Bench Talk

The broad world of DC motors is divided into two basic categories: Brushed and brushless. The brushed motor has been around “forever,” figuratively speaking, and while billions of such motors in use prove it can work well, it has many well-known drawbacks. These include brush wear, electrical noise, low to moderate efficiency, controllability, and more.

Several decades ago, the brushless motor with its electrical commutation became popular. This was largely due to two developments: High-energy, permanent magnets and low-cost, effective power-switch devices (i.e., metal-oxide semiconductor field-effect transistors, or MOSFETs, and insulated-gate bipolar transistors, or IGBTs) for the coils. Pretty soon, it seemed as if brushed motors were relegated to low-end applications where high performance and reliability weren’t priorities. Even the larger brushed motors in the range of hundreds of horsepower transitioned to true brushless designs or variable AC drives (a cousin of brushless), while smaller motors likewise often transitioned to the stepper-motor approach, another relative of the brushless motor. At some point, it seemed that brushed motors were only for low-cost, throwaway toys; window displays; and similar low-end applications.

Therefore, when a new product needs a DC motor, now the tendency is to think brushless, right? Probably so, but that would be a short-sighted approach. This was made very clear to me by the fascinating case study article entitled “Every Drop Counts: Designing Motors to Optimize Home and Ambulatory Infusion Pumps,” published in a January 2018 issue of Medical Design Briefs, in which an engineer at Portescap performs a motor-selection analysis for an infusion pump. This pump—motor, gearing, and pump mechanism—must be small, efficient, quiet, and reliable, as it sits on a pole near its user or while carried by a patient.

The author analyzes why the brushed motor is the best choice in this case, yet he also admits to its relative shortcomings versus the brushless motor. First, of course, he defines the flow-rate specification the assembly must achieve, which translates to torque and other basic performance specifications for the motor. He then qualitatively compares the characteristics of brushed, brushless, and stepper motors that meet these requirements. He also discusses the different gearing arrangements to the pump mechanism that the varying motor types would need, which is a major factor in the motor-selection process.

What impressed me was that the article did not say that the brushless was the best across all attributes of efficiency, compactness, lifetime, noise level, and reliability. In other words—though of no surprise—every design choice is a balance of tradeoffs made in the context of the dependencies among them. This is an engineering reality that is too often glossed over in a design review or discussion, yet it is the essence of the engineering process.

Such an article can teach old and new designers alike about focusing on more than simply matching a choice of options to the design priorities in a specific case. It can also clarify the need to be clear in any analysis when deciding on priorities, their weight, and their technical and dollar costs. In this case, the brushless and stepper motors had issues related to gearing and efficiency at the speed and torque levels required, which made them less-desirable choices even while the brushed motor showed some weaknesses regarding longer life.

Whether making decisions about motors or other key components, going with the “obvious” solution or with conventional, popular wisdom may not be the right choice. Step back and look at the numbers, tradeoffs, relationships, and compromises among parameters and performance with honesty, even as the case study showed for the infusion pump application.