Many areas of appliance design have experienced significant changes during the past 10 years. Electronic controls and electronically controlled and commutated motors have proliferated. Modern clothes washers come with DVD instructions and they feature elaborate, microprocessor-based control systems.

Along with the proliferation of advanced digital controls, motors have also experienced many changes. Shaded-pole motors are being replaced by electronically commutated, brushless motors; and drives and single-phase AC motors are increasingly replaced by brushless PM motors. Not only has the industry experienced significant changes in the technologies, but it is also experiencing a dramatic shift to off-shore manufacturing, due to labor and material cost pressures.

In the area of motors and controls, many exciting trends are emerging that promise to combine performance improvements with cost reductions. Appliance manufacturers must closely follow these trends as they make strategic decisions for the future.

As noted, brushless motors are quickly becoming the motor of choice in appliance design. Most of these brushless motors are currently PM-type motors, although some vacuum-cleaner applications have selected reluctance-type motors. Most PM motors use low-cost neodymium magnets, and, until recently, the cost of these magnets has declined steadily. But that may be about to change, as the worldwide demand for neodymium increases steadily, especially in China. That country holds most of the world’s reserves in neodymium, but is also quickly becoming the world’s largest consumer of the material.

In automotive and industrial applications, internal permanent magnet (IPM) motors are becoming commonplace because they offer higher torque and power for a given amount of magnet material. The IPM motor utilizes the magnetic effects of both the permanent magnet and the reluctance-induced component, making it a hybrid motor. One of the characteristics of the IPM motor is its wide constant power range; it can deliver its rated torque up to a fixed design speed, and then it can maintain constant power at speeds up to six to eight times higher. This is a desirable characteristic in applications such as clothes washers, where high torque is needed to reliably move the drum and where the wide constant power range allows the drive system to obtain high speed during the spin cycle.

Stator Type 2 (SMC core and S23). Photo: KERI

In modern IPM motors, between 70 percent to 80 percent of torque can be the effect of the motor’s reluctance torque, and the balance is due to the PM-induced component. Unfortunately, while the torque of the motor increases, so do the tooling and assembly costs, which can easily offset the savings in the cost of magnet materials. Much attention has been focused lately on the development of other types of hybrid motors that promise lower manufacturing costs, while maintaining the performance advantages of the IPM motor.

Innovative hybrid motor concepts are the focus of much academic research and a major focus for many makers of motors and drives. When developing these new motor concepts, the designers must be conscious of the total system cost, rather than just strive for the lowest cost motor or individual component alone. For example, reducing the phase count may not change the motor cost noticeably, but the impact on the overall system cost can be very significant due to savings in the power electronics. A case in point is the ECM evaporator fan motor, which utilizes a single motor phase and four power devices for a cost-effective and efficient motor solution.  The use of bi-filar windings can further reduce the controller cost at the expense of higher motor costs.

Sintered powdered metals and amorphous materials have also attracted attention as a means to reduce the cost of the motor. While these promises have been largely unfulfilled to date, recent research exploring the use of sintered metals, in conjunction with transverse flux motor (TFM) geometries, appear to be very promising.

The transverse flux motor is an emerging motor technology that should be closely watched by appliance designers because it offers many design features that allow for extremely efficient manufacturing techniques  (i.e. powdered metals), excellent material utilization, and the promise of high motor efficiencies.

While many of the published descriptions of TFMs center around the permanent magnet (PM) type, the transverse flux motor can be constructed in a variety of ways, either as a reluctance motor or a hybrid motor that combines two or more magnetic effects, similar to the IPM motor.

In the construction of a transverse flux motor, there are no end-turns, which makes the motor very efficient and reduces the material cost for the copper. The winding can also be constructed and wound in a very cost-effective way, and it is easy to maintain a high slot-fill to further improve the motor’s efficiency, if desired.

The TFM is basically a single-phase motor construction, and phases can be stacked as required. Recently, more advanced concepts have been written about that even share flux between phases, further enhancing the TFM’s performance properties.

In addition to the TFM, other motor types are emerging that can result in lower system cost. One such area is the development and emergence of other hybrid brushless motor types, most notably those that do not employ permanent magnets. While few results have been reported publicly, there are several companies, including Rocky Mountain Technologies, that are actively working on such developments.

