Despite the ascendance of brushless and stepper motors, the classic brushed DC motor is still viable for many cutting-edge applications.
The broad world of DC motors is divided into two basic categories: brushed and brushless. The brushed motor has been around “forever” and while billions of such motors in use prove it can work well, it also has many well-known drawbacks. These include brush wear, electrical noise, low-to-moderate efficiency, controllability issues, and more. Despite these shortcomings, brushed motors were widely used, as they were the only DC-motor option in most cases.
The motor situation changed several decades ago as the brushless motor with electronic commutation became popular. This was largely due to two developments: high-energy permanent magnets along with low-cost, effective power-switch devices (MOSFETs and IGBTs) for their coils.
Many larger previous brushed-motor applications also went to brushless designs or variable AC drives (a cousin of brushless motors) while smaller motors often migrated to the stepper-motor approach (a close sibling). It seemed that brushed motors were only worth consideration for low-cost, low-end, non-critical applications such as throwaway toys, window displays, and similar where higher performance and reliability weren’t priorities.
Nonetheless, I have a modest confession: I have a “soft spot” for brushed motors. Perhaps it’s because I used to play around with them way back in the day (I often salvaged them from broken toys). Or perhaps it’s the direct and visible relationship they provide between their construction and operation. They are so simple that they are offered as STEM-focused kits for children, Figure 1.
Figure 1 The classic brushed-DC motor is an effective teaching and demonstration fixture for basics of electricity, magnetism, and motion. (Image source: Banggood)
Nonetheless, when a new product needs a DC motor, the tendency is to automatically think brushless in many cases – but that would be short-sighted. This was made very clear to me when I came across a white paper “Every Drop Counts: Designing Motors To Optimize Home and Ambulatory Infusion Pumps” from Portescap, a leading vendor of small motors.
The paper detailed the motor-selection analysis for an infusion pump. This pump (motor plus gearing plus pump mechanism) must be small, efficient, quiet, and reliable, as it sits on a pole near the user or may even be carried by the patient. The author analyzed why the brushed motor is the best choice in this particular case, yet also admits to its relative shortcomings versus the brushless motor (as this vendor offers brushed and brushless motors, he can be unbiased here).
First, of course, he defines the flow-rate specification the assembly must achieve, which then translates to torque and other basic performance specifications for the motor. He then qualitatively compares the characteristics of brushed, brushless, and stepper motors which meet these requirements. He also discusses the different gearing arrangement to the pump mechanism that each motor type would need, which is a major factor in the motor-selection process.
What also impressed me was that the article did not say that the brushed motor was the best option across all attributes of efficiency, compactness, lifetime, quiet operation, and reliability. In other words—no surprise—every design choice is a balance among trade-offs made in the context of the priorities and their ranking, as the author points out in a summary of the relative attributes of various motor types in this specific application, Figure 2.
Figure 2 The relative pros and cons of different motor architectures for the portable infusion-pump design show how the various attributes are weighed and balanced against the application priorities. (Image source: Portescap)
However, brushed motors are not just for highly specialized applications such as this infusion pump. They are also used for a wide range of automotive functions with constrained, defined velocity and torque requirements. As an indication of this, vendors have recently introduced automotive-specific brushed motor drivers, which is clear evidence of their continued viability.
For example, the TB9053FTG and TB9054FTG from Toshiba Electronic Devices & Storage Corporation are new two-channel drivers for brushed DC motors. They target automotive applications such as throttle valve control, engine valves, retractable door mirrors, seat positioning, and door latches, Figure 3. That’s a lot of new-product activity supporting a motor architecture which some may consider outdated and not worthy of serious consideration.
Figure 3 There’s lots of life left in brushed DC motors, as indicated by this array of recent automotive-specific drivers for them from just one vendor. (Image source: Toshiba Electronic Devices & Storage Corp.)
Reading an article such the infusion-pump design story can teach new (and old) designers more than the specifics of matching a choice of options to the design priorities in a specific case. It also reinforces the need to be honest in any analysis when deciding about priorities and their weighting, along with the technical and dollar cost of various solutions. In this case, the brushless and stepper motors had issues related to gearing and efficiency at the speed and torque levels needed which made then less-desirable choices, while the brushed motor did have some weakness in terms of longer life.
My experience is that most motor vendors offer a broad range of brushed and brushless motors and their many sub-varieties. As a result, they can be impartial in helping designers assess the best motor for the application. Most also offer comprehensive application notes to help with selecting a motor type and sizing it to provide the needed torque, velocity, size, and other factors.
Whether making decisions about motors – as well as other key components – just going with the “obvious” solution or conventional, popular wisdom may not be the right choice in the target application. Step back and look at the numbers, tradeoffs, relationships, and compromises among parameters and performance with honesty, as this article shows for this application.
Have you ever had to go “back to the future” to find the optimum solution for your design needs? Did you (or others) ignore or exclude older components and technologies simply because they were assumed to be outdated? Was it hard to convince yourself or the others that these older approaches were not only still viable, but the right choice in your situation?
This article was originally published on EDN.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.