A newly developed motor design program helps optimize the design of custom motors, including new axial gap motors.
Electric motors have been around since the days of Michael Faraday and Nikola Tesla, and there have been many improvements and patents related to the design of electric motors throughout the years.
In recent years, the use of rare earth permanent magnets has become more common. The availability of rare earth magnets with high magnetic flux and resistance to demagnetization has allowed greater power density resulting in smaller and lighter weight motors.
Along with higher strength magnets, powerful modeling software has become available that allows a skilled engineer to accurately predict performance of a unit before it’s built. A skilled designer can model magnetic flows, mechanical stresses, thermal transfer and motor dynamics with such accuracy, that it’s possible to improve the design by iteration before any part of the unit is actually built.
While in theory software tools are technically feasible for a class of motors, in practice there are few experts that have the necessary combination of skills to undertake the task of integrating all aspects of a motor design into a generalized set of programs. Instead, development work is typically focused upon a particular motor project, with no intent of creating a generalized set of design tools.
Computer modeling tools with embedded expertise related to magnetic, mechanical and thermal modeling for application to electric motors has taken a leap forward. These tools allow multiple design iterations to be done quickly at low cost, letting designers quickly explore possibilities and create new designs.
Motor Design Software
The basic design process begins with a 3D mechanical model using Solidworks.™ Mechanical details of the magnetic circuit are particularly important if the result is to be accurate. The magnetic circuit model can be a simplified subset of the actual unit.
Once the mechanical and material parameters of the magnetic circuit are entered into the program, it’s possible to calculate an estimated magnetic field based on the elements that have come from a version of a lookup table based on magnet properties. The mechanical information and the magnetic information are transferred into an Ansys ™ program where the resulting magnetic flux patterns can be graphically displayed.
Designers can change the mechanical dimensions, material selections and other parameters to quickly optimize the magnetic flux patterns if they understand the implications of the display. Once satisfied with the magnetic circuit, the software can be used to calculate torque, back emf, and other performance measures based on design assumptions and winding configurations.
After choosing the winding configuration, it’s possible to do a thermal analysis of the coil when the motor is working. Once the model is assembled, it’s relatively quick to make modifications to the design, or if desired to optimize diameter, length, weight, efficiency or some other parameters using mathematical formulas and multiple iterations of the model.
While many of the magnetic principles that apply to radial gap motors are the same for axial gap machines, the radial gap designs incorporate some features that are quite similar to linear motors. As such it was necessary to develop a set of software tools that integrates mechanical, magnetic, thermal and electromechanical features into a package specific to axial gap motor assemblies.
The process begins with a mechanical design of a magnetic circuit, the number of magnets, the size of the magnets, the placement of the magnets and the geometry of the circuit, all of which must be defined before analysis can begin. The materials are then specified so that magnetic flux calculations can be made. With a simplified model of the magnetic circuit, it’s possible to begin design analysis and improvement by iteration. Looking at a display of field strength, an experienced designer can see problems of magnetic saturation, or misdirected flux patterns, or inadequate flux concentrations, and can then adjust the mechanical design to correct for these issues.
After the mechanical and magnetic designs are brought into an acceptable range, the next step is to define the windings, number of turns, size of wire, location of coils, potting method and materials. From this and the magnetic field information, it’s possible to calculate the motor performance. Again, iterations can move the design to approximate the desired performance. Once the winding parameters are estimated, then thermal analysis of the motor assembly can be accomplished and rapid iterations in the computer model can bring the thermal design into approximate range. Finally, after the basic design is established, using mathematical formulas that have been developed, it’s possible to optimize a particular parameter. For example, one might choose to optimize the design for minimum weight, or maximum efficiency. The computer will then take the information from the earlier design effort and fine tune the results to obtain the optimized design for that particular parameter.
The advent of powerful rare earth magnets enables motor designs that were impractical with weaker magnets, with the axial gap motor being one common example. And improved electronic drives are able to take advantage of the new motor designs, while being flexible enough to address required changes.
The form factor also plays a role. The cost of manufacturing radial gap motors favors smaller diameters resulting in sausage or cigar shapes, while axial gap motors result in a flat pancake style motor. For some systems this form factor has an advantage. Large diameter ring-shaped motors provide hollow centers that offer advantages for optical pathways, beam lines and industrial processes like cable winding or optical fiber wrapping or other continuous web processes where 360 degree rotation around the item is needed.
This same form factor places the magnets on a larger radius that produces a high torque, higher inertia motor, bringing advantages to low-speed, high-torque type applications. The larger diameter increases the speed of the magnet for a given RPM while increasing the torque. The effect is similar to adding a gearbox to a smaller diameter radial gap motor but it eliminates the cost, wear and efficiency loss of the gearing. Axial gap designs have found favor as directly driven alternators for certain types of windmills, and other slow speed renewable energy projects.
The large inertia can be useful as a flywheel in an application like an oil well pump where the energy of the flywheel helps lift the load on the upstroke, while on the down stroke the energy is recovered. This motor, in combination with the proper set of electronic controls, significantly reduces pump power consumption as compared to a geared AC motor.
Axial Gap Motor Designs
An advantage of axial gap designs is that there is no switched iron in the magnetic ciruit. Eliminating the iron losses in the circuit results in higher efficiency, typically an increase of 10 to 15%. Also there is no detent torque when the motor is de-energized, allowing the motor to freewheel when the situation requires it. The magnetic attraction forces are typically supported by a spacer, so that large loads aren’t applied to the bearings.
However, the air gap between the magnet faces results in a curve that decreases the magnetic flux substantially as the gap increases. The general result is that the output of the motor increases as the coil plate gets thinnner. This presents both thermal dissapation and mechanical stress challenges to the motor designer when it comes to the coil plates.
In another variation, iron is inserted into the magnetic circuit where the iron increases the torque of the motor, but at the cost of energy losses in the iron. This design will typically produce 1.5 to 1.7 times the torque of a unit without the iron ring but there will be magnetic losses when back driving the motor, large attractive forces that must be supported by the structure, and lower top speeds as the winding inductance and the back EMF will be higher.
Another variation of the iron ring is a ring with notches. Where the windings are placed in the notches, this brings the iron very close to the magnets allowing machined air gaps similar to those found in iron-based radial gap motors. As such, the torque of such a motor design is greatly increased, cogging torque is much higher, detent torque is significant, iron losses are greater, and magnetic forces are exceptionally large, so overall energy efficiency is reduced. Still, for applications that demand high torque output in a minimum package size, this approach is useful.
Axial Gap Motors
So what are axial gap motors? Like radial gap motors, axial gap or axial flux units come in a variety of designs. There are versions with and without switched magnetic iron, with small and large air gaps, with different types of magnets and magnet shapes, as well as different pole shapes and constructions. The one thing they all have in common is that the direction of the magnetic lines is parallel to the axis of rotation.
Such designs have been used for oil field equipment, vacuum processing equipment and satellite testing. Some of the advantages of these types of machines: Higher electrical efficiency in the units without switched magnetic iron, shorter delivery schedules as the modeling predicts performance quite accurately, pancake form factor is useful in some situations, hollow motor designs are easily accommodated, high torque due to large radius provides good performance as an alternator directly driven at low speeds.
One example of this is alternators. Most alternators on the market are radial gap designs, but this development takes advantage of the strengths that an axial gap design provides in these applications.
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Filed Under: Motion control • motor controls, Software