A brief discussion on the working principle of axial flux motor

The heart of an electric motor consists of a rotor and a stator, around which the rotor spins. Traditionally, stators are made of iron and tend to be heavy, accounting for about two-thirds of the weight of a traditional motor. To reduce the weight of the stator, some have proposed using a printed circuit board (PCB) instead. From the beginning, the idea of ​​replacing iron materials with lightweight, thin, easy to manufacture, and durable printed circuit boards was attractive, but it has not been widely adopted in products such as lawn equipment and wind turbines for more than a decade. Now, however, the PCB stator is getting a new lease of life. You can expect PCB motors to save weight and energy in all electric drive products.

The energy-saving role they play is crucial: Software may be eating the world, but electricity is increasingly what keeps it going. Electric motors consume a little more than half of the world's electricity. About 800 million electric motors are sold each year, and that number is growing 10% a year, according to market researcher Imarc. Electric motors are used in cars, trains, airplanes, industrial equipment, and HVAC systems. Transportation, buildings, and HVAC together account for about 60% of U.S. greenhouse gas emissions; efficient motors can help reduce emissions from these sectors. Despite their many advantages, PCB stators have been slow to gain traction due to several misconceptions. First, there's the misconception that PCBs are only for precision applications. But in 2011, CORE Outdoor Power introduced a leaf blower and lawn mower, both of which use PCB stators, which are rugged and quiet. Second, some people think PCB stators are only for low-power machines. But in 2012, Boulder Wind Power put a printed circuit board stator into a 12-meter diameter direct-drive generator that drives a wind turbine with an output of 3 megawatts and a torque of more than 2 million newton meters. It is one of the smoothest-running high-power generators ever built.

Neither company lasted. Boulder Wind Power ran out of money and failed to secure solid commercial contracts, while CORE Outdoor Power had to fight cheaper competitors in a crowded market and ultimately lost. Still, their groundbreaking work demonstrated the viability of PCB stators. Today, my company, Infinitum Electric (Austin, Texas, USA), has developed a motor with a versatile PCB stator. It produces the same power as a traditional AC induction motor, but is half the weight and size, produces very little noise, and reduces carbon emissions by at least 25%. The motor is now widely used in HVAC, manufacturing, heavy industry, and electric vehicles. Here’s how it works.

Infinitum Electric's motor is an axial flux motor, where the stator's electromagnetic wiring is parallel to the disc-shaped rotor containing permanent magnets. When an AC current is applied, the stator drives the rotor to rotate. The motor is air-core, meaning there are no iron parts to regulate the magnetic flux, only a thin layer of air between the motor's magnetic parts. All of these parts combine to form a hollow axial flux permanent magnet motor.

In the past, there were serious practical difficulties in making such motors. The stator was cumbersome to manufacture, the copper windings were bulky, the support structure for the coils was complex, the air gap was very wide, and only large magnets could generate sufficient magnetic flux.

Instead of using copper windings, Infinitum Electric uses photolithography to etch thin copper sheets separated by epoxy-glass laminates to insulate the coils from one another. Eliminating the iron core and reducing the copper use reduces the motor’s weight by 50% to 65% and its volume by 50% to 67% compared to an equivalent conventional iron-core motor. In addition, the copper and laminate expand and contract in the same way as the temperature rises and falls, avoiding stresses that slowly cause the parts to disintegrate. Because there is no stator core, we can place two identical rotors, each with powerful permanent magnets, facing each other on either side of the stator. This creates a constant magnetic flux. As with other axial flux motors, the flux is parallel to the axis of rotation, not radial to it.

Because the magnetic air gap is narrow, we only need a small magnet, which is why we can get more energy in a given mass and volume. More importantly, PCBs are manufactured through an automated process, which means they are more consistent and reliable than manual winding machines. We make them more reliable by simplifying their topology. Topology is related to the phase of the motor. Phase voltage refers to a sinusoidal AC voltage that can be phase-shifted in time relative to another phase voltage. The changes in multiple phase voltages are synchronized, so the sum of the currents is always zero. When a multi-phase voltage system acts on the application motor, each phase voltage corresponds to a separate winding, and the multiple currents generate a rotating magnetic field in space. This rotating magnetic field interacts with the magnetic field generated by the rotor magnets to drive the rotor to rotate. Previous PCB stators mixed copper wires with different phase voltages in the same layer structure, which could cause short circuits. Therefore, we chose to only correspond to one phase voltage per layer and minimize the number of electrical connections between layers. This layout provides a continuous current path and reduces the risk of electrical failure.

