(Source: Edgar Martirosyan /stock.adobe.com; generated with AI)
When most people think of electric motors, they assume they're motors that provide rotary motion and turn a shaft, as that’s the motor most people are familiar with.
But there’s another kind of electric motor: the linear motor (Figure 1). While not as well known, it is widely used in applications where a straight-line motion is needed. Applications span large overhead traveling cranes and building elevators, mid-size beltless conveyors and window curtains, smaller-scale semiconductor wafer handling in fabrication plants, and even tiny motors controlling the focus in camera lenses. Needless to say, it's a long list.
Figure 1: Linear motors are electric motors that produce a linear force by “unwrapping” the stator. (Source: Schnibbi678, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons)
One important point about linear motors is that they are not a conventional rotary motor with some sort of mechanical arrangement of gears, lead or ball screws, or belts which transform the rotary output of the motor into linear motion. Those are called linear actuators and have an important role in electromechanical system designs.
In contrast, a linear motor has an inherently linear output motion, and there is no rotary motion to be “converted” into linear motion. The linear motor is one way of providing linear motion, along with pneumatic (compressed gases), hydraulic (pressurized fluid), and electric power (Figure 2). These motors started as low-acceleration devices, but modern brushless direct current (BLDC) motor technology and the supporting drive electronics have largely overcome these and other limitations.
Figure 2: The linear actuator family includes pneumatic, hydraulic, and electric options, each having sub-members. (Source: Author)
In a traditional electric motor, the rotor turns inside the non-moving stator. In contrast, in the linear motor, the stator is “unwrapped” and laid out flat, and the rotor moves past it in a straight line. As in the rotary motor, the moving rotor surface does not touch the stator surface, but there may be a guide rail or track to keep them aligned. The stator becomes a track of flat coils made from aluminum or copper, while the rotor becomes the moving platform.
Linear motors are powered by an AC supply and a servo controller, often the same types used for rotary motors. The current phase in the coils is controlled and adjusted to change the polarity of each coil.
The alternating attracting and repelling forces between the current-driven electromagnetic coils in the rotor and the permanent magnets in the stator generate a linear force that induces the rotor to move along the stator. As with the rotary motor, the rate of change of the current controls the velocity of the movement, and the amperage of the current determines the force generated. The resulting motion can be precisely controlled by managing the timing (phasing) of the power to the electromagnets.
There are many parameters that characterize the performance of a linear motor. Among the top-tier parameters are:
One of the main advantages of linear motors is that they don’t incur the variable losses of transmission components, such as gearboxes, couplings, and the associated issues of backlash and motion/resonance error. Therefore, the bandwidth and stiffness of the motion system can be much higher, supporting better repeatability and accuracy.
As noted earlier, linear motors are not the only way to provide well-controlled linear motion. In many cases, the same motion effect—if not performance—can be accomplished using a less expensive solution with a rotary motor and a ball screw or other mechanism as part of a linear actuator.
So why should a design engineer use a linear motor rather than a ball screw or linear actuator? The short answer is that linear motors are better for fast motion, acceleration, and very high accuracy, while ball screws and linear actuators offer higher force and lower cost.
Let’s look in more detail at the relative attributes of linear motors in comparison to linear actuators:
Engineering is about balancing priorities and tradeoffs, and linear motors have their relative drawbacks as well:
Linear motors are also associated with two rather unconventional applications: magnetic levitation (maglev) railways and rail guns. Maglev railways have been promoted for years but have only been built and used in a few locations. These trains can reach speeds ranging from 300km/hr to 400km/hr (roughly 200mph to 260mph) using a linear motor stator as the track and the railway car as the rotor. The train floats (i.e., levitates) above the track, and the absence of contact between the two as a frictionless arrangement is a significant benefit (Figure 3).
Figure 3: Maglev railways, while rare, have a maximum cruising speed of 300km/hr and have reached a top speed of 431km/hr., meaning a 30km trip takes approximately 8 minutes. (Source: Кирилл Макаров: /stock.adobe.com; generated with AI)
Meanwhile, the rail gun for military uses is a concept that has been tested on combat vessels and validated but also put aside for the time being. The system uses a linear motor about 10 meters long to accelerate small, passive projectiles (roughly one kilogram) to speeds up to 2500 meters per second (about 5600mph). The projectile destroys the target via the impact's non-exploding kinetic energy.
However, there are various major obstacles to both maglev and rail gun deployment. The upfront construction costs of maglev trains are much higher than conventional trains, while the ongoing operating costs (especially power) and maintenance are also higher. And while rail gun technology may be revived for land-based use in the future, the challenges and logistics associated with size, setup, wear, and power needs remain daunting.
Linear motors offer unique advantages for applications requiring high speed, fast acceleration, and precise linear positioning. Though they may come with challenges such as higher costs and complexities in design and implementation, their performance benefits often outweigh these drawbacks. Linear motors eliminate issues like backlash and variable losses associated with mechanical transmission components, making them ideal for applications demanding high precision and responsiveness. However, design engineers must carefully evaluate the specific requirements of their applications, balancing the trade-offs between linear motors and alternative solutions like rotary motors with linear actuators. With continuous advancements in technology, linear motors remain a compelling choice for various industrial and specialized applications.
Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.
Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.
He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.