High-speed medical motors

Written by: Paul Fanning | Published:

Advanced medical applications make very specific demands of high-speed motors. Dr. Sab Safi of SDT Drive Technology looks at the challenges.

The electric motor plays a significant role in the modern medical industry. The growing worldwide demand for medical analysis and testing services has created a niche for equipment with greater throughput and high reliability.

To accomplish this goal, drive train components must provide more torque over expanded speed ranges and at higher duty cycles. As medical equipment grows smaller, lighter, there is a trend for high-speed motor drives for power density increase, volume and weight reduction and higher efficiency.

High-speed brushless permanent-magnet motor technology is the most suitable choice for these kinds of advanced medical applications due to a variety of merits. They offer substantial reduction in size and thermally excellent high-power density, which reduces the running costs with good performance and reliability.

Several considerations must be addressed when choosing high-speed motors for medical devices. Torque pulsation has an important role where the medical application is very sensitive to vibration and noise such as devices and equipment or patient care facilities in hospital. The motors used are required to comply with low noise level standards to endorse patient comfort and reduce anxiety. Torque pulsation can excite a system that is not mechanically well damped, and this may lead to destructive consequences.

There are two main sources for torque pulsation in advanced medical equipment: the current pulsation in the stator winding and the cogging torque. Careful consideration must be given to all aspects of motor design when evaluating the design of selected motor topology that minimise or eliminate these pulsation sources and thus low audible noise generation which are very important criteria for medical devices.

Since reliability of medical device products is critical, the motor needs to be designed to be durable and versatile. Many parts have integrated functions to help reduce the number of components and ensure the product is compact and robust. They should be designed to endure hostile environments, caustics fluids, steam, elevated temperatures, vacuum, vibration and mechanical impact.

In order to achieve high power density and reliability, it is not enough to only optimise the cooling capability and the electrical design. All aspects of the motor’s design must be optimised. Thus during the design phase, it is important to take into account the complete system, i.e., blower or pump, motor, driver and feedback.

Finally, OEMs want medical devices that are a compact design and energy-efficient. Power and space are limited in portable equipment. Most devices also need to be light and easy to manoeuvre. One way to accommodate these trends is to use high-speed motors that can deliver the necessary power and performance within the specified footprint.

CHALLENGES FOR

HIGH-SPEED

A number of design challenges arise as a consequence of the motors becoming smaller and faster, designers must consider the potential effects of electrical drive frequencies, mechanical stresses on the quickly rotating shaft (rotor dynamics), magnetic eddy currents, laminated steel core loss, and the inverter.

First, the mechanical integrity of the rotor becomes an issue. Keeping the rotor radius small enough should guarantee operation below critical speed. The introduction of a ring in the air-gap will ensure magnet retention. This results in an increase of the effective air-gap length.

Another design issue occurring at high-speeds is that of increased rotor losses. However, through careful selection of the permanent magnet material and through proper thermal management, the rotor losses can be minimized and a successful design can be obtained.

Additionally, iron and windage losses are significant and become dominating factors in determining the overall rating and efficiency of the high-speed motor. Reducing iron losses in the stator requires a low loss lamination steel grade and to use thinner lamination steel to build up the stator. With higher speed, the iron losses have become the dominant loss source, enhancing the need for accurate iron loss modelling.

Bearings should be chosen according to rotary speed, the radial and axial loads on them, as well as the environment requirement. More importantly, different types of bearings have their own features which will influence the rotor dynamics. Speeds within the range of 20 to 60 krpm are well within conventional bearing capabilities. However, for improved reliability, foil bearings are highly desirable, although the size of the foil bearings is inversely proportional to speed.

Ultimately, the rotor and stator design should be as simple as possible. A simple design has many benefits in terms of manufacturing, but these have to be weighed against overall performance targets. Basically, there are two main stator topologies; slotted and slot-less stator.

In a slotted BLDC motor design, the stator is made of slotted steel laminations that are stacked together with a series of teeth, and copper windings are inserted into these slots. The slotted motor design is simple and inexpensive to manufacture, but can exhibit cogging torque. Cogging is the jerky, non-uniform angular velocity particularly apparent at low speeds in motors with a small number of poles. It occurs when the magnetic poles of the rotor pass the stator slot openings (air gaps) and approach the stator’s metal teeth, magnetic force pulls the rotating part forward.

In a slotless BLDC motor design, there are no iron teeth to support the windings. Instead, the stator lamination is constructed of steel rings that are stacked together, and the windings are encapsulated in an epoxy resin, which gives the winding structure shape and rigidity. This “self-supporting” winding is placed in the air gap between the stator lamination and the rotor. The configuration eliminates stator teeth and minimises cogging.

Slotted BLDC motors still hold some advantages. For example, the air gap in a slotted motor is smaller than the air gap in a slotless design (which must accommodate the self-supported winding assembly). This means that the flux density is higher in a slotted motor, and torque production is more effective and efficient.


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