility of the spring drive elements and to proximity to the roadbed. This type is likewise limited to high speed service on account of its limited torque capacity.