Bench Talk

For most electronic engineers, nearly all software engineers, and even many mechanical engineers, the world of electric motors and their many variations is a blend of mystery, magic, and fear. There's no minimizing the fact that there are so many sub-types and sub-sub-types, and each one has its own idiosyncrasies, attributes, virtues, and weaknesses. The motor "family tree" just begins to tell the story in Figure 1.

Figure 1: The motor "family tree" is both fascinating and confusing; it starts with a basic division between AC-versus-DC power source and then branches out into many variations. (Image source: Lessons In Electric Circuits)

Any discussion of motor selection begins with the basic division between AC and DC motors, and then expands into many major and minor branches of the tree. Looking at the above tree brings up four questions:

1. Why are there so many motor types and sub-types?

Motor design and performance is guided by the laws of physics, primarily electromagnetics, with a constant interplay between electric currents and magnetic fields. While these laws allow us to develop equations and models which characterize an ideal motor, the reality is that there are many real-world obstacles to creating that ideal motor. Motors also bring in mechanical issues such as moment of inertia, rotating masses, and centrifugal force, all of which complicate the design and create a gap between theory and execution.

Also, engineers are clever. Over the 100+ years of electric motor history, they have developed subtle variations and shadings of each motor type to overcome specific shortcomings that make a motor design unsatisfactory in a given class of application. Complicating the situation, many of these variations were devised long before the availability of modern control electronics (MOSFETs/IGBTs), microcontrollers, and FPGAs. Today's advances offer very different sets of tools to overcome those inherent weaknesses, while those earlier engineers had to work with what they had available, and devised some impressive but hard-to-grasp designs.

2. Why are most engineers somewhat fearful when considering or dealing with motors?

Beyond the very basics, principles of magnetism and electromagnetism are not taught in most EE programs. These principles are less "tangible" than more-familiar, easier-to-measure electronic concepts with parameters such as voltage, current, and resistance. Certainly electromagnetic and magnetic fields are fully defined by Maxwell's equations and others, and are analogous to those of electric fields.

Yet the reality is that magnetics are much harder to grasp, measure, and visualize for most engineers. Very good models of motors are available (and have been for decades), but many engineers feel that it is like trying to learn a very difficult, new language that uses strange symbols with a totally different grammatical structure.

3. Is this fear and reluctance justified?

To a large extent, yes. The many issues involved in motor selection and drive are complex and subtle, and exist at the intersection of electric fields, magnetic fields and mechanical considerations. This makes nearly every motor-related decision a challenging balance among tradeoffs that may be hard to grasp and harder to decide.

Even what seems like a simple question has many answers: should you use a high-speed motor and gear it down to slower speed (bringing issues of cost, weight, size, backlash and positioning errors), or do you go with a motor that can inherently be operated at slow speed (but has issues of torque, start/stop time, and more).

Complicating the challenge is the fact that the ways in which motor performance is quantified are very different than how electronics are assessed. Motors are judged by their RPM, torque, resonances, position performance, and acceleration/deceleration, among other factors; these are very different from conventional electronic-performance factors such as drive level, distortion, linearity, and accuracy. In addition, motors are current-driven devices, and most electrical engineers are much more comfortable with voltage-driven components.

4. What can you do if you need to choose or use a motor?

In some cases, the designer is fortunate: there is a body of experience or an accepted approach to help guide the choice. For example, if you are designing a subsystem to move a printhead across a page or in a rapid-prototyping 3-D printer, a stepper motor is the most logical choice with very few exceptions.

But many situations are not so clear cut. Even after the first decision of whether to use AC or DC is already established by the system design, there are still many issues to consider in terms of motor type, driver circuitry, control algorithms, and driver and motor protection. For these reasons, it's often a good idea to consult with an experienced motor- or motion-control applications specialist at a distributor or vendor early in the cycle. The good ones have technical understanding and insight, which can get you pointed more quickly in the right direction and so avoid a lengthy, error-prone, and often painful learning curve. Doing so allows you to focus on optimizing a fairly good to very good solution, rather than trying to first determine what that solution looks like.

Don’t be afraid of motors: literally billions of them—from tiny to huge—are successfully designed into countless products. After completing some successful motor-related designs, you might even find them fascinating and exciting as well.