Referred to by many design engineers as the ideal replacement for the more commonly used brushed DC electric motor, brushless DC (BLDC) motors are more frequently finding their way into an increasing number of medical applications every year. Challenges that once hindered these types of DC electric motors from being designed into medical equipment such as the cost of drive electronics are being addressed by the electric motors and controls industry. Furthermore, customer push to develop more efficient, more compact, and more reliable medical equipment has subsequently led to the emergence of brushless DC motors within medical applications.
What is a Brushless DC motor?
By definition, a brushless DC electric motor is a synchronous electric motor powered by DC power and is electronically commutated. BLDC motors are a derivative of the most common used DC motor, the brushed DC motor. Mechanically, BLDC and brushed DC electric motors are direct opposites. The windings of a BLDC motor are affixed to the brushless DC motor’s housing, whereas, the windings of a brushed DC motor are affixed to the motor’s shaft. In addition, the magnets of a BLDC motor are affixed to the motor’s shaft, whereas, the magnets of a brushed DC motor are affixed to the motor’s housing. Another primary difference between brushed DC and BLDC motors is the method in which each type is commutated. Commutation is the act of changing the motor phase currents at the appropriate times to produce rotational torque. Due to its mechanical structure, brushed DC electric motors use brushes to commutate the motor. BLDC motors do not have brushes and function through electric commutation.
Sensorless BLDC Drive Technology
Ordinarily, brushless DC motors utilize Hall Effect devices (HFD) to sense the rotor position which subsequently enables an electronic device known as a drive to commutate the motor. The development of lower cost yet highly intelligent sensorless BLDC drives is helping to increase the popularity of BLDC motors within the medical design community. The evolution of the sensorless brushless DC electric drive is widening the brushless DC motor’s appeal within the medical design community.
Sensorless BLDC drives do not require HFD’s to provide feedback from the motor. Instead, sensorless BLDC drives detect a motor’s naturally produced back-EMF to determine the best method of commutation. The most common technique uses a ramp-up pattern to energize two motor phases while monitoring the third phase for back-EMF. This technique requires the drive to initially orient the rotor to a known position. Next, two phases are energized based on a ramp-up pattern such as AB-AC-BC-BA-CA-CB. As the motor increases speed, a proportionate amount of back-EMF is produced by the motor on the non-energized phase. When the amount of back-EMF becomes sufficient enough for the drive to sense, the drive will switch over from the ramp-up technique to a back-EMF, “zero-crossing”, technique.
Sensorless brushless DC drives have two primary disadvantages:
- The DC motor must rotate at a minimal speed to generate sufficient back-EMF for the
drive to sense.
- Sudden changes in the load can cause the back-EMF loop to become out of sync
and result in a loss of speed and torque.
Application: Sleep Apnea Treatment
The treatment of sleep apnea requires the use of Positive Airway Pressure (PAP) respirators. Most PAP respirators use a brushless DC motor to drive the blower fan. The blower fan creates positive airway pressure to aid the patient in breathing while they are asleep. In this application, the blower fan will increase or decrease the patient’s airway pressure in response to their breathing pattern. When the patient inhales, the blower fan
must increase the volume of air into the lungs, therefore the motor must accelerate. When the patient exhales, the blower fan must reduce the volume of air entering the lungs, therefore the motor must decelerate. Sensorless brushless DC drives are ideal for blower fans used in sleep apnea equipment because the motor is never required to operate below the minimum threshold speed of the drive. In addition, brushless types of DC electric motors are ideal for this type of application because there is no risk of a sudden change in load.
