It is possible to use as a motor any direct - current dynamo, whether the field - magnets be series - wound, shunt - wound, separately excited, or permanently magnetized. There is this curious point of difference. Suppose the dynamo to be arranged so as to work as a generator, and then to be supplied with currents from an exterior source, to make it work as a motor. If the dynamo is series - wound, it will run the reverse way (or against its brushes), no matter which way the currents run through it. If shunt - wound, it will run with its brushes, whichever direction the current runs through it. The direction of rotation taken by the separately - excited and the magneto - machine will also be with the brushes, if the current is in the right direction, through the armature. These points have to be taken into account in any attempt to combine the different systems.
In applying to motors rules and suggestions such as were applied to generators, it will be found that, whilst some of them apply directly, others are singularly in contrast. For example, it is advisable, for the sake of steadying the currents in generators, to use large and long field - magnets with plenty of iron, and with heavy pole - pieces. In the case of motors there is no such necessity, for we want here to produce a uniform steady rotation. Even if the impulses be intermittent, the mechanical inertia of the moving parts will steady the motion. Electric currents have no such inertia (except in so far as the self - induction in a circuit exerts an influence like that of inertia), and hence the precautions for generators. In the case of generators, to produce steady currents, we had to multiply coils on the armature in many separate paths, grouped round a ring or a drum, involving a complicated winding, and a collecting apparatus consisting of many segments. In motors, no such necessity exists, provided only we arrange the coils that there shall be no dead - points.
For large motors, it may be advisable to multiply the paths and segments for other reasons (as, for example, to obviate sparking at the collectors), but for securing steady running, the inertia of the moving parts spares us (at any rate, in small machines) the complication of parts which was expedient in the generator. Some of the most successful of the little motors that have recently appeared - those, for example, of Deprez, Trouve, and Griscom - have for their armatures the simple old shuttle - wound Siemens armature of 1856, and in these is the disadvantage of dead - points to take into account. Deprez, in his first motors, placed this armature longitudinally between the poles of a horse - shoe magnet, with the axis parallel to the limbs. He has also constructed motors with 2 such armatures on one spindle, one of the coils being 90° in advance of the other, so that while one was at the dead - point, the other should be in full action. The same suggestion has been carried out in Akester's motor. Trouve has tried to get over the dead - points by utilizing the method of oblique approach. The Griscom motor, which has little copper rollers as commutator brushes, has for field - magnets a compact tubular electro - magnet wound in series with the armature.
It has the disadvantage of dead - points. There is, in all these motors, the disadvantage that at every half - revolution the magnetism of the armature core is reversed; and as in all these forms this core is of solid iron, there must be waste by heating in the cores. In fact, to the rotating armatures of motors, as to those of generators, apply all the rules about slitting to get rid of induced eddy - currents, avoiding idle coils and useless resistances. The rules about proper pole - pieces, adjustable brushes, and multiplication of contacts, are mostly applicable to motors also as well as to generators.
In order to meet the case of a handy and reliable motor, Prof. Thompson designed a machine in which the field - magnets, which also constitute the bedplate of the motor, are of malleable cast - iron, of a form that can be cast in one, or at most two, pieces. Their form is that of a Joule's magnet, with large pole - pieces, and wound with coils, are ranged partly in series, partly as a shunt, in certain proportions, so as to give a constant velocity when worked with an external electromotive force of a certain number of volts. As an armature, he employs a form which unites simplicity with efficiency for the end desired; he modifies the old Siemens armature by embedding, as it were, one of these shuttle - shaped coils within another, at right angles to one another. And having duplicated the coils, he duplicates the segments of the commutator, which therefore becomes either a 4 - part collector or else a double collar, according to circumstances. There are no solid iron parts in the armature, but the cores are made of thin pieces of sheet iron, stamped out and strung together.
Reckenzaun's motor is interesting, because its armature, though a drum - armature in form, in reality consists of independent coils, connected, like those of the Brush dynamo, to separate commutators. There are, in fact, 4 commutators, grouped as 2 twos, and 2 pairs of brushes in contact with them.
De Meritens employs a ring armature very like that of Gramme, but places it between very compact - and light field - magnets, which form a framework to the machine. There is one point about this machine of great interest, which is, however, a later addition. It is provided with a reversing gear. In it are 2 pairs of brushes; the 2 upper are fixed to a common brush - holder, which turns on a pivot, and can be tilted by pressing a lever handle to right or to left. The 2 lower brushes are also fixed to a holder. Against each brush - holder presses a little ebonite roller, at the end of a bent steel spring, fixed at its middle to the handle. The result of this arrangement is that, by moving the lever, the brushes can be made to give a lead in either direction, and so start the motor rotating in either direction. Such a reversing gear is obviously a most essential adjunct for industrial applications of motors, and if the difficulties of sparking at the brushes, caused by the sudden removals of them from the collector, be obviated, must prove much better than any mechanical device to reverse the motion, by transferring it from the axle of the motor through a train of gearing to some other axle.