When most people think of electric motors, they think of motors that provide rotary motion and turn a shaft. This is because these are the motors that many people are familiar with.

But there is another type of electric motor: the linear motor (Figure 1).

Although lesser known, these motors are widely used in a variety of applications that require linear motion. Applications include large overhead cranes and building elevators, medium-sized beltless conveyors, window curtains, small equipment for handling wafers in semiconductor manufacturing plants, and even micromotors for adjusting the focus of camera lenses. Needless to say, it has many other uses as well.


One of the important features of linear motors is that they are not devices that convert a conventional rotary motor into linear motion by adding mechanical devices such as gears, lead screws, ball screws, or belts. These conversion devices are called linear actuators and play an important role in the design of electromechanical systems.

On the other hand, linear motors are essentially motors that output linear motion directly, with no rotational motion to be 'converted' into linear motion. Linear motors are one way to provide straight-line (linear) motion, but there are also pneumatic (compressed air), hydraulic (pressurized fluid), and electric power methods (Figure 2).

Although these early linear motors started out as low acceleration devices, recent advances in brushless direct current (BLDC) motor technology and high-performance drive electronics have largely overcome these limitations.


■ Linear motor that moves with the stator unfolded

In a traditional electric motor, the rotor rotates inside a stationary stator. In a linear motor, on the other hand, the stator is 'unwrapped' and lies flat, and the rotor moves in a straight line over it. As in a rotary motor, the moving rotor surface does not touch the stator surface, but there may be guide rails or tracks to align it. The stator is a track of flat coils made of aluminum or copper, and the rotor is a moving platform.

Linear motors are driven by alternating current (AC) power and a servo controller, often the same type as rotary motors. The current phase in the coils is controlled and adjusted to change the polarity of each coil.

Alternating attractive and repulsive forces between the current-carrying electromagnetic coils in the rotor and the permanent magnets in the stator generate a linear force that moves the rotor along the stator.

As with rotary motors, the rate of change of current controls the speed of movement, and the magnitude of the current (amperes) determines the force produced. Consequently, precise movement can be controlled by managing the timing (phase) of the power supply to the electromagnets.

■ Determining linear motor performance with multiple parameters

There are several parameters that determine the performance of a linear motor. The main parameters are as follows:

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

■ Support for repeatability and accuracy

One of the major advantages of linear motors is that they do not have the variable losses of transmission components such as gearboxes and couplings, and they do not have the backlash and motion/resonance errors associated with these components. This allows for much higher bandwidth and stiffness of the motion system, resulting in better repeatability and accuracy.

As mentioned earlier, linear motors are not the only way to provide well-controlled linear motion. In many cases, the same motion effect (although not the same performance) can be achieved using a less expensive solution that combines a rotary motor and a ball screw or other mechanism as a linear actuator.

So why should design engineers use linear motors instead of ball screws or linear actuators? Simply put, linear motors have the advantage of providing faster motion, acceleration, and very high accuracy, while ball screws and linear actuators provide higher force and lower cost.

A closer look at the relative characteristics of linear motors compared to linear actuators is as follows:

- Higher speed with resolution: High-performance linear motors can achieve speeds of 3 meters per second (3 m/sec) and resolution of 1 micron (1 ㎛), and speeds of 5 meters per second (5 m/sec; about 200 inches/sec) or more are possible, but resolution may be somewhat reduced.

- High precision: Accuracy, resolution, and repeatability are mainly determined by the performance of the feedback device, the bandwidth of the control system, and cost.

- Fast Response: The inherent response speed of linear motors can be 1 to 2 times faster than that of mechanical transmissions, resulting in faster acceleration and shorter settling times.

- Stiffness: The stiffness of a linear motor is determined by the loop gain and current drive, and is higher than that of a mechanical link.

- Zero backlash: Since there are no mechanical transmission parts, there is no backlash or loose movement.

- Zero wear: There is no wear or friction because there is no direct contact between the flat stator tracks and the moving rotor. However, guide tracks are subject to wear, but can be managed through proper design and sizing.

