Brushless dc Motors in Medical Applications

It used to be that brushless-dc (BLDC) motors just weren’t an option for most medical applications. But that situation is changing as the cost of BLDC drive electronics falls. Furthermore, a quest for more-efficient, compact, and reliable medical equipment has put BLDC motors on the prescription list for a variety of applications.

By definition, a brushless-dc motor is a synchronous electric motor powered by a dc-power source. While derived from the brushed-dc motor, an electronic commutation circuit takes the place of the standard commutator and brush assembly. In terms of design, though, BLDC and brushed-dc motors are almost direct opposites. The windings of a brushed-dc motor wind around the motor’s rotating shaft or armature, whereas the windings of a BLDC motor are affixed to the motor housing. Likewise, the magnets of a brushed-dc motor attach to the motor housing. But the magnets of a BLDC motor are on the rotating shaft, now called the rotor of the motor.

Of course, the primary difference between brushed-dc and BLDC motors is the method in which each type is commutated. Commutation is the act of reversing the polarity of the phase currents in the motor windings at the appropriate time to produce continuous rotational torque. Without commutation the magnetic fields of the windings and permanent magnets would align and lock the rotating shaft in place. The polarity of the windings must reverse at the right time to change from magnetic attraction to magnetic repulsion and back again to keep the motor shaft spinning.

Brushed-dc motors use brushes and a commutator that acts as an electromechanical switch to connect the windings in the proper polarity. The mechanical switch is replaced with electronic switches in BLDC motors with the polarity-reversal timing controlled by an electronic circuit.

Ordinarily, BLDC motors use Hall-effect devices (HFD) to sense rotor position and control the electronic drive of the motor. However, by monitoring motor back- EMF, it’s possible to eliminate the HFD and create a sensorless BLDC-motor drive. The fact that motors without HFDs can be less expensive is one of the driving forces spurring the adoption of BLDC motors within the medical- design community.

BLDC motors used for sensorless operation typically contain three windings or phases, similar to a 3 ac motor. But without a sensor to detect the rotation angle of the permanent magnet in the rotor, it’s impossible to know which phases to energize with a given polarity. The most common technique starts by energizing two of the phases with a given polarity. Current is applied gradually using a ramp-up action to orient the rotor gently to a known prestart position.

Once the two orientation phases reach full power, the electronic control reduces the power in one phase while ramping up power in the third phase. Phase power begins to cycle, always keeping two of the three phases energized. For example, a motor with three phases labeled A, B, and C, phase power might follow the pattern AB-AC-B’C-B’A’-C’A’-C’B-AB. The labels A, B, and C indicate normal current polarity is applied to those phases while A’, B’, and C’ means the current polarity is reversed in that phase. The effect produces a rotating magnetic field between the windings.

The rotor begins to spin as its magnet tries to stay aligned with the rotating magnetic field created by the phase windings. As the rotor picks up speed, the magnetic field of the rotor starts to generate a back-EMF in the third, nonpowered winding. Once the amount of back- EMF becomes great enough that the drive can sense it, the drive switches from the ramp-up technique to a back- EMF, “zero-crossing” technique. As motor speed climbs, the amount of back-EMF rises proportionally with rpm to provide feedback for speed control and regulation.

Sensorless BLDC drives have two primary disadvantages: First, the motor must rotate at a minimal speed to generate sufficient back-EMF for the drive to sense. Second, sudden changes in the load can cause the back-EMF loop to lose sync resulting in a loss of speed and torque.

Treating sleep apnea
The treatment of sleep apnea requires the use of Positive Airway Pressure (PAP) respirators. The patient dons a special breathing mask attached to the PAP respirator. A blower fan within the respirator pressurizes the air in the mask to create positive airway pressure that helps the patient breathe while asleep. The blower fan must raise or lower the patient’s airway pressure in response to their breathing pattern. When the patient inhales, the blower fan must speed up to supply a larger volume of air to the lungs. When the patient exhales, the blower fan slows down to reduce the volume of air and let the patient breathe out. Sensorless BLDC motors and drives are an ideal power source for the blower fans: The motor never need operate below the minimum threshold speed of the drive and there is no risk of a sudden change in load.

Another advantage of BLDC motors is that they are quiet. Motors in hospital equipment or patient-care facilities must be quiet to comply with low-noise-level standards. Motors in sleep-apnea equipment operate at high speeds and yet must comply with even lower noise level standards because the equipment is in the patient’s bedroom at home. So the absence of a commutator and brushes in BLDC motors removes an additional source of motor noise.

Power density and reliability
There’s no question that recent events have put a strain on the world’s medical analysis and testing services. Reason’s include the continuing development and improvement of medical technologies in the areas of disease detection, prevention, and treatment. Moreover, there’s been a double-digit increase in the number of people needing medical care over the last decade. 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. Compared to brushed-dc motors, brushless-dc motors do exactly that.

As medical equipment grows smaller and lighter, the volume of air surrounding the motor drops. In such cramped quarters, temperature gradients between the motor and the surrounding air diminishes at a proportional rate. The heat-transfer efficiency of a motor directly impacts its performance in areas such as rated torque and duty cycle.

One area where BLDC motors excel is in the elimination of waste heat and how that impacts on equipment design. The majority of heat transfers from a motor at the surface of its housing via convection. A major source of heat in motors is the I²R losses created by current flowing through the motor windings. The ability to eliminate that excess heat goes a long way in keeping a motor cool and reliable.

As the windings of BLDC motors are affixed to the motor housing, the BLDC motor is efficient at getting rid of its heat. When compared to its brushed counterparts, the higher heat-transfer efficiency of a BLDC motor lets it run cooler in tight spaces. This not only boosts the overall throughput of medical equipment, it also helps reduce the cost of existing designs.

One final advantage BLDC motors have over their brushed counterparts arises from the nature of their design. Because BLDC motors have no brushes to wear out and replace, their expected life is double that of brushed motors. BLDC motors easily reach their life expectancy of 10,000 hr compared to a brushed-motor life expectancy of 2,000 to 5,000 hr.

Medical analyzers
Medical analyzers are multifunction machines that test human bodily fluids such as blood and urine. Fluid samples within the analyzers move from station to station for various tests. Generally, medical analyzers are totally enclosed. The temperature within them will rise to well above ambient temperature during periods of peak operation. Medical analyzers are designed to test thousands of samples annually and to run a minimum of 8 hr daily.

Stepper motors, a close relative of the BLDC motor, are the most common type of motor in medical analyzers today. Stepper motors are also considered to be brushless-dc motors. However, the major difference is the construction and pole count of the rotor assembly. Stepper motors generally have high pole counts, typically 50 poles or more. On the other hand, a BLDC motor has a typical pole count of six. For example, stepmotors rarely exceed 1,000 rpm because of their high pole count. Yet BLDC motors can easily reach top speeds of 5,000 rpm or more.

Machines with higher throughput need motors that rotate at speeds above the capability of stepper motors. BLDC motors fill that need for medical analyzers because they combine high-speed operation, high heat-transfer efficiency, and long life.

All in all, a need for high throughput and reliability in medical machines will continue to challenge the capabilities of brushed-dc motors. In addition, the trend toward squeezing more equipment functions into less volume promotes the use of smaller motors able to dissipate heat in small spaces. BLDC motors can meet these demands now and should continue to do so into the future.