MOTORS, FANS, BLOWERS & PUMPS: Quiet Power
Kinetic Art & Technology Corp. (KAT), Greenville, Ind., has developed a unique architecture for permanent magnet electric motors and controllers known as Segmented ElectroMagnetic Array (SEMA) technology, designed to deliver higher power density (especially peak torque) and higher efficiency than many existing motor designs. A SEMA motor is also designed for easier control and lower noise. The SEMA motor and controller technology is being commercialized by KAT subsidiary Lynx Motion Technology Corp., also in Greenville.
Low torque ripple is another beneficial trait of the SEMA motor, a benefit related to the motor's quietness. Conventionally designed motors often possess magnetic characteristics that inherently produce low-frequency torque oscillations and acoustical noise. In particular, the slotted magnetic core in such machines introduce cogging, magnetic saturation, armature reaction, and hysteresis phenomena that all contribute to these effects. Furthermore, even in the absence of such phenomena, any non-sinusoidal nature to the motor's field distribution will also create low-frequency torque pulsations unless custom-designed current commutation waveforms are used.
SEMA technology is based on an axial gap (disk) motor topology that uses a unique coil design along with innovative manufacturing techniques to produce a motor with significantly improved specific power over competitive machines, while being very economical to produce. The motor's performance characteristics - high power density, high reliability, high efficiency, low noise and vibration - are achieved in several ways.
1. The motor uses a double air-gap design, which eliminates the need for a rotor yoke, making the machine lighter and smaller.
2. The magnetic forces in its axial-gap design act at a larger mean radius compared with those in a conventional radial-gap arrangement, allowing these forces to be utilized more effectively in the production of torque.
3. The armature windings are wound on bobbins without an iron core. This approach eliminates hysteresis losses and magnetic attraction between rotor and stator, which in turn reduces bearing loads.
4. The machine uses a toothless core design that eliminates the magnetic losses, cogging, and saturation effects associated with iron teeth. In addition, Lynx and KAT have developed a method of eliminating torque ripple in SEMA motors, making nearly vibration-free operation an inherent aspect of SEMA designs. The toothless design also increases the area available for copper conductors (for higher electrical loading), boosts peak torque capability, and allows high-coercivity permanent magnets to be more effectively utilized.
5. The coils are fully potted in a high-strength, thermally conductive epoxy. This arrangement not only gives the machine structural integrity, but also effectively dampens the high-frequency vibrations that might otherwise be produced due to PWM excitation.
Motors based on SEMA architecture may be operated at constant or variable speed, steady state or intermittently, and can be reversed, with the same quality of performance in all modes. Since there is no cogging -- even at low speeds -- power is delivered to the motor in a quiet, continuous manner. Many power sources can be used as input to the motor's power electronics, including DC, single-phase AC, or three-phase AC.
Under microprocessor control, the motor can be controlled precisely in torque and speed, and can be positioned with high accuracy and repeatability. There are no brushes or contacts to malfunction or wear, spark, or cause losses inside the motor. The SEMA motor's unique stator construction reduces wire-resistance losses and eliminates iron saturation and hysteresis losses. The inherent modularity and scalability of SEMA technology facilitates configuration of motor components to support a wide variety of applications. This allows a wide range of motors to be produced from a small set of mass-produced components.
The SEMA motor's extremely high torque and efficiency make it practical as a direct-drive motor in applications which usually require a speed reducer. The direct-drive method eliminates gear noise, positioning errors from backlash (common in gear drives), and lack of torsional stiffness (common in harmonic drives), yielding a quieter and smoother system. Since precision speed reducers or conversions to hydraulic actuation often cost much more than the motors which drive them, motor systems using SEMA-based electric motors may also offer a cost savings over non-direct drive motors.
While many of the SEMA's attributes recommend it for use in electric drives, it is the uniform production of torque that makes this motor technology inherently quiet. Two major attributes of the SEMA yield this result. First, the production of torque within the SEMA motor remains proportional to current. Second, the motor's torque is uniform with respect to rotor position.
ConfigurabilitySEMA-based designs may be configured around several unique coil structures and magnetic circuits, all of which have been developed with increased torque and efficiency in mind. These permit SEMA configurations to be made for conventional three-phase designs as well as more advanced multiphase designs and architectures requiring complex, customized control implementations. The versatile control potential of SEMA motors owes much to these coil designs.
