It is necessary to control motors. That is, motors must be started, stopped, and reversed, and, within limits, it must be possible to control their speed.
The controls needed to perform these various functions on d-c motors are designed to alter the quantity or the direction; of the current in the field or in the armature of the motor. On d-c controls, these functions were shown to consist mainly of resistances, switches, and solenoids.
It is possible to control motors, not only electromechanically and electromagnetically, but also electronically, by means of vacuum tubes or gas-filled tubes. Some electronic devices can be made to operate a relay which in turn controls a motor.
Others influence the amount and direction of current flowing in the motor circuit and thus the action of the motor itself. Both types may be found in one control device.
