Airgap motors were developed over 40 years ago in Switzerland. But until recently, they have remained in relative obscurity due to the lack of high volume manufacturing capability that could make them cost effective in other than fractional horsepower (FHP) ratings. Now, by employing modern manufacturing techniques, these motors can be employed in a number of different applications. Indeed, design, manufacturing and cost constraints can, and have been, overcome to produce motors with ratings as large as 2.2kW.
The axial airgap motor offers the opportunity for the design engineer to exploit the short axial length of the motor in the OEM equipment, which may add either an additional engineering feature or a marketing benefit to the finished product.
The motor itself (Figure 1) has a disc-shaped stator and rotor cores that are adjacent to each other. Flux linkage is across an axial air gap. Electromagnetic induction leads to the production of torque as a result of electrical energy input to the coils around the stator.
Unlike the squirrel cage induction motor, where both stator and rotor are built on stacks of individually punched steel laminations, the axial airgap cores can be produced from a tightly formed coil of steel strip. Close control of the coil rotation in relation to a slot punching operation leads to the creation of slots that can be either radial or skewed.
For the stator, coils of copper wire are inserted into the slots and connected to form the winding. For a rotor, the aluminium conductors are created by die casting.
The stator and the rotor cores are the principle elements of the motor which are then combined with the enclosures, shafts and bearings that are consistent with the mechanical requirements of the application.
Axial air gap motors have similar design criteria to other ac electric motors, but there are some specific considerations that designers should bear in mind.
Since the axial air-gap stator presents proportionally less surface area for heat transfer, it is at a disadvantage to a conventional squirrel cage motor with its die-cast aluminium ribbed frame. However, the geometry and position of the rotor makes it more accessible for dissipation of rotor heat, whereas a conventional motor has the problem of heat dissipating from its centre. Taking these factors into account, it is possible to achieve designs with an output that is competitive with a conventional motor.
The creation of laminated cores for the stator and the rotor of axial air-gap motors of differing electromagnetic designs can be achieved with less capital investment than a conventional squirrel-cage motor (typically £10K against £100K for volume production tooling). This provides the opportunity for electrical designers to design custom motors for high-volume applications. This is another advantage over conventional motors that have compromised lamination design to provide a general purpose motor suited to many different applications.
A significant axial force is developed between the stator and rotor when the motor is energised. The force of attraction acts to close the working air-gap between the two components and is a principle consideration in the mechanical design of a motor that uses air-gap technology. To overcome this difficulty, twin rotors or stators can be used to create equal and opposite forces that cancel out, or a bearing arrangement can be used that is capable of accommodating the force. Automotive hub bearings have been found to be ideally suited for this purpose.
The geometry of the rotor core of an axial air-gap motor, being disk shaped, leads to a higher rotor inertia than on a conventional squirrel cage induction motor of equivalent rating. This could be of advantage where a flywheel effect is desirable, but must be borne in mind where fast response in starting or stopping is required. Figure 2 shows how a compact centrifugal pump that relies on a relatively flat impeller can be produced using a fractional horsepower axial motor integrated within it. The pump shows how the stator winding can be isolated by the motor shape.
In the design of products that use axial motors, the whole assembly must be viewed, rather than just focus on the detail of the motor. In this instance, thermal considerations come to the fore. Because the pump unit will be immersed in water with a predictable ambient temperature, the water itself improves the heat transfer from the pump. Electrically, the unit is a fixed speed device that can be optimised accordingly. Clearly, water provides an excellent cooling medium. Compound injected into the plastic casing seals the stator and provides rigidity but renders it irreparable.
The geometry and the manufacturing processes associated with the axial airgap motor leads to more effective use of materials and less wastage. However, the high thrust loads require more substantial bearings than found on a conventional radial airgap machine of equivalent rating.
The criteria for the potential success of the industrial axial airgap motor is considered to be cost-competitiveness (although not necessarily cost parity) with the volume squirrel induction motor. Any cost differential between the two types of motor is more than compensated for by the added value in the equipment.
Figure 1: Axial motor components
Figure 2: A compact centrifugal pump