What factors need to be considered in the selection of linear motors

The market share and usage rate of linear motors are increasing, partly due to the end users needing higher throughput and higher accuracy. The understanding of linear motors is usually that they can run at high speed, have long travel and high positioning accuracy, which other driving mechanisms cannot achieve. However, in fact, the advantages of linear motors are not only limited to these, but they can also complete extremely slow, smooth, and precise movements. More precisely, linear motor technology provides a wide range of powerful features: thrust, speed, acceleration, positioning accuracy, and repeatability, to the extent that there are few solutions or applications that cannot be achieved using linear motors.

When linear motors are the best choice in applications, the following three things need to be considered when selecting the initial motor.

1、 Is there an iron core or no iron core?
There are two main types of linear motors: with iron core and without iron core, which refer to whether the winding of the primary part (similar to the stator in a rotating motor) is installed in a laminated iron layer or in epoxy resin. Determining whether an application requires an iron core or a non iron core linear motor is usually the first step in design and selection.

Iron core linear motors are most suitable for applications that require extremely high thrust. This is because the lamination of the primary component contains "teeth" (protrusions), which gather magnetic flux to the magnet of the secondary component (similar to the rotor in a rotating motor). The magnetic force between the iron core of the primary section and the permanent magnet of the secondary section enables the motor to provide higher thrust.

Coreless linear motors typically have lower thrust, making them unsuitable for applications with extremely high thrust requirements such as stamping, machining, or forming, but they are more adept at high-speed assembly and transportation.
Coreless linear motors are sometimes referred to as "U-shaped" linear motors because their secondary parts have a shape similar to a "U" shape, with two magnetic plates installed opposite each other. The main part (also known as the "thruster") is located in the U-groove between two magnetic plates.

The disadvantage of a design with an iron core is the tooth groove effect, which reduces the smoothness of motion. The occurrence of tooth slot effect is due to the slotting design of the primary component, which gives it a "preferred" position when moving along the magnetic line of the secondary component. In order to overcome the trend of alignment between primary and secondary magnets, the motor must generate greater force to counteract the trend, which leads to fluctuations in speed, known as the cogging effect. The fluctuation of force and velocity can reduce the smoothness of motion, which may be a major issue in applications where the motion quality (not just the final positioning accuracy) within the stroke is highly required.

Manufacturers use multiple methods to reduce tooth groove effects. A common method is to arrange the position of the magnet (teeth) in an inclined manner, which makes it smoother when the primary passes through the secondary magnet. In addition, changing the shape of the magnet to a slender octagon can also achieve a similar effect.

Another method to reduce the tooth slot effect is called segmented winding. In this design, the laminated teeth of the primary coil are more than the magnets in the secondary, and the laminated stacking has a special shape. These two designs work together to counteract tooth groove forces. In addition, this problem can be solved through software solutions. The algorithm for reducing the cogging effect allows servo drivers and controllers to adjust the current provided to the primary coil, thereby minimizing changes in force and speed.

Coreless linear motors do not have tooth slot effects because their primary coils are encapsulated in epoxy resin instead of being wound around the iron core. The quality of a coreless linear motor is lower (epoxy resin is lighter than iron core, but has lower hardness), which enables the highest acceleration, deceleration, and operating speed to be achieved in electromechanical systems. The stability time of motors without iron cores is usually better (lower) than that of motors with iron cores. The primary coil does not use an iron core, and the disappearance of related tooth slot effects or speed pulsation also means that a coreless linear motor can provide very low speed, stable motion, usually with a speed fluctuation of less than 0.01% during low-speed operation.

2、 How integrated is the linear motor?
Like a rotary motor, a linear motor is just a component in the motion system. A complete linear motor system also requires bearings (guide rails) to support and guide loads, cable management, feedback (usually linear encoders), servo drives, and controllers. Experienced equipment and machine manufacturers, or users with very unique design or performance requirements, can build complete systems with internal features and ready-made components from different manufacturers.

The design of linear motor systems can be said to be simpler than systems based on belts, gears, racks, or screws, with fewer components and fewer labor-intensive assembly steps (without aligning ball screw brackets or tensioning belts). Linear motors are non-contact, so designers do not have to worry about preparing for lubrication, adjustment, or other maintenance of the drive unit. But for original equipment manufacturers and machine manufacturers looking for turnkey solutions, there are countless options for complete linear motor driven actuators, high-precision stages, and even Cartesian and gantry systems.

For DIY enthusiasts, when choosing a suitable linear guide for a cored linear motor, it is necessary to consider the attraction between the primary and secondary components, which may increase the load on the linear guide. But there is no such problem with a coreless linear motor, because not using an iron core in the primary part means there is no attraction between the primary and secondary parts.

3、 Is the working environment suitable for linear motors?
Linear motors are often the preferred solution in special environments, such as cleanrooms and vacuum environments, as they have fewer moving parts and can be paired with almost any type of linear guide or cable management to meet the particle generation, degassing, and temperature requirements of applications. In extreme cases, the secondary (magnetic track) can be used as a moving component, while the primary components (winding, including cables and cable management) remain stationary.

However, if the environment is composed of metal debris, metal dust, or metal particles, a linear motor may not be the best choice. This is especially true for linear motors with iron cores, as their design is essentially open, exposing the magnetic tracks to pollution. The semi enclosed design of the coreless linear motor provides better protection, but care should be taken to ensure that the slots in the secondary components are not directly exposed to pollution sources. The design of enclosed linear motors with and without iron cores can solve the problem of pollution, but it will reduce the motor's heat dissipation capacity and may replace one problem with another.