The current global health crisis only serves to underscore the ongoing challenges faced by medical equipment OEMs. Consider the all-important ventilator. The system not only needs to adjust speed to augment patient breathing, it also needs to be suitable for bedside operation. It needs to be quiet and compact. Efficiency should be maximized, not just to minimize heat dissipation but also to reduce power consumption. Above all, systems need to be reliable, as the life of the patient depends upon the lifetime of the equipment. By making an informed choice of motor, OEMs can make great strides toward achieving these goals.

The most fundamental decision in specifying a DC motor for medical applications involves commutation type: brushed or brushless. This article focuses on the trade-offs involved, including efficiency, lifetime, maintenance, robustness, size, and cost.

BRUSHED DC MOTORS

An electric motor consists of two basic elements: a rotor and a stator. Both rotor and stator generate a magnetic field, and the interaction between the two produces torque that enables the motor to do useful work. Although a wide range of design variations exists, including linear motors, for purposes of this article, we will focus on rotary motors in which the rotor (armature) is mounted on bearings and free to turn, while the stator is fixed.

Let’s start with a permanent-magnet brushed DC motor. In these designs, the stator field is created by permanent magnets. The rotor consists of an iron core wound with a series of coils that alternate in direction. As a result, when the coils are energized by current, each coil forms an electromagnet of opposite polarity from its neighbors. The force of attraction between a given coil/electromagnet and the nearest stator magnet applies a torque to the rotor, causing it to turn.

Torque is a cross product, however, which means that when the stator field and rotor field are parallel, applied torque falls to zero. In theory, that means that the velocity of the rotor falls to zero, as well. (In reality, the rotor has some angular momentum that will carry it past the zero point, although it may not be enough to keep the motor turning.)

To avoid the zero torque problem, we switch the direction of the current in the rotor coils, which switches the polarities of the electromagnets and puts the two fields in quadrature again. Now, instead of torque falling to zero, it returns to maximum. If we continually switch the direction of the current as the rotor turns, the applied torque will remain constant, as will the output torque of the motor. This process of switching current direction is known as commutation. The commutation approach is one of the primary differences between brushed DC and BLDC motors.

The simplest way to commutate a motor is with a mechanical switch assembly called a commutator. A mechanical commutator consists of a number of switch contacts attached to the rotor, with each pair of contacts wired in series with a rotor coil. One or more sets of complementary contacts called brushes, which are affixed to the motor housing, contact the commutator to deliver current to the coils through the commutator (see Figure 1). The brushes are connected to a voltage source so that one brush is positive and the other is negative. The commutator and brushes form a sliding switch assembly that energizes the rotor coils, reversing the direction of the current that passes through each set of windings as the rotor turns, switching the polarities of the electromagnets.

Up to this point, we have been describing permanent-magnet brushed DC motors. In these designs, the stator consists of permanent-magnet segments mounted in a steel tube. Permanent-magnet brushed DC motors are simple, robust, and easily controlled. They can operate on DC or rectified AC power sources. They have linear speed-torque curves.

The wound-field brushed DC motor, as the name suggests, generates the stator magnetic field using windings rather than permanent magnets. These motor types are more rugged than permanent-magnet designs, remain magnetically stable even at high temperatures, and can operate on DC or AC power sources.

BRUSHED DC MOTOR PERFORMANCE TRADE-OFFS

Brushed DC motors are simple, rugged, low-cost options for many industrial applications. They are widely available in a variety of configurations (see Figure 2). They do not require onboard electronics for commutation, making them good choices for high temperature, high radiation, and high shock and vibration environments. They are well suited for portable applications because they can be powered directly by battery, although typically they require intervening switches and/or a control mechanism, such as series resistors, for motor activation and control.

At their simplest, brushed DC motors operate from a fixed DC power supply to provide constant-speed operation. They can be used in conjunction with DC motor controls to provide variable-speed operation. Paired with servo drives/controllers and feedback, brushed DC motors can operate as servo motors to yield precise, accurate, repeatable position or speed control.

Brushed DC motors work best in moderate- to low-speed applications, from a few hundred to a few thousand RPM. They are good solutions for medical mobility equipment such as power wheelchairs, stair lifts, and motorized patient beds.

As always, there are trade-offs. The addition of the mechanical commutator increases the size and weight of brushed motors relative to their brushless counterparts. That won’t necessarily matter for a stair lift or patient bed but could be important for a power wheelchair. Certainly, it could be problematic for portable medical devices like ventilators or tabletop instrumentation such as blood-gas analyzers.

