Axial-Field Electrical Machines

An electrical machine translates its input electrical power into an output mechanical power i.e. it is an electromagnetic energy conversion device. Electrical machines have been available and working for nearly a century. During this period many extensive efforts have been made by researchers worldwide to develop and improve design, configuration, and performance of electrical machines. They may be categorized according to their conductor geometry and field orientation as:

1. The Linear machine, where the mutually perpendicular flux and the conductors are arranged along a linear path.
2. Axial-field electrical machine, where the conductor is radial and the air gap flux is axial.
3. Radial-field electrical machine, where the conductor is axial and the air gap flux is radial.

The history of electrical machines shows that the earliest machines were of the axial-field type. Based on the principle of electromagnetic induction, Faraday invented the Faraday disk in 1832, which is also called the homopolar machine. Because of the strong magnetic force existing between the stator and the rotor, these machines were soon replaced by radial-field machines. These radial-field machines have been and are still used to a large extent. One example of a popular axial-field machine is the printed circuit servomotor. As mentioned above, one drawback of the axial-field design is the strong magnetic force between its stator and rotor. This problem may be alleviated by using a sandwich configuration with a stator placed between two rotors or a rotor sandwiched between two stators as shown in the below figure.

Some of the commonly used applications where the axial-field machines used are radiator cooling fan, auxiliary power unit, wind- power generator, electric vehicle, a high-speed generator driven by a gas turbine, adjustable-speed pump drive, lawnmower motor, car heater blower etc.

Classification of Axial-Field Machines:

In theory, each type of radial field machine will have an analogous axial-field machine. Therefore, an axial field machine can operate as a d.c.machine as well as an a.c. synchronous or induction machine. These machines can be constructed in one of the following ways in correspondence with the design of the magnetic circuit:

1. The single stator and single rotor (one air gap), as shown in Figure (a).
2. Central-stator machine (double air gaps), as shown in Figure (b)
3. Central-rotor machine (double air gaps), as shown in Figure (c).
4. Multi-disc machine (multiple air gaps).

Machines constructed using a single stator/rotor experience a strong magnetic pull between the stator and rotor and therefore, the sandwich configuration appears much more viable. Of the two types of sandwich construction available, the central stator machine produces more torque per length of stator conductor, since both the working surfaces of the stator core are used. In axial-field machines, the electromagnetic torque is a function of the machine outer diameter. The multi-stage arrangement is suited to overcome the restriction on the machining diameter and to meet the torque required at the machine shaft.

Differences between Axial-Field and Conventional Machines

Axial-field machines differ from conventional machines in the following:

1. The air gap flux is axial in direction and the active current carrying conductors are radially positioned.
2. The stator and the rotor cores are of the disc type.
3. A higher percentage of the stator winding produces torque. This is due to the fact that the stator winding is wound in a toroidal configuration. Thus, the two working surfaces of the core are both active for the electromechanical energy conversion such as in the Torus machine.
4. The two rotating discs (in the central stator machine) act naturally as fans, thereby removing heat produced by copper and iron losses. This can be accomplished by means of holes positioned near the mechanical shaft so that the air flows radially through the machine air gaps. This machine configuration, therefore, permits an exploitation of active materials than in conventional machines.
5. Axial-field machines can be designed to possess the high power to weight ratios with more saving in the core material especially if permanent magnets are used. They have also a larger diameter to lengthratio.6. Floor space required for manufacturing and assembly line is significantly reduced for the axial-field machine.

The short axial length of the axial-field machine results in a larger diameter than that of the radial field machine and hence a higher inertia for the same power rating. This limits their application to systems requiring a slow response.

Applications of The Axial-Field Machines

Axial-field machines are particularly appropriate for the development of compact integrated design showing to their disc-shaped. Some potential applications of the axial- field machine include the following:

1. Auxiliary power units

Using the Torus configuration a number of auxiliary power units for military applications have been developed in association with Dornier GmbH

2. Wind power generator

The Torus generator can be direct coupled to a wind turbine allowing the elimination of the gearbox. This brings about reduced nacelle weight and noise, and improved reliability and efficiency. These considerations led to the proposed use of direct-coupled Torus generators for small-scale stand-alone generating systems in remote areas. 10 kW stand-alone wind/photovoltaic generating system prototypes have been constructed. Below figure shows the layout of a system that uses a wind-turbine-driven permanent-magnet machine and a photovoltaic array as power generating units.

The principal features of the system are:

• The permanent magnet wind generator is directly driven and has the high torque to weight ratio and high efficiency.
• The double-input, single-output, d.c.-d.c. converter combines the power generated by the wind and photovoltaic array with high efficiency.
• The d.c.-d.c.converter is used for charging a storage battery and supplying the user a.c. load via a voltage source inverter.

3. Water-cooled EV drive

The arrangement with a water-cooled ironless stator is particularly suitable for direct-coupled wheel drives demanding high torque density. The motor construction could be totally enclosed to provide protection against the environment. A single-stage demonstrator machine was first developed and tested to prove the principle, and a two-stage machine was then designed for the application in an innovative city car. The power loss within the winding is removed by assisted circulation of cooling water. This results in a relatively low over-temperature between the inlet and the outlet of the cooling duct.

4. Electric scooter drive

An axial field slot less permanent-magnet motor was used as motor-in-wheel drives for an electric vehicle to replace the 3HP two-stroke engine in a standard production scooter. The motor drive arrangement uses microprocessor-controlled IGBT power converter, which is fed by a lead-acid battery. The motor was supplied from a 96 d.c. voltage via a microprocessor-controlled IGBT power electronic interface consisting of a bi-directional d.c.-d.c. converter and a current-regulated PWM inverter. The use of a bi-directional converter allows suitable control of both motoring and regenerative braking operations and also for on- board battery charging from the main supply.

5. High-speed generator driven by a gas turbine:

High speed enables a high power output to be obtained from a relatively small and lightweight machine and allows the generator to be directly coupled to a gas-turbine engine. A prototype machine was driven at 60, 000 r.p.m. by a small gas turbine engine. The generator is rated at 50 kVA. The machine could be used in an electric vehicle with suitable batteries as the electrical power source for the traction motors. Moreover, the machine could be used also as a small stand-alone generator set.

6. Adjustable-speed pump drive

The design and construction of a 3 Nm, 2800 r.p.m., slotless, axial-field machine prototype using ferrite permanent magnets for application in adjustable speed pump drives. This prototype drive possesses several advantages over the conventional three-phase induction motor such as higher efficiency, compactness, and lightness. Furthermore, it was shown that the manufacturing cost of the prototype, on a large scale, can be as low as that of an induction machine drive having the same power rating (for low power ratings)