There is a current trend and an increasing number of requests for designing brushless DC motors into high-speed applications. New impeller technology, for instance, is changing the way modern respirators are designed, allowing them to be more compact and quieter. Such performance requires motors that rotate up to 50krpm to 60krpm and are capable of delivering high acceleration and deceleration in synchrony with a patient's breathing pattern. Other examples include surgical and dental hand tools where motors have to be ever stronger and smaller.

One way to achieve this level of performance is to use high-speed brushless motors that can deliver the necessary power and performance, within the preferred footprint. Mechanical power is the product of torque and speed, and, to increase power, there is a need to either increase torque or speed. But generally, for a given technology, continuous torque is related to the motor's size and is often limited by thermal considerations.

Without considering high-speed constraints, a motor designer will try to optimize the torque the motor can dissipate for a given power dissipated by Joule effect. The figure of merits R/k2 is a good factor to characterize a motor, and a good motor should have a small resistance and a high torque constant. One way to increase the torque constant is to use stronger magnets such as NeoFe.

In order to decrease Joule losses, the objective is to have as large a wire section as possible, to create the lowest copper resistance. After optimization of R/k2, maximum torque is still limited for a given motor size by its thermal limitation. Consequently, the other parameter to increase the power is to increase the speed. 

Copper losses (Joule losses) vary with the load in proportion to the square of the current, or R*I2. Iron losses are the result of magnetic energy dissipated when the motor’s magnetic field is applied to the stator core.

In theory, it seems easy to increase the speed by simply increasing the voltage of the power supply. However, increasing speed will generate more heat due to iron losses, bearing friction losses, and current ripple that also creates losses.

Iron losses due to Eddy current are the losses generated by the current circulating in the lamination created by magnetic flux. To decrease these losses, thinner laminations with high electrical resistance are used to increase the resistance within the lamination.

The magnetic material used to conduct the magnetic field also produces losses due to hysteresis. Every time the magnetic flux is reversed, the hysteresis of the material will generate losses. During the design, engineers can optimize hysteresis losses by using specific magnet materials such as FeNi.

Iron losses depend on the square of the induction in the iron laminations, and on the square of the frequency. For this reason, motors generally having a high number of pole-pairs will have speed limitations. In many cases, according to the specific design, a motor with two pole-pairs will have more iron losses than a motor having one pole pair, but it's likely this motor will have a better R/k2.

Different brushless DC motor types
In terms of technology, there are two major types of brushless DC motors, slotted and slotless, which utilize different stator designs.

In slotted stator motors, the coils are wound within the slots. Magnetic induction in the lamination is high since the air gap between the laminations (stator) and magnet is small. The volume of the copper is limited by the slot space and by the difficulty to wind within the slot.

Having the coil inside the stator slots offers the advantage of reducing the thermal resistance of the coil/stator assembly and makes the motor robust. Without current, the rotor has preferred magnet positions in front of lamination, generating a cogging or detent torque. One way to decrease the detent torque is to skew the lamination. By design, it's also possible to build motors having a large ratio of length to diameter.

The two major types of brushless dc motors, slotted and slotless, utilize different stator designs.

With a slotless motor, the coil is wound in a separate, external operation and is a "self-sustaining" type. This coil is then inserted directly into the air gap during motor assembly. In this design, the magnetic induction in the coil is decreasing since the air gap is increasing. Therefore, the motor diameter is usually optimized to have the ideal magnetic induction with the optimum copper volume. Usually by design, induction in these motors is much smaller than in a slotted brushless motor, and a larger magnet is used to compensate for the loss of induction. Since the inertia of a rotor follows the square of its diameter, the inertia in a slotless motor is usually higher than the slotted motor.

In terms of R/k2, a slotless motor has merits since induction versus copper volume is optimized while a slotless motor doesn't have any cogging/detent torque. By design, iron losses at high speeds are greatly reduced in slotless motors.

High-speed medical/surgical applications
A motor in a respirator application has to be able to ramp up from a few thousand rpm to 50krpm in milliseconds, in synchrony with a patient's breathing pattern. The torque needed to spin the impeller is in the range of a few ounce-inches. Most of the torque is used to accelerate and decelerate the impeller. Portescap recently developed a new motor family for this specific application where Joule versus iron losses have been optimized to fulfill stringent needs.

Brushless dc motors are being used in high-speed applications for surgical and dental hand tools where motors have to provide high power, small sizes, and high reliability.

Motors used in surgical hand tools have to run at high speed to produce power in a lightweight package. They also need to run at low temperature for the surgeon's comfort and be able to survive the autoclave sterilization process.

Optimization of the magnetic circuit has allowed Portescap to design a 16mm diameter motor capable of delivering a few ounce-inch of torque at speeds of up to 80,000rpm without exceeding 43C on the motor housing.