The technology is also compatible with existing motor technology. The simplicity of SEMA construction makes it possible to manufacture many variations of the design with conventional manufacturing techniques for electric motors. A SEMA-based motor, despite its high power density and efficiency, may be brought to market at a cost equal to the cost required to implement a conventional design. As high-volume production techniques are employed, the cost of SEMA motors may even become become lower than their conventional counterparts.
ApplicationsThe properties of the SEMA motor technology make it well-suited for military, aerospace, vehicle and industrial automation applications, but there are any number of appliance products that could benefit, as well. For example, one of the very first projects initiated was the design of a motor for cordless circular saw operating off a 24 V battery pack. The customer's economic circumstances prevented the product from making it to market, but the design and development work performed by Lynx proved the concept.
A cordless circular saw using the Lynx motor was tested against an off-the-shelf cordless circular saw, with both using the same saw design and 24 V battery pack. Only the motors were different.
"We cut a large number of 2x4s with the saws to see how many you could cut on a charge," says Roy Kessinger, president of KAT. "With the store bought saw we cut about 100. With the saw using our motor, we cut about 200. In addition, our motor cut the boards faster because we used a DSP chip to control speed. The DSP adjusts the torque for constant velocity so the blade doesn't slow down as it is pushed into the wood. So the motor would be especially good for professional power tools where you don't want to change battery packs that often. Size and weight are also considerations in that segment, and our motor is considerably smaller and lighter than the conventional brushed motor typically used in such tools."
Kessinger says that Lynx has also performed some research and development on motors for driving refrigeration compressors, with the goal of achieving higher efficiency and variable speed operation. The work was sponsored by the Department of Energy. The research showed that the compressors powered by Lynx motors could achieve much higher levels of efficiency than conventional systems.
Lynx has also developed a 2 kW motor for a compressor application where the customer desired a total system efficiency (motor and controller) of better than 90 percent. Lynx met the requirement. The end application is still under development.
Because the Lynx motor is capable of efficient, reliable direct drive, the motor would also be suitable for clothes washers, eliminating the need for belts, pulleys, or transmissions.
"With our controller, we can provide speed ranges for both low-speed agitation and high-speed spin," Kessinger says. "We can produce high torque at low speeds, and can also reverse direction."
When using a 4-quadrant controller, the motor can also be used to slow down an inertial load. During that braking, the motor generates electricity that goes into a capacitor. That feature would be beneficial for a motor-reversing application, as on a vertical-axis clothes washer agitator, since it would regenerate on half the cycle, recovering some of the energy use.
Another obvious appliance application for the motor is commercial floor care.
"A floor scrubber is a natural for our motor," Kessinger says. "Our motor's pancake shape begs for a scrubbing pad to be put on to it. The unit would have extremely low profile, eliminating the the big motor hump on top like you see on most commercial floor scrubbers. That hump prevents it from being able to go under things. Our motor would only be about 2 in. tall in a scrubber, so it would slide under furniture or ledges easily."
In the past, the Lynx motor would not have been economically feasible for the floor care segment, but Kessinger says that is now changing due to the decreasing price of control electronics and the falling prices for the neodymium-based magnets the machine uses.
"Neodymium magnets used to be around $100 per lb.," he says. " Now they are down to about $10 per lb. in high volume and $25 per lb. in lower volume."
Because the SEMA motor technology is scalable, there may be other appliance applications that can benefit from it. Lynx has built motors as small as 3 in. in diameter, and as big as 33 in. in diameter. They have built motors all the way from 1/3 HP up to 200 HP.
The business model for Lynx is to design and develop motors using the SEMA technology, then license the design to another company to produce, whether it be a motor manufacturer or OEM. Lynx only produces prototype quantities of the motor.
The first licensee for the SEMA technology was Bodine Electric, based in Chicago. Bodine calls the motor the e-Torq and currently markets it in three frame sizes, 7 in., 9in., and 14 in. Some of the markets Bodine envisions for the e-Torq motors include:
- Foodservice appliances where size and energy efficiency are major concerns. Ventilation blowers and conveyorized cooking equipment are examples.
- Medical equipment, such as large scale diagnostic equipment, where size, torque, and quality of motion are critical.
- Business machines, such as high-resolution imaging and printing equipment, where size and quality of motion are very important.