The thermal management is another challenge. The armature (rotor) has losses in the laminated steel core, the windings, the brushes, and at the commutator-brush interface. There is no good thermal path to the exterior of the motor to dissipate the heat generated by these losses, which limits performance.

Arcing at the commutator-brush interface can generate electromagnetic interference (EMI), which can present a problem for sensitive medical UI electronics. Probably the biggest issue for brushed motors is that friction/stiction at this interface can cause the brushes and commutator to wear over time. The process can generate particulates. Eventually, the brushes need to be replaced and the commutator may even need to be resurfaced.

The issue limits lifetime and compromises performance. It also increases cost of ownership in terms of parts and maintenance hours. Because improper brushed replacement can ruin a motor, some manufacturers will opt to use an inaccessible brushed design when possible to avoid warranty problems from improperly serviced motors. Users should always check the documentation and warranty of a motor before opening the housing for service.

In the industrial environment, brush wear can be a nuisance but for a critical medical application like a ventilator, it could be a matter of life and death. Brushless designs provide an alternative.

BRUSHLESS DC (BLDC) MOTORS

In the brushless version of our rotary DC motor, the windings are on the stator and the magnets are on the rotor (see Figure 3). Because the windings are fixed, direct electrical connections can be made to them easily. As a result, there is no need for a mechanical commutator and brushes.

So how do we keep the two magnetic fields in quadrature in a brushless motor? Somehow, the system needs to detect (or estimate) the angular position of the rotor magnetic field. The drive can use that information to generate a complex commutation signal that will control the stator winding currents in the stator coils to maintain the stator field in quadrature with the rotor field under all expected operating conditions.

Once the BLDC motor type is selected, then, it is necessary to choose a feedback approach to determine rotor position. Feedback technologies include back electromotive force (BEMF) sensing, Hall-effect sensors, sensorless vector control, and closed-loop feedback using encoders or resolvers. Commutation types include six-step and sinusoidal.

BLDC MOTOR PERFORMANCE TRADE-OFFS

BLDC motors offer several benefits compared to brushed motors. They have greater torque density and can operate at higher speeds. They also exhibit flatter speed-torque curves. Exact numbers vary from motor to motor but in general, a good brushed motor can control speed to within about 10 percent, while a good BLDC motor would be closer to 5 percent. Paired with a high-performance drive and carefully implemented, top-of-the-line BLDC motors can deliver consistent speed control to within 1-2 percent (see Figure 4).

In general, BLDC motors are more efficient than brushed designs. Moving the windings to the stator creates a shorter, lower resistance thermal path for heat dissipation to the ambient air. For high strength rare-earth magnet designs, rotor inertia drops considerably, leading to better dynamic response.

Electrical commutation removes the need for brushes, which addresses a number of issues. Arcing and EMI are reduced, as is the need for maintenance. The motors run quieter and do not interfere with nearby medical electronics. Eliminating brushes also cuts friction losses, further improving efficiency. BLDC motors are generally more compact than their brushed counterparts as well as more reliable. Because the only wearing parts are the shaft bearings, BLDC motor service life can easily top 20,000 hours. These attributes make BLDC motors highly effective for critical applications like ventilators, which typically operate in ICUs.

On the downside, BLDC motors are more complex than brushed designs. They require some type of commutation controller in order to operate. That means an electronic drive, which in turn requires power and cables between the motor and drive, adding cost and points of failure. Motors with integrated commutation and control electronics (see Figure 5) address most of these concerns, but they introduce trade-offs of their own. The heat generated by the motor can impact the operation of the electronics, reducing lifetime and leading to a derating of the integrated motor compared to separate motor and drive combinations.

BLDC motors have traditionally cost more than their brushed counterparts, but as the technology has matured, the price differentials have become more reasonable. From a cost point of view, a BLDC motor is a system that includes motor and drive. For an accurate comparison, be sure to include the cost of the drive with the brushed DC servo motor.

No two categories of medical equipment have the same requirements. For gross positioning equipment such as beds, stair lifts, and wheelchairs, brushed DC motors may be effective solutions. For the most demanding applications, such as ventilators and medical robotics, BLDC motors provide a number of advantages, including higher efficiency, reduced contamination and EMI, lower maintenance, smaller size and weight, and improved heat dissipation. Perhaps most important of all, they minimize cost of ownership while maximizing lifetime and reliability, characteristics that are all important in the medical market. That said, there are no hard and fast rules. OEMs should always work closely with their vendors to ensure that they choose the best possible motor for each project.

From：medical design briefs