DC Motor Drive Basics - Part 5: DC servo drives

D.C. SERVO DRIVES
The precise meaning of the term 'servo' in the context of motors and drives is difficult to pin down. Broadly speaking, if a drive incorporates 'servo' in its description, the implication is that it is intended specifically for closed-loop or feedback control, usually of shaft torque, speed, or position.

Early servomechanisms were developed primarily for military applications, and it quickly became apparent that standard d.c. motors were not always suited to precision control. In particular high torque to inertia ratios were needed, together with smooth ripple-free torque.

Motors were therefore developed to meet these exacting requirements, and not surprisingly they were, and still are, much more expensive than their industrial counterparts. Whether the extra expense of a servo motor can be justified depends on the specification, but prospective users should always be on their guard to ensure they are not pressed into an expensive purchase when a conventional industrial drive could cope perfectly well.

The majority of servo drives are sold in modular form, consisting of a high-performance permanent magnet motor, often with an integral tachogenerator, and a chopper-type power amplifier module. The drive amplifier normally requires a separate regulated d.c. power supply, if, as is normally the case, the power is to be drawn from the a.c. mains. Continuous output powers range from a few watts up to perhaps 2"3 kW, with voltages of 12, 24, 48, and multiples of 50 V being standard.

Plate 4.2 High-performance permanent-magnet brushed d.c. servo motors with integral tachno/encoders. (Photo courtesy of Control Techniques)