Unlike motor designations such as DC-motor or induction motor, the term servomotor or servo motor does not describe an actual physical operating principle. Thus, the difference to other motors is not in the motor itself but instead solely in its actuation, which takes place in a closed control loop.
Only servomotors with integrated servo amplifier can independently regulate torque, speed or position without external components after a supply voltage is applied in servo operation. This occurs according to specifications via a bus connection or via digital or analog inputs. Nevertheless, motors that require an external servo amplifier are commonly referred to as servomotors. As in the past, these make up the majority of servomotors.
The combination of motor and amplifier is generally called a servo drive. The modeling servo, which consists of only a servo and which is likewise operated in a closed control loop, is, however, excluded from the term servomotor in this case. Thus, the term servomotor does not describe one certain type of motor construction but can rather refer to servomotors of a wide variety of design types.
Permanently excited synchronous motors, asynchronous motors and brushed DC-motors are common types of servomotors. AC servomotors are generally motors that are operated on servo amplifiers supplied directly by an alternating voltage (single phase or three phase). These are usually power supplies from approx. 230 V to over 600 V. Standing in contrast to these are servomotors that can be operated directly on a DC voltage of up to approx. 48 V; these are known as DC servomotors. According to this definition, permanently excited synchronous motors are used both as AC and as DC synchronous motors. Asynchronous motors and reluctance motors are used primarily as AC servomotors; permanently or separately excited brushed DC-motors, on the other hand, are used as DC servomotors.
Servomotors in combination with a servo controller (motion controller) matched to the motor can be accelerated to their rated speed in just a few milliseconds. In the same time, they can be braked and their direction of rotation reversed. They are used as highly dynamic, acceleration-optimized drive units for controlling angles, positions and paths.