The applications in which electric motors are used can be roughly divided into two main groups, and this division also gives us the first criterium for selecting the right type of motor. The first group consists of drive applications in which the motor is used to set systems, such as conveyors, pumps, and compressors, in motion and keep them moving. In the second group, controlled movements are needed for positioning loads and other motion control applications. In the first group, the emphasis is often on power, while in the second group accuracy is the main aspect.
There are many additional selection criteria that can be taken into account, such as the available supply voltage, the required torque or power, and the inertia of the load. Cost of ownership can also play an important role, especially when energy costs are a focal point.
One of the classic motors in electric drive technology is the asynchronous induction motor. The term asynchronous refers to the rotor (the rotating part of the motor) turning at a slower speed than the magnetic field that is produced by the alternating current in the stator (the stationary part of the motor). The biggest advantage of asynchronous motors is their sturdiness and their ability to self-start; this means that they do not require an external drive, although it is possible to use a soft starter or a frequency drive to avoid a high inrush current. A disadvantage of an asynchronous induction motor is their value for money, which is usually quite low. However, modern versions in efficiency class 4 (according to standard IEC 60.034) can achieve a high level of efficiency, even up to 95%.
The classic induction motor is traditionally used in applications such as pumps and fans, compressors, conveyors, and various household appliances.
Just as the two previous types, the classic synchronous motor is often used for drive applications without any specific motion control. The rotor of the classic synchronous motor usually has electromagnets, which translates into a very powerful performance. The electromagnets are fed with a regulatable direct current via the slip rings. Furthermore, the advantages of the synchronous motor mentioned earlier, such as a precisely controlled speed and a high energy efficiency, also apply to this type of motor. It is not self-starting and so it is usually controlled by a frequency drive.
This type of motor is often used in heavy industries where huge amounts of power are required. A special application of the classic synchronous motor is also to compensate for the power factor demand/duty cycle in electricity networks.
The stepper motor is especially important in the many applications in which controlled movements are needed. A stepper motor can be designed as a reluctance motor with a toothed rotor or they can have a rotor with permanent magnets. There are also hybrid stepper motors with toothed magnets, which is the type that is most commonly used in industrial applications. The stepper function is realised by activating the phases in the stator one at a time, which makes the teeth of the rotor jump one by one. In principle, the stepper motor does not require any feedback for carrying out controlled movements even though there are versions that have an encoder that supplies the controller with information on whether or not each step has been executed correctly.
Applications in which stepper motors are found are those requiring less power, such as printers, compact CNC applications, and robotics.
The synchronous servo motor is the most commonly used motor in applications with controlled movements. It normally has a powerful permanent magnet as rotor, resulting in greater efficiency than that of motors in which the magnetic field is produced by induction. This motor type is characterised by its compact design, high power density and accurate speed and torque control.
Synchronous servo motors are often used in robot applications and advanced CNC applications.
Last but not least, the linear motor. This is a type of motor in which electrical energy is converted directly into a linear motion. There is thus no need for a toothed belt or ball screw to create a linear movement. The motor can be synchronous or asynchronous, with magnets along the full length of the axis. Linear motors have a high dynamic response, are very accurate but can often be very expensive for large movements.
The linear motor is used primarily in applications requiring high precision, such as in electronics and printing systems.
The range of electric motors offered by Festo is very much geared towards stepper motors and synchronous motors. Since there are many variations and accessories, the choices are plentiful. That is why an interactive tool was developed to help the user select the best possible solution.
This tool enables the user to define the application and the motion cycle in just a few steps, while taking into account the physical dimensions, several load characteristics, and the time spans in which the subsequent movements need to be carried out. Once these parameters have been entered, the tool provides a range of options as well as a list of actual products with which the application can be realised. It also provides the parameter settings and all the files and documents that are needed. In addition, the connectivity to the PLC and the drive configuration are also included.
Finally, we would like to mention a new trend in electric actuation, namely the use of a hybrid stepper motor, actuated by a servo drive. This behaves like a brushless DC motor and combines considerable power and high dynamic response with extremely accurate motion control and a very attractive price.
Festo offers this type of motor with multiprotocol drives, with special functions such as velocity control and field weakening. This motor can also be decelerated in a controlled way in case of an emergency stop, and thanks to its precise control, can be used in many applications with 3D interpolation.
A last trend in electric drive technology is the use of decentralised drives that form a unit with a motor and can thus be installed in the field as an alternative to placing separate drives in a control cabinet. The big advantage is that the daisy chain principle can be used with just one cable going from one motor unit to another; this provides the voltage supply as well as the communication via a drive. This represents a considerable saving when compared to the classic approach in which every motor in the field has its own cable connecting it to the drive in the control cabinet.
In addition, Festo offers the option of connecting IO modules to the decentralised drives that can also be used to connect sensors in proximity of the motors to the central controller in the cabinet via a daisy chain. This allows very extensive and complex systems to be equipped with their power supply and communication in a simple and clear layout.