GENERAL THEORY.—Dyvamo machine and electric motor are convertible terms. Any dynamo can be used practically as a motor, and in most cases any motor can be used to generate a current. On purely theoretical grounds this should be possible in all eases, but in practice it is found that the speed which is required to make some small motors act as self exciting dynamos is so high as to render that application mechanically impossible. The reason for this is, according to Kapp, that in small motors the polar surfaces are of very limited extent.and consequently the magnetic resistance of the path traversed by the lines of force is excessively high. requiring more electrical energy to excite the field magnets than the armature is capable of developing at a moderate and practical speed.
Dynamos wound and connected for working as generators of continuous currents may be used in all cases as motors, but with some difference. A series dynamo set to generate cur rents, when run right-handedly (and therefore having a forward right-handed lead), will, when supplied with ticurrent from an external source, run as a motor, but runs left-handedly, against its brushes. To set it right for motor purposes requires either that the connections of the armature should be reversed, cr that those of the field magnet should be reversed (in either of which cases it will run right-handedly), or else the brushes must be reversed and given a lead in the other direction (in which ease it will run left-handedly). A shunt dynamo,
set ready to work as a generator, will, when supplied with current, run as a motor in the same direction as it ran as a generator ; for if the current in the armature part is in the same direction as before, that in the shunt is reversed, and vice versa. A compound-wound dynamo, set right to run as a generator, will run as a motor in the reverse sense, against its brushes, if the series part be more powerful than the shunt, and with its brushes if the shunt part be the mere powerful. If the connections are such (as in the compound dynamos) that the field magnet receives the sum of the effects of the shunt and series windings when used as a generator, then it will receive the difference between them when used as a motor.
In several respects it is even more important that the rules laid down for the good design of generators (see DyNAmo ELErritic MActuNns) should he observed for motors. Eddy currents must be even more carefully eliminated. According to Mordey, in a generator the self-induction in the sections of the armature coil, and the eddy currents in the core, are antagonistic ; in the motor they tend to increase one another. Also. the greatest attention must be paid to proper mechanical arrangements for transmitting to the shaft the forces that are thrown by the magnetic field upon the condlicting wires amnia].