Engineering is a process of balancing priorities and trade-offs, and linear motors do have their relative drawbacks.

- Cost: Linear motors are more expensive in part because they require rare earth magnets along the entire length of the travel rail, whereas rotary motors only require magnets around the rotor. Similarly, linear feedback systems are more expensive than rotary motor feedback systems because of the length of the travel path.

- Coordinated power cables: Power cables connected to moving parts of the motor must bend and move with the motor, which can create design problems.

- Protection: Since linear motors are relatively exposed to the outside world, it can be difficult to protect and enclose them from their operating environment.

- Bandwidth: Since linear motors have no mechanical reduction gear between the motor and the load, they require drives and loop control with higher bandwidth. The servo response bandwidth must also be faster.

- Force versus size: Linear motors are not as compact a 'force generator' as rotary motors. While rotary motors can gain mechanical advantage through gearing, linear motors have less volume relative to their power output.

- Self-heating: In most linear motor applications, the motor is directly attached to the load, so power loss (I²R) is also transferred to the load. In some cases, additional thermal management is required, especially since the epoxy used to hold the coil in place does not dissipate heat effectively.

■ Non-traditional applications of linear motors

Linear motors are also used in two rather unconventional applications: magnetic levitation (maglev) railways and rail guns. Maglev railways have been under consideration for many years, but have only been built and used in a few places.

This train can reach speeds of 300 to 400 km/h (approximately 200 to 260 mph) by using the stator of a linear motor as the track and the train body as the rotor. The train floats (i.e. levitates) above the track, and the frictionless arrangement has the significant advantage of no contact between the two (Figure 3).


Meanwhile, the military rail gun is a concept that has been tested and proven on warships, but is on hold for the time being. The system uses a linear motor about 10 meters long to accelerate a small, non-explosive projectile (a projectile) weighing about 1 kilogram to 2,500 meters per second (about 5,600 mph). The projectile destroys its target solely through the kinetic energy of impact, without an explosion.

However, there are several major obstacles to commercializing maglev trains and railguns. The initial construction cost of maglev trains is much higher than that of conventional trains, and ongoing operating costs (especially power) and maintenance costs are also high. Furthermore, while railgun technology may be explored again for ground-based deployment in the future, there are still challenges to be addressed in terms of size, installation, wear, and power requirements, as well as logistical issues.

■ Despite high cost and complexity, the performance benefits are greater

Linear motors offer unique advantages in applications that require high speed, rapid acceleration, and precise linear positioning. Although there may be design and implementation challenges such as higher cost and complexity, the performance benefits often outweigh these disadvantages. Linear motors eliminate backlash and variable loss issues associated with mechanical transmission components, making them ideal for applications that require high precision and responsiveness.

However, design engineers must carefully compare alternative solutions, such as linear motors and rotary motor-based linear actuators, and evaluate the requirements of their specific applications. With the continued advancement of technology, linear motors remain an attractive choice for a wide range of industrial and specialty applications.

# Author
Bill Schweber is a contributing writer and electronics engineer for Mouser Electronics. He has written three textbooks on electronic communication systems and hundreds of technical articles, opinion columns, and product articles. Previously, he was a website manager for EE Times, managing several topical technology websites, and also served as an Executive Editor and Analog Editor for EDN. He worked in marketing communications at Analog Devices, Inc. (a major supplier of analog and mixed-signal ICs), where he was also involved in the technical PR function, communicating the company’s products, stories, and messages to the media, while also serving as a media outlet. Prior to his MarCom role at Analog, he was an associate editor for a respected technical journal, and worked in the product marketing and applications engineering groups. Prior to that, he worked directly with Instron Corp. on analog and power circuit design and systems integration for materials testing machine control. He holds an MSEE from the University of Massachusetts, a BSEE from Columbia University, and is a Registered Professional Engineer. He also holds an Advanced Class amateur radio license. Bill has also designed, written, and taught online courses on a variety of engineering topics, including MOSFET fundamentals, ADC selection, and LED driving.