While the TFM development is still in its infancy, some of the reported results are impressive. The Industry Applications Research Lab of the Korean Research Institute (KERI) has developed several TFM designs for robotics and material-handling applications, as well as a clothes washer motor for a large, Asian appliance maker. The latter demonstrated that a TFM can be built for this application using only 50 percent of the active materials compared to a PM AC motor with comparable performance. Other companies have also announced the commercial development of TFM machines, including a generator by LDW in Germany and a 2.5 MW TFM shipboard drive that is under test in France.

Reduced phase-count motors represent another area of research and development interest. A three-phase brushless motor requires six power switches for its operation. These power switches and the related drivers account for a substantial portion of the system’s overall manufacturing cost. However, a switched-reluctance motor requires only four power switches for a three-phase motor. A conventional two-phase brushless AC or PM motor requires eight power switches, which offers no cost advantages to offset the two-phase motor’s potentially lower efficiency.

Concepts using bi-filar windings have been introduced that can reduce the cost of the power switches in exchange for a reduction in copper utilization and efficiency. Such designs can allow for a very low system cost for the motor and drive. Furthermore, advances in controls and power electronics may expand these concepts and lead to the development of innovative, single-phase, hybrid motors that can develop starting torque in every position and, combined with advanced DSP-based controls, ensure the proper starting direction. Such a single-phase system can offer significant cost savings for refrigeration compressors and other similar applications, such as pumps and fans that require less than full torque at startup. These developments are just beginning, and it may be possible to construct a suitable hybrid motor that can develop full starting torque in the future.

Driving

Actual parts for a 25 N.m, 300 rpm, transverse flux motor. Here and next image are the rotor and housing. Photo: Keri

In the past 10 years, most of the advances in the area of drive design have focused on the development of lower cost, increasingly powerful processors that deliver more functionality, and the implementation of more advanced motor-control algorithms. In the area of power, there is a trend towards packaging and bonding of silicon in an effort to reduce the system cost.

A part of the rotor and housing. Photo: Keri.

In most appliance applications, IGBTs have replaced MOSFETs as the power switch of choice, even though the PWM-switching frequency of IGBTs is much lower that that of FETs, which can easily reduce the system efficiency and the power output of the motor.

Stator. Photo: KERI

Some manufacturers have offered motors with integrated power electronics that greatly reduce the installation cost, but there are some concerns about thermal management, reliability, size, weight, and cost that have limited the proliferation of these integrated motors.

Base of stator. Photo: KERI

Recently, a new power device technology has emerged that could significantly change the way power electronics are designed, applied, and packaged: silicon carbide (SiC) technology. Several manufacturers have already devised SiC power switches and diodes and published test data about them. The military has already embraced this technology.

SiC technology can operate at very high junction temperatures, up to 350 DegC. It can withstand very high breakdown voltages and it can switch extremely fast. As a result, this technology requires little cooling. The high breakdown voltage permits smaller, less expensive capacitors, and helps eliminate snubbers and related losses and heat generation. The high switching frequency can improve the motor efficiency and motor utilization, and it reduces switching losses (less heat generated), which can reduce the total active material required for a power switch.

While there are still significant technical challenges that prevent the widespread proliferation of SiC semiconductors, this technology should be closely watched, as it will quickly transition into manufacturing once it becomes cost effective and more readily available, due to the many advantages that it offers for the design of motor drives.

Finally, one cannot ignore advances in the area of controls. Some trends, such as diagnostics and pending failure prediction, will continue to grow as faster, more powerful processors become available without cost penalty. The processors and control algorithms can, in some instances, modify the behavior of the controller to extend the life of the unit until service can be scheduled. For example, a bearing defect might be accommodated by lowering the speeds at which the appliance is operated.

Sensorless algorithms and controller implementations will continue to proliferate for brushless PM and SR motors, eliminating the cost of the sensor, its installation and related warranty issues. Eliminating Hall sensors and other feedback devices that are typically required by many controllers can lead to significant cost reductions, not only in hermetically sealed compressors where connections are expensive, but also in many other applications where removing the sensors reduces total system cost.

In the near future, many of the recent trends in motor and drive development will continue to improve the performance and cost effectiveness of brushless-motor technology and vector-controlled AC drives. Over time, however, more significant changes are likely to take place as new technology, such as SiC semiconductors and novel, hybrid motor concepts, advance from the lab to the production lines of appliance manufacturers. Those companies that pay attention to these trends and prepare for the quick and early adoption of these emerging technologies stand to reap great benefits.

For more information, email: George.Holling@RockyMountainTechnologies.com