Motors used in hospital equipment or other patient-care facilities are required to comply with low noise level standards to endorse patient comfort and reduce anxiety. Nevertheless, electric motors used in sleep apnea equipment operate at high speeds and are expected to comply with even lower noise level standards. Brushless DC motors are ideal for noise sensitive environments because they do not have brushes, which emit audible noise
Optimizing Power Density
The continuing development and improvement of medical technologies in the areas of disease detection, prevention, and treatment; coupled with a double digit increase in the number of people requiring medical care over the last decade, has created a strain on the world’s medical analysis and testing service providers. The worldwide increase in demand for medical analysis and testing services has resulted in the need to develop
medical analysis and testing equipment with greater throughput and of high reliability. To accomplish this goal, a machine’s drive train components must be capable of providing a greater amount of torque over a larger speed range and perform at higher duty cycles.
When compared to brush DC electric motors, brushless DC motors provide exactly that:
- Greater amounts of torque over a larger speed range and capable of
performing at higher duty cycles
Heat Transfer Efficiency
While increasing the overall throughput of medical equipment is the primary objective
for many design engineers, there is also an increasing market trend to reduce the cost of
existing designs. Reducing cost often creates additional challenges for design engineers
such as a reduction in the amount of space available for the drive train and electronics.
The majority of heat transferred from a motor occurs at the surface of its housing by the
mode of convection. As the volume of air surrounding the motor is reduced, the
temperature gradient between the motor and the surrounding air diminishes at a
proportional rate. The heat transfer efficiency of a motor directly impacts its performance
characteristics such as rated torque and duty cycle.
When compared to their brushed DC counterpart, brushless DC motors have:
- Higher heat transfer efficiency which allows them to run cooler in crowded spaces
There is a direct correlation between the location of an electric motor’s windings and the rate in
which heat is dissipated from a motor. For optimal heat dissipation, the ideal location for
the windings is affixed to the motor’s housing. Being that the windings of the brushless DC motor
are affixed to the motor’s housing, the BLDC motor is very efficient at transferring heat
away from the motor.
More Reliability = Brushless
Medical applications typically require motors to long life. A BLDC motor is more
reliable than a brushed DC motor because it does not have any brushes to wear out and
replace. The life expectancy of the brushless type of dc electric motors is over 10,000 hours, whereas brushed DC
motors have life expectancies of 2,000 to 5,000 hours.
Application: Medical Analyzer
Medical analyzers are multi-function machines used to test human bodily fluids such as blood and urine. In a medical analyzer, fluid samples are transported from station to station to conduct various tests. Generally, medical analyzers are totally enclosed and the temperature within the machine will rise to well above ambient temperature during periods of peak operation. Medical analyzers are designed to test thousands of samples
per year and to run a minimal of eight hours per day. How can the brushless type of DC electric motors be used to optimize this process?
Today, stepper motors, a close relative of DC electric motors, particularly the brushless kind, are the most common type of motor used in medical analyzers. Stepper motors are also considered to be “brushless
DC” motors. However, the major difference is the construction and pole count of its rotor
assembly. Stepper motors have a higher pole count than that of a brushless DC motor. The more
poles a motor has, the lower its top speed. The need to develop machines with higher throughput requires the motor to rotate at speeds above the capability of stepper motors. Brushless DC motors are ideal for medical
analyzers because they combine the characteristics of high speed operation, high heat
transfer efficiency, and they have long life expectancies.
The BLDC motor will continue to emerge within medical applications.
As sensorless brushless DC drives continue to develop and costs are reduced, the attractiveness
of brushless DC motors will continue to increase. Customer demand for medical equipment
capable of handling even higher throughput and of higher reliability than the machines of
today will continue to challenge the capabilities of brushed DC electric motors. Lastly, the trend
of reducing the size of medical equipment will require the motor’s overall size to
continue to become smaller.
Notes on BLDC Motors
What this discussion of the brushless type of DC electric motor in medical applications should illustrates is that the uses of the DC motor, particularly the brushless kind, which is more efficient, quieter and longer lasting than the brushed DC motor, are myriad. The brushless variety of DC electric motor can be effective in industries ranging from computer parts and accessories to audiovisual equipment. In any area where motors are used, the utility of a brushless motor should be considered.