How to Make High Efficiency Motors Affordable Abstract: In many segments of the motor industry the demand for high efficiency motors is increasing due...
How to Make High Efficiency Motors Affordable Abstract: In many segments of the motor industry the demand for high efficiency motors is increasing due to a variety of reasons: energy cost, cooling and power/weight density. Switched reluctance motors are now routinely pushing 94%-96% and IPM motors run as high as 98+% using standard steels and magnet materials. The paper discusses some of the design trends that are used for these high efficiency motors and gives an outlook as to which technologies and motor efficiencies will become available in the near future. Background: Currently most brushless motors used in the motion industry are based on rare-earth magnets. These magnets have experienced dramatic increases in the price of rare-earth minerals and thus, the price for these magnets has increased by several hundred percent over that past two to three years. China is the main supplier of these materials today and the signals are mixed as to whether prices will continue to decrease or whether limited allocation will keep magnet prices high. Although magnet prices have eased since the heights that we experienced in 2011 they are still high in absolute terms. Due to the economic conditions it is still difficult for suppliers to raise prices and motor manufacturers are looking at alternatives to design cost competitive products that ideally do not use any of these magnet materials or at best significantly reduce the amount of such materials. Reluctance motors offer such an alternative in a variety of forms which will be discussed in this paper. Technology: The most widely know and associated motor with a “reluctance” motor is the switched reluctance motor or more correctly the variable switched reluctance motor (VSR). Figure 1 shows an example of a”classic” VSR machine. The VSR machine was invented in the mid 1800s but due to the lack of drive electronics was not commercialized until the 1970s when the principle was successfully used in stepper motors and as a typical VSR machine in a plotter.
Figure 1: Example of a 3 phase (6/4) VSR machine
It is beyond the scope of this paper to explore the physics and design of VSR motor in detail which is covered well and in great detail in the literature. While the VSR motor is still mainly designed for and used in custom applications these motors are available commercially in blowers (Ametek), for traction (Rocky Mountain Technologies) and as general purpose variable speed motors (Emerson). A detailed comparison of motor technologies shows that the VSR machine competes well with brushless and AC induction motors (ACI)[ HEND91] although with modern, high energy magnets the brushless motor can yield much higher power densities today. Modern VSR motors will run from 92% to 96% efficient and a well designed traction motor can show as little as 5% torque ripple with a simple controller and even less if more advanced control methods are employed. The main perceived drawback of the VSR machine is its perceived “acoustic noise”. Acoustic noise in the VSR machine can be reduced through a range of mechanical design features. Since acoustic noise applies to all reluctance based machines we will discuss it in more detail here. Acoustic noise is generally considered caused by a deformation of the lamination when the rotor is in or near the aligned position when the rotor teeth are aligned with the stator teeth (shown in Figure 1). The radial forces in the airgap are significant and they are typically as high as 10x the tangential force which causes torque. These high forces cause the stator to deform and the tooth the elongate slightly. When the exciting current is removed these forces decay rapidly, thus causing the teeth and stator to return back to their original position and thus create a snapping sound, which causes acoustic noise. This problem can be attenuated by mechanical imbalances between the opposing gaps especially when a very small airgap is used as is the case with classic VSR motor designs. Good mechanical construction and large airgap designs can help to greatly reduce acoustic noise in reluctance machines. Electronic schemes have also been proposed and while they work well under certain conditions anecdotal evidence suggests that they lack the robustness and may thus be of limited value. We have compared the noise in a vehicle cabin between a brushless and a VSR motor at speeds from 30 MPH to 60 MPH and found that the noise between these motors varies by +/-3dB over this range giving neither motor type a clear advantage in this test. While most authors advocate the simple, cost effective construction one must caution that some of these gains are offset by the need for thinner laminations – SR based machines use 29 ga. versus 24 ga laminations commonly used in brushless motors – and other mechanical issues such as tolerances, bearings etc. These factors can offset any savings for a very small motor while the VSR motor clearly has the advantage for larger machines. VSR machines are proving themselves to be a real alternative to brushless PM and even brush DC machines. Our partner in China is now introducing a design to compete with brush motors in 3-wheeled vehicles with a 128 mm OD motor that delivers up to 80 Nm peak torque with low torque ripple. The next alternative technology is the synchronous reluctance motor (SYR). Figure 2 shows examples of a SYR. The left construction shown in Figure 2 was classically used which presents many technical problems. However, lately the right hand construction has become more widely used, which is based on common laminations that can be stamped in a standard lamination die.
