Generally, the operation of servomotors is characterised by frequent changes in speed and torque, operation at standstill to hold positions and short-term operation with high overloads. One example of a servo-driven application is a web process machine that unwinds, processes and rewinds films. In addition to the two winding drives, comprising three-phase asynchronous motors and frequency inverters, the machine has five servo drives for positioning, transporting and processing the material.
Servomotors are generally characterised by a slim-line design with a high power density, low inertia, and high efficiency. They offer optimal drive behaviour with high dynamic performance and accuracy, and are tailored for operation with servo drives.
In comparison with standard three-phase motors, servomotors are considerably more compact and have significantly lower moments of inertia. Hence they accelerate much faster and provide the power for automation tasks within a small space. The active components for torque generation only occupy a small proportion of the total volume of the motor case. Generous roller bearings, the holding brake and the shaft position sensor make up the majority of this volume. To ensure compliance with UL specifications, large insulation clearances are required.
Plug connectors allow fast and error free connection of the motors. In contrast to three-phase AC motors without speed feedback, the servomotor not only has three power terminals and one earth connection, but also at least six connections for the shaft encoder and two connections for temperature monitoring. The large number of connections can easily lead to errors in the connection process. Plug connectors mean that connection errors can be eliminated and time saved during commissioning.
Servomotors have a sensor for angle and speed, the signal from, which is evaluated by the drive. This enables precise and dynamic control of motor speed and position. The setting range for the speed and the achievable dynamics are considerably higher than when using a frequency inverter with a three-phase ac motor.
Servomotors are available in both asynchronous and synchronous configuration. Asynchronous servomotors have a wide speed range with reduced torque available above the rated speed due to field weakening operation. Because of their higher inertia compared with synchronous servomotors, they are generally employed on less dynamically demanding applications such as travel drives with toothed belts.
Synchronous servomotors have a rotor with high energy magnets. They achieve a lower inertia than asynchronous servomotors that have the same rated torque, are smaller in size and do not require any magnetising current. The result is a higher dynamic performance with greater acceleration.
Servomotors are operated in a closed control loop for speed and angle of rotation. To provide feedback, a sensor is required to record the actual speed and the position angle of the rotor. Synchronous servomotors, for example, can be equipped with resolvers, incremental encoders, single-turn Sin-Cos absolute encoders, or multi-turn Sin-Cos absolute encoders.
Resolvers depend on the angle-dependent magnetic coupling between the windings in the rotor and those in the stator, the angle and speed being determined from the relationship of the induced currents. Resolvers are robust and retain absolute angle values within one revolution. The output can also be used to generate the commutation information required for current regulation in the servo drive.
Incremental encoders are optical devices that generate a given number of pulses per revolution. Two signals are generated with a phase difference of ½ pulse length, which enables both angle measurement and direction of rotation to be determined. These are more suitable for asynchronous, as opposed to synchronous servomotors, as they do not provide commutation information. Also optical in operation, the sin-cos absolute encoder partly generates incremental signals and is able to achieve very high angular resolution.
This high resolution ensures smooth motor running at low speeds. Single-turn encoders determine the absolute phase information within one revolution, so that all signals for operating synchronous motors are provided, similar to the resolver. Multi-turn encoders often have additional gearing and sensors and are able accurately to determine position in as many as 4096 motor shaft revolutions. This means that a machine can start immediately after it is switched on without first having to be 'homed'.
A servomotor is often equipped with a brake that is used to hold its position if the power supply to the drive is switched off. Servomotors are normally brought to rest under the control of a drive, with the brake only being used in emergency stop situations - in the case of a mains failure, or where the duty involves the secure holding of a load. Two types of brakes are used: spring-applied, which apply the braking force using spring pressure via an armature plate on the brake rotor, and permanent magnet brakes that generate braking torque by exerting the action of the permanent magnets on a rotating armature plate.
The advantage of permanent magnet brakes is their backlash free properties. The braking torque is transmitted to the shaft using a metal disc with no play. This means that high positional accuracy can be achieved. On the other hand spring-applied brakes have lower inertias, and can continue to operate even after there has been extensive wear from repeated dynamic braking of the drive.
This article was provided by Lenze, which supplies servomotors similar to those described in the text. Examples include the Lenze MCA asynchronous and MCS synchronous ranges, which are equipped with appropriate encoders and brakes, as described in the article.