Variable speed blowers offer a unique solution to several medical device systems requiring a fluctuating air flow. However, device designers are unlikely to be all that familiar with it. Therefore, this article outlines the key features of blowers that are used in medical devices, explains why brushless is preferred over brush commutated, and reviews the applications in which the blowers are most commonly used.
By Jeffrey L. AnzevinoJeffrey L. Anzevino is a sales engineer for Blower Products-Medical at AMETEK Technical & Industrial Products. His responsibilities include product development for the global medical market. Anzevino can be reached at 330-673-3452 or email@example.com.
Opportunities abound in the world of medical devices and healthcare equipment for designers to prescribe variable-speed blowers. Among notable applications:
- Respiratory equipment-Blowers perform in ventilators and sleep apnea machines to deliver forced air to a patient's lungs.
- Therapeutic beds-Many bed applications engage blowers both for inflation of the mattress (for therapeutic support) and to sustain airflow across a patient's skin.
- Dental aspirators-Blowers exhibiting high vacuum capabilities enable HVE (high-volume extraction) of particles during drilling procedures.
- Fume evacuation devices-Blowers serve to remove smoke plume and bio-contaminants during cauterizing and/or electrosurgery operations.
Although every application presents particular operating and performance demands, all variable-speed blowers in medical applications and settings ideally should provide high-purity air, minimize maintenance requirements, offer high efficiency and long life with little noise, and fit within ever-shrinking design envelopes. Designers can achieve positive outcomes in meeting these requirements by specifying brushless technology for blower applications.
The heart of a variable-speed brushless blower is the brushless DC motor, which carries distinct advantages over brush-commutated types.
A brushless DC motor operates by converting electrical energy into mechanical energy through the interaction of two magnetic fields. A permanent magnet assembly produces one field and an electrical current flowing in the motor windings produces the other field. The relationship between these two fields results in a torque that rotates the rotor. As the rotor turns, the current in the polyphase winding is commutated-or switched-to produce a continuous torque output. In short, brushless motors achieve commutation electronically via a permanent-magnet rotor, wound stator, and rotor-position sensing scheme.
This method of achieving commutation is in stark contrast to brush-commutated motors. Brush DC motors use brushes (typically graphite with metal content) as part of the commutation process and ongoing brush wear (caused by the interface between brush and commutator) is the leading cause of premature motor failure. A secondary cause of failure can be attributed to dust from brushes contaminating the motor's bearings. This effectively reduces bearing life and, in turn, restricts motor life.
Even the conventional mounting configuration of brushes to DC motor assemblies can add to the headaches. The usual method involves soldering the brushes onto standard cantilever springs. This spring design, however, results in force levels diminishing over time, often ending in premature motor failure.
The foregone conclusion is that brushless motor technology is the way to go. These blowers exhibit:
- Greater life expectancy-Medical equipment applications typically require long life. Brushless DC blowers can address this need by providing service life expectancies in excess of 10,000 hours. In contrast, the expected lifetime for brush-commutated DC types collapses dramatically to 2,000 to 5,000 hours of operation, due to brush wear.
- No contaminants or sparking-Brushless DC blowers bypass risks associated with the carbon dust generated by brush types. Such contamination cannot be tolerated in medical applications. In addition, from the safety perspective, brush technology provides an added spark-free advantage.
- Flexibility in size and speed-Blowers driven by technologies other than brushless DC motors (including AC induction motors) fail to offer the necessary size and speed ranges required for the applications. High rotational speeds for brushless DC motors often will be limited only by the mechanical integrity of the rotor construction, speed-related internal losses, and bearing selection. Speeds in excess of 10,000 rpm (and even much higher) are possible (with appropriate designs) and speeds below 1,000 rpm can be achieved, depending upon drive capabilities.
Among the most influential parameters to consider in brushless blower selection, designers should initially estimate the requirements for pressure and flow rate, the available design envelope (governing blower size), desired service life, input voltages, and speed control scheme.
(Especailly Small blowers.)
Illustrating how some of these factors can help narrow the field, smaller, high-speed blowers have emerged as the norm for respiratory equipment (ventilators and sleep apnea machines) because of relatively limited "real estate."
Blowers for respiratory equipment additionally must be able to accelerate and decelerate quickly to enable the blower output to correspond to the patient's breathing pattern. Fast deceleration ensures that the patient does not have to exhale against the blower pressure, but then the blower must accelerate immediately to force air into the lungs upon inhalation.
As a result, blowers for these applications should possess the capabilities to deliver high-pressure (up to 50 in. H2O) and low flow rates (less than 20 CFM). Their approximate diameters usually will range no more than 3 to 5 in. (76 to 130 mm) to fit conventional design packages.
For therapeutic beds, blowers will inflate and pressurize a mattress, which is perforated throughout its surface. The tiny holes allow the air inside the mattress to leak out for the purposes of keeping the patient's skin dry. Bed sores are primarily prevented by the pressure-reducing nature of the "air" bed on the body's pressure points. Blowers for these applications should provide sustained capabilities to deliver constant and reliable flow rates.
High vacuum capabilities for dental aspirators will suggest multi-stage blowers providing vacuum capabilities up to 154 in. H2O. Equally strong vacuum capabilities will be required when specifying blowers in fume evacuation devices.
Invariably, the blower's speed control scheme will play a vital role in virtually every medical equipment application. A brushless DC motor's electronic commutation technology inherently allows for accurate performance control and rapid transient response time. In turn, these promote faster power availability crucial in medical settings.
In standard control schemes, the main supply voltage (typically 10-28 V, depending on the application) is used to power the blower and the blower speed is controlled usually with a 0-4 V or 0-10 V command signal. The blower speed is directly proportional to this command signal. (Translation: 0 V equals zero speed and 4 V equals full speed.)
The command signal can be generated from a sensor for closed-loop speed control. For example, the application can sense pressure somewhere in the air circuit and adjust the blower speed command according to the pressure, or the speed command value can be a function of a timer or manual adjustment.
Most brushless DC blowers for medical applications can be designed for this type of speed control or configured for the blower speed to be set manually with a potentiometer located on the controller.
Some blower types have been engineered with an even simpler control scheme; the blower has only two wires. In these versions, the controller is "onboard" the motor and the wires serve as a DC power supply connection. (More sophisticated blower types, too, can incorporate "onboard" controllers.) The blower's speed is directly proportional to the supply voltage and a separate speed command signal is unnecessary.
With a wide range of options and customization capabilities, the prognosis is good for proper selection of a variable-speed brushless blower. When designers partner early in the specification stage with an experienced manufacturer, outcomes can become even more positive.
For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or AMETEK Technical & Industrial Products at www.ametektechnicalproducts.com.