Another advantage of the new layout is that it gives designers the freedom to choose whether to connect the coils in series or in parallel. Series coils are suitable for three-phase industrial applications and next-generation electric vehicles; parallel coils are more suitable for low-voltage applications, such as auxiliary motors in electric vehicles.

Similar to other permanent magnet motors, our axial flux motors require a variable frequency drive (VFD) to start the motor smoothly and accelerate it as required. The VFD can also control the motor's speed and torque as required by the application. However, the air core design allows the motor to have very low impedance (typically only 5% to 7% of a traditional iron core motor) because air holds much less magnetic energy than iron, so less magnetic energy is available to smooth out the voltage changes that the VFD brings to the motor.

To compensate for this shortcoming, we added another component: an integrated variable frequency drive that precisely operates low-impedance motors. Our variable frequency drives use high-efficiency silicon carbide metal oxide semiconductor field effect transistors (MOSFETs) to reduce losses and improve overall efficiency. In addition, the variable frequency drive can monitor the performance of the motor and report the results through cloud functions if the customer wishes; the motor's software can also be updated in this way. This remote monitoring provides a variety of ways to save energy, manage performance, and predict when maintenance is needed. Ultra-thin printed circuit boards provide a high surface area to volume ratio, which can improve cooling efficiency. We can apply 2 to 3 times the current with a certain amount of copper. Air cooling can be achieved through heat sinks between the outside of the motor and the electronic components.

Eliminating the iron core eliminates the losses caused by the periodic magnetization and demagnetization of the iron, and also eliminates the eddy current losses in the metal. As a result, our air-core motors can operate efficiently in the range of 25% to 100% of the rated power. Eliminating the iron core also means that the magnets on the rotor will face a constant reluctance and a constant magnetic field as the rotor turns. This design eliminates eddy current losses in the magnets and rotor, so the rotor can be made of standard non-laminated composite mild steel sheets.

For ordinary electric motors, both the stator and rotor are made of ferromagnetic materials. Once current is applied and a rotating magnetic field is formed, these magnetic fields will generate two forces: one is a useful torque force that rotates the rotor; the other is a radial force that pulls the rotor toward the stator, causing the stator slots where the copper coils are placed to jump. The radial force is not only useless, but also exacerbates noise and vibration.

Here's how it happens. The forces created by the magnetic flux start out pointing in the same direction as the rotor's motion, then as the rotor spins, the rotor pole positions change relative to the stator slots until the forces are in the opposite direction. This alternating force creates torque fluctuations that can cause metal fatigue in the motor and the machinery it drives. Infinitum Electric's motors don't generate these alternating magnetic forces. This and other efficiency features are why Infinitum motors are, on average, about 5 decibels quieter than conventional motors. That may not seem like a lot less noise, but that noise is often particularly harsh and annoying.

Infinitum Electric’s motors combine the lightness of an air-core motor with the high torque density of an axial-flux motor, making them ideal for ventilation and HVAC systems in buildings. They are especially useful now that the COVID-19 pandemic has made indoor air purification a top priority. Heat pumps for heating and cooling are another application where motors can save energy, simplify installation, and reduce noise. According to a recent test by the General Services Administration (GSA) and the U.S. Department of Energy, if GSA HVAC plants were equipped with Infinitum Electric’s motors, they could save up to $8 million per year. Electric vehicles are another big market for this new type of motor. The U.S. Energy Information Administration predicts that electric vehicles will account for 31% of the global vehicle fleet by 2050. Our company is working with a leading automotive supplier to develop an oil-cooled motor for a long-range gasoline-electric hybrid vehicle.