Miniature Motors for Medical Ventilation
The medical sector today mandates mechanical ventilation in cases where a patient's spontaneous ventilation is inadequate to maintain life. This is done to prevent an imminent collapse of other physiological functions, or ineffective gas exchange in the lungs. Mechanical ventilation therapies in hospitals and in the field have contributed to dramatic improvement in life expectancy of patients during and after surgeries, or patients suffering from accidental disruption of lungs, or even patients suffering from acute chronic pulmonary diseases. More recently, home care applications previously considered as luxury or comfort therapies are showing an interest in race to extend people’s life. For instance, low end mechanical ventilators devices known as CPAP are used at night to treat sleep apnea, a disorder once considered to be a simple discomfort and which is now recognized as a source of major medical complications such as increased blood pressure, diabetes, etc.
Medical ventilation technology has further evolved from stationary units originally relying on oxygen and compressed air drawn from the hospital supply lines, to autonomous portable devices, capable of providing their own controlled pressure and flow. This evolution was accelerated with an increased need to allow patients to recover in the comfort of their own homes after their pulmonary surgeries when affected from acute Obstructive Sleep Apnea (OSA) syndrome, or even from more severe conditions such as acute respiratory failure, or Chronic Obstructive Pulmonary Diseases (COPD) – some of the leading causes of death globally.
Consequently, former valve or piston-driven solutions progressively gave way to high-speed micro-turbine blower-driven ventilators, which can now be distinguished in 4 major categories: intensive care, home care, transport ventilators and neo-natal ventilators.
Miniature Motors for High Efficiency Blower Systems
High efficiency blower systems including a motor and fan, and have been designed such that inspiration pressure (IPAP) and expiration (EPAP) pressure can be generated and controlled by acting only on the motor speed as shown below.
Typical Medical Ventilator Set Up
By ramping motor speed up and down ranging from 15,000 to 60,000 rpm in a matter of 100 - 120 milliseconds, the newest ventilators are capable of ventilating modes very close to a patient’s natural breathing pattern thus maximizing their therapy acceptance, and are also capable of invasive ventilation not possible earlier with turbine blower designs. Pressure of 100 CmH2O and flow in excess of 200L/min are now common requirements which blowers equipped with Portescap brushless motors can reliably support. Indeed, brushless motor technology offers a longer life span at such speeds. Collaborative work with ball bearing manufacturers warrants the best solution for each application requirement (speed, load, oxygen content, noise level).
Because of the increased demand in ambulatory ventilation solutions mentioned above, a longer motor battery life and thus the need for a higher efficiency motor is very critical.
Understanding Motor Efficiency
To understand motor efficiency, let’s review the types of losses that can be generated by a motor under load - Eddy current losses and Joules losses. Indeed both losses if not minimized, contribute directly to poor battery life, as well as to a rise in motor temperatures which adversely affect bearing life and temperature of other internal components in a ventilator. The latter situation often leads to an increased need for cooling fans and vents which contribute to a higher ventilator noise and increased costs.
Joule losses are equal to I².R where R is the motor resistance and I the current consumed. Hence, an efficient motor under constant load will consume little current to perform a task, while a less efficient motor will consume a lot of current to perform the same task and as a result will heat up more. Thanks to their specific coil design and optimized magnetic circuits, Portescap slotless brushless motors are particularly current frugal which allows us to size down batteries and power supplies, and thus further enhance the portability of ventilators.
Eddy current losses are the other type of major losses generated in a running motor. These losses occur when a magnetic field rotates in front of a piece of iron and generates heat. This phenomenon is exactly what happens inside a motor when its rotor magnet spins at high velocity in front of its stator laminations. Eddy current losses are proportional to the square of frequency. With the increased requirement in pressure and flow of modern ventilators, thus in motor rotational speed, it is important to consider this while designing the best motor customized for each ventilation application. The number of rotor magnet pole-pairs in a motor is as much important as is the rotational speed of a motor. As an example, Portescap brushless motors have been optimized with 1 pole pair, thus dividing by 4 the magnitude of Eddy current losses typically generated by 2 pole-pair motors.
Lamination material and thickness selection is also an important step in the design of the best motor for a given application as they both dramatically affect losses generated at high speed. Low core-loss steel lamination material and thinner lamination can reduce operating temperature of motors, but they can rapidly affect its cost as well.
Brushless Motors – The Slotless Coil Technology
Slotless motor self - sustaining coil technology offers the added benefit of not requiring to be hosted by the lamination. Indeed, typical slotted brushless motors stator coils are wound around the lamination teeth created by their profile.
Slotted Versus Slotless Brushless Motor Selection
The benefit of the slotless motor design is that it does not introduce iron in the air gap (area between magnet and copper wire) which further reduces iron losses. This design also yields a smoother rotation as the magnet is not attracted in any preferred position created by the stator teeth. Vibration and noise levels are typically reduced to a great extent.
Consequently, both reduced these residual losses (Eddy & Joules) in some available miniature motors allow operating temperatures to decrease by as much as 25% under the same ventilation requirements (pressure, flow, BPM) than with previous generation motors.
Length is another factor to consider when selecting a motor. A longer motor increases torque constant and reduces the current needed under a same load as compared to a shorter motor. It also has the benefit of better dissipating the temperature generated by its losses. However on the other side, Eddy current losses also increase with the amount of lamination material and inertia is also marginally negatively affected as motor length grows (as opposed to being considerably affected when its diameter grows).
Miniature Motor Selection Considerations
In conclusion, the selection of the best ventilator motor comes down to a balancing act of application requirements. For its ability to continuously deliver full pressure and speed, maximize reliability with best thermal management, and where cost is not as sensitive as in home care solutions, typically larger in size ICU ventilators can accommodate a full length 22 mm slotless BLDC motor.
Home care and transport ventilators designers typically favor a medium length 22mm motor for its compactness, efficiency for longer battery life, reliability in ambulatory applications, more modest cost and the flexibility it offers in providing temporary high pressure & flow or high dynamics.
Dedicated neonatal ventilators, usually running smaller fan impeller will best be designed with lower inertia motors such as Portescap 16 mm brushless slotless motor solution allowing for high speed and acceleration and repeated step function cycles.
Finally, motor and blower integration is key for the best overall flow generation function. Mechanical features of a motor such as shaft, front flange etc need to be customizable to accommodate for the best union of different components.
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