Steve Meyer is President of Solid Technologies – Lakewood, Col.
Electric motors consume more electricity than any other machine, lighting system, or type of load. In the industrial world, some experts estimate that as much as 62% of all electricity generated is used for this purpose. So it is not surprising that hundreds of millions of dollars have been spent by the Department of Energy (DOE) and private companies in the quest to make electric motors more efficient.
Recent improvements in motor technology are many, but they have always come at a cost. For example, new magnet materials such as neodymium-iron-boron magnets account for higher torque density in brushless dc and stepping motors. Some induction motor manufacturers include
magnetic poles on the rotor to increase torque, but this has to be balanced against the cost constraints of the application.
NovaTorque’s target markets are driven by applications that require outstanding energy efficiency, such as battery-powered tools, scooters, and electric vehicles. Other applications that require extremely high energy density and small-size motors with low rotor inertia include aerospace actuators and industrial servo systems.
Lamination steels, too, have undergone significant improvement in recent years. So-called “exotic” alloys have higher saturation flux and produce more torque for the same size motor. And silicon steels are more efficient and have lower eddy-current losses, which mean less waste heat is generated in the motor and more flux is converted to torque. But exotic laminations and silicon steel cost more, so the gains have to be balanced against the costs.
Another factor is tighter machining tolerance, which can make the motor’s air gaps smaller to generate higher output torque. But precision machining requires more expensive equipment, and amortizing the cost of such equipment into the motor just makes the motor that more expensive.
A cutaway view clearly shows the geometry of the rotor’s conical magnetic end hubs and the windings surrounding the field poles.
Beating the odds
But one company, NovaTorque, Inc., North Highlands Calif., has found a way to improve efficiency and lower manufacturing costs in the same motor design. The results of this new motor design come from rearranging the magnets and stator in a rectangular flux path. This technique eliminates the stator back-iron normally used to conduct the magnetic field through the outer ring of the motor. The new magnetic path is shorter and has lower losses than any conventional design currently in production. Shorter magnetic circuits mean higher torque output for the same size of motor.
The flux path shown in this cross-section view passes through the stator field poles and the rotor magnets.
Because most motor manufacturers have always stuck with the same traditional stator design formed with complex circular paths, they have never had the benefit of using the type of grain-oriented steel that makes transformers more efficient. While conventional motor laminations are rolled thin so that the bulk magnetizing loss of the steel is minimized, transformer steels are more efficient because their core grain aligns in the rolling direction. This reduces magnetizing losses in the material by 7 to 8%. NovaTorque’s new, “straight-line flux path” stator design takes advantage of the much easier to use transformer lamination steel. This reduces heat loss and makes the stator more efficient, which shows up in higher efficiency over a range of speeds.
There are other benefits: Reduced heating typically extends operating life, or allows higher current carrying capacity for more demanding applications. In keeping with other brushless designs, the stator and its thermal path are closer to the outside of the motor housing for improved cooling. Heat doesn’t get trapped in the rotor as it would on a brush-type dc motor.
The stator windings and geometry for the motor are unique and compact, and all contribute to the motor’s exceptional efficiency rating.
In addition to these major magnetic and electrical improvements, the cost to wind individual, rectangular pole pieces in a straight-line is much less than that to wind complex circular profiles. These segmented pole stators are wound one pole piece at a time and then assembled into a stator ring. Winding a single pole piece is so easy that it can be done on inexpensive “bobbin” winders at high speed and low cost, therefore most of the capital equipment normally employed for electric motor manufacturing is not needed. Consequently, the amortized cost of making a “high performance” servo motor is largely eliminated and much of the process and labor costs connected with motor manufacturing are reduced as well.
The torque vs. speed curve illustrates the motor’s capability for high efficiency over a wide speed range.
Moreover, the segmented pole requires no end-turn copper on the stator that connects one phase to another. This alone can reduce copper usage by another 1/3 or more, depending on the individual motor design. Because end-turn copper contributes no torque in the motor, eliminating it reduces winding losses, improves efficiency, and increases the maximum speed available. In addition, the space required to accommodate the end turn is eliminated so the motor can be made smaller in axial length. Less copper also means a lighter weight motor for a given power output. And in the current economic environment of rising copper costs, eliminating end turns helps keep motor costs down.
Showing how the output power varies with speed and torque can help in selecting the precise operating range needed for a specific application.
Finally, the motor’s total magnet geometry is simple. By making the magnets triangular or conical, the straight pole piece easily makes the corners work to huge advantage. The angle of the air gap can be manipulated to create many different relationships, each of which can be used to take advantage of the characteristics of different magnet materials. The triangular air gap path also generates more torque-producing area than a comparable motor with a simple cylindrical air gap.
Cumulatively, improvements in materials and copper utilization and reduced magnetic circuit length all work together to produce some astounding results. Testing shows the motor is more than 90% efficient over 90% of its speed range. Most electric motors are highly efficient only around a relatively small speed range. A peak efficiency point is not as valuable as a range of high efficiency because the on-time at varying speeds usually produces lower overall efficiency. This improved design can be a big benefit, especially in battery-powered systems and hybrid vehicles where efficiency directly impacts operating time per charge.
The initial development of this motor design was driven by a customer request for a brushless dc motor for a pumping application, but the cost had to compare to that of a stepping motor. After several attempts using conventional motor designs, John Petro and Ken Wasson developed the new stator and rotor configuration, and with Don Burch, the three set out to create a motor manufacturing company that could deliver the highest possible performance at lower costs than have previously been done. NovaTorque’s engineering team has over 100 man-years of experience in the field of electric motors and recently tooled up for serious manufacturing during 2008.
Filed Under: Electronics • electrical, Motion control • motor controls, Motors • ac, Motors • dc, Motors • servo