Bench Talk

(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)

Operating Principles

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.

Key Parameters and Performance Characteristics

There are many parameters that characterize the performance of a linear motor. Among the top-tier parameters are:

• Continuous thrust force: the maximum rated current that can be supplied to the motor windings without overheating.
• Peak force: The maximum force of the linear motor.
• Overall stator length: The length of the fixed magnet track.
• Slider or carriage travel: The actual range of travel of the moving coil.
• Maximum speed: The maximum speed of the linear motor.
• Maximum acceleration: The maximum acceleration of the linear motor.

Relative Pros and Cons

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:

• Higher speed with resolution: High-performance linear motors can achieve speeds of three meters per second with one-micron resolution and over five meters per second (about 200 inches/sec) but with coarser resolution.
• High precision: The combined accuracy, resolution, and repeatability is largely controlled by the feedback device performance, control-system bandwidth, and cost.
• Fast Response: The inherent response speed of the linear motor can be one-to-two orders of magnitude faster than a mechanical transmission, resulting in faster accelerations and crisper settling times.
• Stiffness: The stiffness of a linear motor is determined by loop gain and current drive and is higher than that of a mechanical linkage.
• Zero backlash: There is no backlash or slack due to the absence of mechanical transmission components.
• Zero wear: As there is no contact between the flat stator track and the moving rotor, there is no wear or friction. While there can be wear on the guide track, but this is managed by suitable design and sizing.

Engineering is about balancing priorities and tradeoffs, and linear motors have their relative drawbacks as well:

• Cost: Linear motors are more expensive partially due to the need for rare earth magnets along the entire length of the travel rail rather than just around the rotor of a rotary motor. Similarly, the linear feedback arrangement is more costly than the same for a rotary motor, again due to the length of the travel path.
• Coordinated power cables: The power cables for the motor's moving part must flex and travel along with the motor, creating design issues.
• Protection: Linear motors are relatively exposed and can be challenging to enclose and protect from their operating environment.
• Bandwidth: Linear motors require higher bandwidth drives and loop controls, as there is no mechanical reduction between the motor and the load; also, the servo response bandwidth must be faster. 
• Force versus size: Linear motors are not as compact “force generators” as rotary motors, where the transmission provides a mechanical advantage—their output versus volume is lower.
• Self-heating: In most linear-motor applications, the motor is attached to the load, so any losses (I²R) are also coupled to the load. In some cases, additional thermal management is needed, especially as the epoxy used to hold the coils in place does not dissipate heat well.

Unconventional Applications of Linear Motors

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.

Conclusion

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.