Design and Manufacture of DC Motors for Control

Generally, DC servo motors have permanent magnets, and their construction employs brushes and commutators for passing electric current to the rotor, which is the armature for generating the electromotive force. As a feature of the motor itself, one of the requisites is that it must have excellent control characteristics by which the number of revolutions can be lineally controlled in keeping with the increase and decrease of the generated torque. Normally, DC motors have low rotor inertia in proportion to the generated torque, a speed build-up response within a range of a few milliseconds to a few tens of milliseconds during starting, a rotational speed-detecting tachogenerator, an encoder for discriminating the rotational angle and direction of rotation, and a braking device for holding the motor stationary. Further, besides its use as a servo motor, it is required to start and stop repeatedly, rotate smoothly at all speeds, operate equally in forward and reverse, and show resistivity to environmental conditions for example temperature, humidity, vibration etc.