Basics: Structure and components of a servo motor

Matthias Beetz, Baumüller Nürnberg GmbH

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What is a servo motor?

The design of a servo motor enables extremely dynamic motion at high effective torque as well as high power density and energy efficiency. The minimum constituents of any servo motor are a housing and a stator inside the housing. The stator contains the winding, which is made of electrically conductive wire that generates an electromagnetic field when energy is fed. This causes the rotor to rotate along with the motor shaft, which can then drive a machine. In many cases, other important components include the cooling system and the servo controller.

The following glossary entry covers:

At a glance: What is a servo motor?

A servo motor is a motor whose functionality is essentially based on electromagnetism. An energy field generated in the interior drives a rotor. Thanks to continuous development, the former auxiliary drives can now be used in many areas as the main drive for industrial machines and much more. A modern servo motor has a very compact and space-saving design, is flexibly adaptable and allows highly dynamic and precise processes.

Servo motor: Structure and functioning

The servo motor consists of a number of essential main components. These basic structural components include:

  • Housing
  • Stator
  • Rotor
  • Motor winding (inserted in the stator)
  • Cooling (optional)
  • Power connection
  • Feedback system (encoder)
Structure of a servo motor

Further components are assigned to the motor itself. Among these, the servo controller is particularly important. This acts as an intermediary between the motor and digital control elements (e.g. PLC), as it converts the signals as required. Only in this combination is the servo drive complete. The following sections will take a detailed look at some of the components.

Motor shaft and motor winding

The shaft and the winding form the structural core of a servo motor. They function as the rotor and the stator. Looking at the servo motor from the outside in, the stator (with the motor winding) is located below the housing. The latter consists of an electrically highly conductive material and can have various designs: The wire can vary in thickness or diameter, which in turn influences the winding intensity. The thinner the wire, the more and tighter windings can be implemented. These have a significant influence on the specifications of the motor. Decreasing wire thickness and increasingly tight windings yield the following effects:

  • amplified magnetic field
  • increased maximum torque
  • higher countervoltage with increasing speed
  • reduced efficiency
  • increased waste heat, which requires appropriate cooling 
How a servo motor works

How a servo motor works

This means that a tighter winding of the stator results in extremely high power being converted at low speed. Extruders, shredders and many more machines require a lot of power at low speeds, which is why a tight winding is used in these, for example. At higher speeds, there is a massive drop in power, but this drop is insignificant in these cases. The type of motor winding depends on the specific application.

The rotor is the motor shaft located inside the stator. It is set in motion by the electromagnetic field of the winding as soon as energy is supplied from outside. The shaft then transmits the motion to the machine driven by the servo motor.

The right cooling – especially efficient with water

Cooling represents an essential part of a servo motor. In terms of applications, various forms of cooling come into question. They can work via air, oil or water. With water cooling, the highest nominal power can be implemented in a compact design. This means an increase in power density. Considering the increased costs entailed by higher space requirements as well as the loss of design flexibility, this increased power density is an important economic factor in the operation of any industrial machine.

Baumüller's servo motors are available with water cooling in sizes 45-560 and oil cooling in sizes 45-200. Their space-saving design makes it possible to operate them in a housing of the same design as used with uncooled variants.

Our Baumüller servo motors are available in different variants, which can also be designed without cooling. Especially for very dynamic requirements, however, water cooling should be selected. Oil (HLP-46 hydraulic oil) can also be used instead of water. In this case, the cooling effect is somewhat reduced compared to water depending on the size and operating mode of the motor. Liquid cooling saves space compared to air cooling (by eliminating the need for a fan) and also requires much less maintenance. This reduces costs for servicing and downtime. Water cooling is also particularly important in group installations that afford little space. If multiple motors are installed next to each other, they heat each other up. Liquid cooling effectively prevents this problem.

Interaction with the servo controller

The servo controller is effectively the communication medium between the control unit (e.g. PLC) and the motor. It converts signals from both sides and transmits them further. The control of the motor behavior can be set particularly exactly. For example, the Baumüller b maXX 5000 family allows an extremely precise determination of torque, speed and position. This means that targets can be reached and maintained, enabling an extremely high degree of dynamics in the movement.

Baumüller servo controllers

Summary – everything at a glance

The servo structure of a drive system essentially consists of a stator and rotor or the combination of a motor winding and a motor shaft. The motor is driven by electrical energy flowing through the winding, creating an electromagnetic field that causes the shaft to rotate. Other important components include cooling and the servo controller. While the former is not essential, it is recommended for most industrial applications. Strictly speaking, the controller is not part of the motor, but is nevertheless essential for its precise operation.

Do you have any questions?

Matthias Beetz
Training Engineer Academy I Baumüller Nürnberg GmbH

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