In recent years, there has been a surge in demand for brushless dc motors for high-speed applications. A variety of factors have contributed to this trend. For instance, new impeller technology has changed the way modern respirators are designed, making them quieter and more compact. Such performance requires motors that rotate as fast as 50–60 krpm (thousands of revolutions per minute) and are capable of delivering high acceleration and deceleration in synchrony with a patient’s breathing.
Other examples adding to this demand include surgical and dental hand tools, which—owing to market demands—are becoming increasingly strong and small. One way to accommodate these trends is to use high-speed brushless motors that can deliver the necessary power and performance within the specified footprint.
Mechanical power is the product of torque and speed. To increase power, either torque or speed should be increased. Generally, for a given technology, the continuous torque is related to the motor size. Continuous torque is often limited by thermal considerations.
Without considering high-speed constraints, a motor designer will try to optimize the torque that the motor can expend for a given power dissipated by the Joule effect, which represents the relationship between the heat generated by current flowing through a conductor. The figure of merits R/K² (where R represents coil resistance and K is the torque constant) is a good factor to characterize a motor. Ideally, a motor should have a small resistance and a high torque constant.
One way to increase the torque constant is to use strong magnets, such as those made of neodymium. To decrease the Joule losses, the wire section should be as large as possible; this will lower the copper resistance. After optimization of R/K², maximum torque for a given motor size is still constrained by its thermal limitation. Consequently, the other parameter that can be used to increase power is speed.
In theory, it seems easy to increase the speed by simply raising the voltage of the power supply. Increasing speed, however, will generate more heat resulting from the following factors:
- Iron losses.
- Bearing friction losses.
- Current ripple–creating losses.
This article discusses losses and tradeoffs for high-speed motors, especially those used for medical applications where issues such as sterilization are present. To read it, click here.
About the author
Norbert Veignat, Ph.D., is the vice president, technologies, at Portescap, West Chester, PA.