Blowers working in patient-care settings must satisfy operational requirements while performing reliably, efficiently, and quietly. Combining brushless dc-motor drives with high-efficiency fans let brushless dc blowers do exactly that.

Why choose BLDC blowers

Of course several factors influence blower selection. But even when specing the design, it's not too early to estimate requirements for pressure and flow rate, the available design envelope (this governs blower size), required service life, input voltages, and control scheme, among others.

The motor at the heart of brushless dc (BLDC) blowers offers distinct advantages over competing technologies. A BLDC motor converts electrical energy into mechanical energy through two interacting magnetic fields. One is produced by a permanent-magnet assembly and the other by an electrical current in the motor windings. The relationship between these two fields provides a torque that rotates the rotor. As it turns, the current in the polyphase winding is commutated, or switched, to produce a continuous output torque.

Brushless dc motors commutate electronically by a permanent-magnet rotor, wound stator, and rotor-position sensing scheme, which runs counter to other motor designs. The benefits from this technology include:

Greater life expectancy

Medical equipment applications typically require long life and BLDC blowers have service life expectancies in excess of 10,000 hours. In contrast, brush-commutated DC types have life expectancies of 2,000 to 5,000 hours, because of the natural wear of the commutation work handled by the motor's brushes, usually graphite with metal content.

No contaminants or sparking

BLDC blowers avoid risks associated with carbon dust created by brushes. Such contamination cannot be tolerated in medical applications and, from a safety perspective, BLDC technology provides an added spark-free advantage.

Flexibility in size and speed

Blowers driven by other motor designs, including AC induction motors, fail to offer the necessary size and speed ranges for various medical equipment. High rotational speeds for BLDC motors will often be limited only by the mechanical integrity of the rotor construction, speed-related internal losses, and bearings. Speeds below 1,000 rpm and in excess of 10,000 rpm are possible depending on drive capabilities.

Less noise

To promote patient comfort and relieve anxiety, low noise is a must, especially for beds and respiratory equipment. By their design and construction, BLDC blowers minimize noise levels. Most noise comes from the impeller (blade-pass tones and resonances) and air turbulence.

Speed control

A BLDC blower motor's electronic commutation allows for accurate performance control and rapid transient response for faster power availability.

BLDC control

A typical BLDC control profile works like this: The main supply voltage (typically 10 to 28V, depending on application) powers the blower, and speed is usually controlled with a 0 to 4V or 0 to 10V command signal. Blower speed is directly proportional to this command signal.

A sensor for a closed-loop speed control generates the command signal. For example, the application can adjust the blower-speed command according to a pressure sensor somewhere in the air circuit. Or the speed-command value can be a function of a timer or manual adjustment. Most BLDC blowers for medical applications can be designed for this type of speed control or they can be configured to manually set the blower speed with a potentiometer on the controller.

Some blowers have an even simpler on-off control scheme. In these, the controller is “onboard” the motor and need only connect to the DC power supply. More sophisticated blowers would have their speed directly proportional to the supply voltage making a separate speed-command signal unnecessary.

A few applications

BLDC blowers are used in respiratory equipment such as ventilators and sleep-apnea machines that deliver forced air to a patient's lungs. For these applications, smaller, high-speed blowers have become the norm. In these cases, blowers are called upon to accelerate and decelerate quickly so they correspond to the patient's breathing pattern. Fast deceleration ensures 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.

Blowers must deliver high-pressure, up to 50-in. H₂O, at low flow rates, less than 20 cfm. Approximate diameters usually range no more than 3 to 5-in. (76 to 130 mm) to fit conventional-design packages.

Other noteworthy applications include dental aspirators using multi-stage BLDC blowers with high-vacuum capabilities (up to 154 in. H₂O) to extract particles during drilling, and fume-evacuation devices with equally strong vacuums to remove smoke plume and biocontaminates during cauterizing or electrosurgery operations, or both.