A direct-current motor

The speed of a direct-current motor is determined by both the field strength and the load. If there is no load, the speed is such that the voltage produced in accordance with the third principle exactly balances the applied voltage, and the armature current is zero. As the load is increased, the speed decreases to allow current to be drawn so the necessary electrical power can be converted. When the motor stalls, it is exerting its maximum force. Therefore, the speed of a shunt motor, or one in which the field is produced by a permanent magnet, is determined by the applied voltage, and can be adjusted finely.
If voltage is applied to a series motor without a load, the motor speeds up. As it does so, the field current decreases so the motor must speed up some more to generate the same back voltage. This keeps up until the motor flies apart. The loss of load on a series motor is a serious thing, and must be guarded against. When a loaded series motor is rotating with the maximum voltage applied to it, the current just produces the required amount of force with the existing field strength. If the field is weakened by reducing the current in it (by putting a resistance in series with it, for example) the motor must speed up to compensate. This is one method of speed control for direct-current motors.
A large direct-current motor must not be started by applying the full voltage across it while it is at rest, especially a series motor. The heavy current and field will create a great jolt that may damage the motor and its mechanical connections. A starting resistance is used to limit the initial current to only the amount necessary to put the motor into rotation. As it speeds up, the starting resistance can be removed in steps. In normal operation, the starting resistance should be removed, since it represents a significant loss of power. For further speed control when more than one motor is used, as on a streetcar or locomotive, the motors can be connected in series to start, and in parallel to run . In each case, the field can be weakened to give a higher speed. With four motors, series, series-parallel, and parallel connections, with field weakening, gives six speed levels that can be designed for service requirements. This could give, for example, speeds of 10, 15, 20, 30, 40, and 60 mph with starting resistance switched out.
A direct-current motor can be reversed by reversing the direction of the current in either the field or the armature. There is more to the story, however. The small demonstration locomotive of Werner Siemens of 1879 reversed by means of gears, and some authors have implied that it was not yet known how to reverse a DC motor, which is absurd. The problem lay in the brushes and commutator. The brushes must be placed so that their switching action takes place at the moment when the current in the windings being switched is zero. As the load on the motor increases, the armature current increases, and the magnetic field it produces causes the total magnetic field to change so that the brushes must be shifted to a new point of zero current. If this is not done, there is sparking at the commutator, which is rather destructive. The brushes on Siemens' locomotive were set at an average position for the load, so that if the motor were reversed, the brushes would have been at an improper position, and sparking would result. Therefore, he used reversing gears and the motor continued to rotate in the same direction.
A better way out of this difficulty was soon found. Small poles, called commutating poles, were placed between the main field poles. These windings are in series with the armature, and proportioned so that they cancel the varying field of the armature. The optimum brush position then becomes independent of motor load or direction of rotation.