Brush DC VS. Brushless DC Motors For Disposable Surgical Power Tools
Disposable tools are a fast-growing segment within the surgical hand tool market, driven by cost and hygiene benefits.
For tool design engineers, the motor technology is often a critical choice, specifically the selection of either a Brush DC or Brushless DC motor.
Surgical power tool designers must choose whether to take a disposable or reusable design approach. Advances in component design and manufacturing techniques have made it increasingly possible to produce equipment with the capability required to perform complex surgery at a price point where disposal of the tool after a single surgery can be justified. These advances extend to motors. Design and manufacturing improvements for both Brush DC and Brushless DC (BLDC) motors have reduced price points while increasing performance – thus making it possible to achieve a sufficiently low per surgery tool cost with a disposable design. In addition, for some tools there may be an opportunity to increase safety by removing the risk of infection due to inadequate sterilization.
Motor Performance Requirements for Single-Use Tools
While motor performance requirements are similar for reusable and disposable surgical tools, the lifetime and cost requirements are vastly different. A motor specified for a reusable tool may have a lifetime requirement of hundreds or even thousands of surgeries and thus must utilize premium components and materials to achieve this remarkable feat. A motor for a disposable tool needs to provide similar performance – albeit for only a single surgery – yet must be available in high volumes and at a competitive price.
When specifying motors for disposable tools, design engineers should consider the possible advantages of conventional Brush DC motors over the more advanced BLDC technology. Due to the nature of the design and inherent reliability advantages, BLDC motors are the typical choice for use in reusable power tools. The advantages of BLDC do unfortunately increase costs vs Brush DC; thus, it is often infeasible to specify a BLDC motor for a disposable tool. Designers should work with a motor supplier well-versed in both technologies so that the performance and cost trade-offs between a Brush DC and BLDC motor technology are correctly identified.
Brush DC vs. Brushless DC Motors
If the goal of a new project is to maximize performance and reliability, a design engineer is likely to gravitate to BLDC technology. Brushless technology makes it possible to operate at high speeds (up to 100k RPM) over a long operating life. In BLDC, commutation is achieved without the use of mechanical brushes (i.e., via magnetic Hall sensors or sensorless drive with a brushless motor controller) – thus the contact between the rotating components and the stationary components in the motor is limited to the ball bearings. This means the lifetime of the motor is primarily related to the longevity of the bearing, and the motor can operate at high speed for an extended period.
By comparison, in a Brush DC motor, commutation is achieved through mechanical brushes (graphite or precious metal) making physical contact with the rotor to complete the electrical connection. In this case, the lifetime of the motor is primarily limited to the lifetime of the brushes, with higher speeds leading to premature wear. For a disposable tool, the higher speed may not be an issue given the short lifetime requirement – however this will depend heavily on the duty cycle and speed requirements of the application.
The performance of a Brush DC motor will also vary significantly depending on the design and materials used. For example, Portescap Brush DC motors are ‘coreless’ (most lower cost brush motors will feature an iron core), meaning that the rotor is only composed of a coil and a single shaft. The coreless design offers lower inertia which results in higher performance regarding acceleration and efficiency. It also eliminates detent torque (cogging torque), which can cause reduced smoothness of rotation at slower speeds.
For reusable surgical tools, the lifetime and speed requirements often make BLDC the ideal solution. However, for some applications leveraging a single-use design, a Brush DC motor can provide an attractive solution.
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