The linear motor: The "unwound" rotary motor

Basics about linear motors The linear motor: The "unwound" rotary motor

08.06.2019Editor: Theresa Knell

Linear motors are the drives of choice when high dynamics are required for translational propulsion in automation or handling technology. These direct drives are not new, but only gradually did they establish themselves in linear technology. They prove themselves especially in highly dynamic positioning tasks.

Linear axis with an integrated linear motor.
(Source: Hiwin)

The linear motor is derived from the rotary motor and was already realized in the middle of the 19th century. In the 1950s and 60s, the English engineer Eric Laithwaite further enhanced the linear motor and even applied the principle in a prototype in the form of the Maglev (magnetic levitation).

Layout of a linear motor

In contrast to the rotary motor, the stator, i.e. the magnets or electromagnets, in the linear motor is not arranged in a circle around the rotor, but - as the name suggests - in a linear way. Thus the linear motor could be referred to as an “unwound” rotary motor. In principle, the travel path can be any distance and can also be curved. For this reason, the linear motor is also suitable for powering trains.

Gallery

As with the rotary motor, in most cases the linear motor consists of a combination of alternating current driven electromagnets and permanent magnets. Acceleration and speed can be continuously adjusted via the frequency of the alternating voltage. If the electromagnets are located in the stator, this is referred to as a traveling magnetic field that pulls the rotor. Slightly offset spools make it possible to change the running direction.

There are two different design systems: Either with fixed permanent magnets and moving coils (Moving Coil Motor) or with fixed electromagnets, whereby the component is moved by the permanent magnets (Moving Magnet Motor). With the latter, maintenance is simplified by the fact that there is no need to carry a power supply with the moving part. This can also extend the service life. However, the moving coil system can achieve longer travel paths.

All types of linear motors have in common that, in addition to the actual drive, they require a guide for the rotor and a measuring system for position determination.

Characteristics of linear motors

Compared to other types of linear drives, linear motors have some outstanding features. For instance, they don’t need a conversion from rotary to linear motion. This is why we speak of direct drives. This results in lower mechanical losses on the one hand and on the other, the linear motor also permits curved movements. This cannot be achieved, for example, with toothed belt drives or ball screws. In the case of indirect drives, the travel path is limited. Although toothed belt drives achieve speeds similar to those of direct drives, they have a clear disadvantage in terms of acceleration and, in particular, precision.

However, toothed belt drives have a decisive advantage over linear motors for applications in which feed rates with medium traverse paths and medium precision are required: They are much cheaper. In particular, when very high precision is required for positioning, the linear motor requires inexpensive magnet systems to be replaced by optical systems for position determination - which increases costs. Furthermore, the price for the magnet systems rises with increasing travel path distance.

Material heating during construction

Since linear motors can heat up at peak loads, the resulting material heating must be taken into account, as it leads to inaccuracies in positioning, whereby position sensors in turn play an important role. On the other hand, the heating leads to stresses in the material or on the platform on which the motor is positioned.

Special couplings and clutches are available for a wide variety of applications. Their common feature is the transmission of rotary movements. (SKF Group)

In addition to their high positioning accuracy down to 1 µm, linear motors permit travel speeds of more than 10 m/s and accelerations of up to 250 m/s². This also places high demands on the carrying structure of the respective machine. Especially when larger masses are moved, very high acceleration forces occur, which the structure has to absorb.

Depending on the design, linear motors also place high demands on the bearing and guide of the rotor. In addition to slide rails, rolling bearings or wheels can also be used on the mechanical side. Air cushions are also used to maintain the distance to the magnetic rail. In the Transrapid, for example, the train, which is the rotor of the linear motor system, is held almost frictionless in the guide by magnets.

Types of linear motors

Depending on the application and requirements, different linear motor designs are used. For example, in an environment with sensitive electronics, the strong external magnetic field generated by the motor can be avoided by a stator consisting of a cylindrical magnetic winding with a permanent magnet. Apart from the low external magnetic field, this so-called cartridge motor is characterized by a high power density.

A common design is that of the flatbed motor. In particular, the permanently excited flatbed linear motor has a high power density. The rotor moves with the traveling field in a rail equipped with permanent magnets. It is important that the rotor has a robust bearing, which must ensure its stability within the strong magnetic fields. Depending on the environment, the motors must also be shielded from the outside.

Drop forging is the most widely used forging process. (Deutsche Edelstahlwerke)

Another design is the ironless linear motor: A coil without an iron core is located as a movable element between two magnetic tracks. Due to the low mass of the rotor, this design enables extremely high accelerations. However, heat dissipation poses a problem, so that no high continuous forces are possible.

Additional Information

APPLICATION: MAGNETIC LEVITATION TRAINS

The largest versions of linear motors are installed in magnetic levitation trains. In this case, the magnetic principle serves on the one hand to guide the train and on the other hand to move it. The train itself or a part of it functions as the runner. In most magnetic levitation trains, the current-carrying alternating current magnets are installed in the guideway, as otherwise the train itself must be inductively supplied with the necessary voltage.

Already at the beginning of the 20th century first models of magnetic levitation trains were developed. In 1934, the German engineer Hermann Kemper filed a patent for a magnetic levitation train. However, the system was not realized until the second half of the 20th century. In the 1970s, Eric Laithwaite developed a new arrangement, the "Magnetic River", which made it possible to integrate a guide in the rail and generate thrust with a linear motor. The first commercial monorail system was opened in Birmingham in 1984. Since then there have been many prototypes and test trains.

However, the magnetic levitation train only became commercially established on a large scale in Japan with the Shinkansen. In 2015, it reached a speed record of over 600 km/h for the first time. A suspension railway system has also been in operation in South Korea since 1993. Other well-known types are the Japanese JR-Maglev and the German Transrapid. New developments are planned in South Korea, for instance.

Applications of linear motors

As mentioned before, linear motors are often used when high dynamics or a high degree of precision are required. In handling systems, for example, they can reduce cycle times through high speeds and thus contribute to cost savings. In this case, the acquisition costs are often offset in a short period of time. Precise positioning is required in machine tools. But in hard disks this characteristic of linear motors for reading/writing heads also comes to bear. Since linear motors contain few or no mechanically frictional components, they are often used in clean rooms - an aspect that is also relevant for hard disks. The combination of precision and speed is important, for example, in laser, water or plasma cutting, where linear motors are used to position and guide the cutting unit.

This article was first published by MaschinenMarkt.

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