DC Servomotors

The force which rotates the motor armature is the result of the interaction among two magnetic fields (the armature field and the stator field). To generate a constant torque from the motor, these two fields might remain constant in magnitude and in relative orientation. It is achieved by constructing the armature as a series of small sections linked in sequence to the segments of a commutator. Electrical connection is built to the commutator through two brushes. Since successive commutator segments pass the brushes, the current in the coils linked to those segments alter direction.

This commutation or switching influence results in a current flow in the armature that occupies a fixed location in space, independent of the armature rotation, and permits the armature to be regarded like a wound core along an axis of magnetisation fixed in space. It gives rise to the manufacture of a constant torque output from the motor shaft. The axis of magnetisation is resolute by the location of the brushes. If the motor is to have same characteristics in both directions of rotation, the brush axis might be positioned to generate an axis of magnetisation i.e. at 90° to the stator field.

DC servomotors are high performance motors and useful as prime movers in numerically controlled machine tools where begin and stops must be made rapidly and accurately. The lightweight and low inertia armatures of DC servo motors respond quickly to the excitation voltage changes. Also low armature inductance in these motors results in a low electrical time constant (typically 0.05 to 1.5 ms) that additional sharpens motor response to command signals.