Mike Howell, PE
EASA Technical Support Specialist
Most EASA service centers encounter very few axial-flux machines. They are rare enough that it is worthwhile to describe what they are and how they differ from the typical radial-flux industrial motor or generator. Figure 1 shows a cutaway of an axial-flux machine on the left and a radial-flux machine on the right. The gold regions represent the energized stator windings and the green regions represent the rotor windings or permanent magnets. Note that the axial-flux machine shown has two rotors; a rotor winding on either side of the stator. The radial-flux machine is what most EASA service centers are accustomed to; a rotor separated from a stator by an air gap in the radial direction and a magnetic field that crosses that air gap to link both windings (or windings and permanent magnets) in a way that can produce useful torque.
The primary attraction to an axial-flux machine is that they have a relatively short axial length. They have sometimes been called pancake motors for that reason. Axial-flux machines are also not new, and they have been used widely in some applications – think of the hard disk drives used for electro-mechanical data storage. And, even though rare in industrial applications, EASA members do encounter axial-flux machines. Figure 2 shows the stator for a 1.25 hp 4 pole axial-flux motor before rewind. Figure 3 shows a more common axial-flux brake design.
Although the windings and cores shown in Figure 2 and Figure 3 are reoriented from radial to axial, their general construction is very similar to their radial counterpart that we see in most rotating machines repaired by EASA service centers. That is, they typically have cores constructed from laminated, electrical grade steel and round enameled copper magnet wire windings. There are many challenges with hand-winding or machine-winding stators like the one shown in Figure 2. The end-windings on the inside are severely constrained, making the work very tedious. Assembling the cores is also more challenging than for a typical radial-flux stator.
In the last few years, a couple of manufacturers have introduced axial-flux, permanent magnet motors with printed circuit board (PCB) stators. These machines are air-cored. That is, they have no stator core laminations to direct the magnetic field. Rather, a PM rotor is placed on either side of the PCB stator to facilitate the flux linkage and production of useful torque. Machines such as those shown in Figure 4 are currently in production up to around 15 hp (11 kW) with typical speed range of around 500 rpm up to 4200 rpm. The PCB stator construction differs between OEMs but some of the more robust designs have stator phases in different PCB layers, reducing the risk of phase-to-phase faults. These machines tend to have integrated variable frequency drives (VFDs) with common input power specifications (e.g., 460 V 3-phase, 50/60 Hz). However, the enclosure and mounting configuration are not standardized, so they are not “drop-in” replacements for NEMA or IEC standard frames. Common applications thus far are fans and pumps where the high efficiency over a large speed range is advantageous.
The current approach from PCB stator OEMs seems to be direct sales and service to end users, but that is often the case with new product launches. Whether distribution sales opportunities or service opportunities develop for EASA members, it is worthwhile to be generally familiar with these machines should you encounter them in the field or be asked about them by a customer.
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