Figure 2: Examples of the Synchronous Reluctance Motor ABB has recently introduced a line of variable speed SYR with excellent performance specifications [ABB11]. The SYR performance is very similar to that of the VSR machine but is suffers less from audible noise and claims a simpler mechanical construction at the expense of a more complex electronic drive. A further advantage is that the SYR can be driven from a standard AC vector drives while the VSR requires a special inverter. These features make the SYR an ideal candidate to directly replace the brushless AC motor. The internal permanent magnet motor (IPM) finally is the third candidate machine. IPMs have magnets embedded in the rotor can be designed in 2 basic forms: a flux channeled design and a “hybrid” design. In the flux channeled design the magnets are focused to increase the strength of the PM field yielding basically a “strong” conventional brushless PM AC motor.
Figure 3: Reluctance Torque Contribution of the “Hybrid” Internal Permanent Magnet Motor
Figure 3 shows the total torque output of the IPM and the component due to the permanent magnet’s field. This graph shows the torque output of the motor as the angle of magnet material in the rotor is widened (more material is added as the magnet grows). As we can see from this graph the maximum torque output of this motor is achieved with 50% of the magnet material of a conventional motor and yet the total torque output of the motor is twice as much as that of the PM only motor. This is due to the added torque from the motor’s reluctance. When we compare the structure of a typical “hybrid” IPM motor we can see that it incorporates both the synchronous reluctance motor and the permanent magnet as shown in Figure 4.
Figure 4: Design of a “Hybrid” Internal Permanent Magnet Motor The design in Figure 4 contains many magnets and while these are small in the aggregate their number represents an assembly problem and they also amount to a significant amount of magnet volume. While this design must be chosen in some applications more commonly a simplified structure for the “hybrid” IPM can be used as shown in Figure 5.
Figure 5: Alternate Design of a “Hybrid” Internal Permanent Magnet Motor
The design in Figure 5 contains a much smaller amount of magnet, although the shape and size of the magnets greatly depends on the final design specifications, thus impacting the total magnet volume. Our company has designed and built such “hybrid” IPM motors with above 98% efficiency when driven with a conventional D-Q vector drive and even higher efficiencies can be achieved with different control algorithms. The main drawback of the “hybrid” IPM motor is the fact that its torque distribution has inherent ripple which must be compensated for. In smaller motors, however, some amount of ripple can be tolerated and thus a simple and inexpensive drive can be used. Figure 6 shows the compensated torque ripple for a 35 kW IPM motor with 230 mm OD and a 70 mm stack and Figure 7 shows the efficiency curve for this motor.
Figure 6: Compensated Torque Ripple for the 35 kW IPM motor at 145 NM
Figure 7: Efficiency Plot for the 35 kW IPM The total magnet weight for this motor is approximately 1.5 lb for this motor. Even at this reduced volume it still represents the single highest cost component.
Research is ongoing to replace the amount of magnet material while maintaining the same or only slightly reduced motor performance. Figure 8 shows the efficiency map of a similar size motor that does not contain any rare-earth magnets and which will deliver substantially the same output torque.
Figure 7: Efficiency Plot for the 35 kW IPM without Rare-Earth Magnets Thus, if we can tolerate slightly reduced motor efficiency novel concepts are emerging that may have a significant impact on the design of IPM machines. Even with more conventional designs the VSR, SYR and the IPM are cost effective design alternatives which should be considered for many application. Summary: The paper has shown that alternatives to the surface PM machine exist and that these can deliver comparable or even better performance than the conventional PM machine performance. For new motor designs Synchronous Reluctance Motors and Internal Permanent Magnet machines along with Variable Switched Reluctance motors should be considered viable design alternatives that can help to reduce the motor’s cost and shield the manufacturer from severe fluctuation in the price of rare-earth magnet materials. Literature: HEND91
Hendershot, J.R., "A Comparison of AC, Brushless & Switched Reluctance Motors", Motion Control, April 1991, pp.16 to 20
