Inventing the Future-Proof Motor
New ultra-efficient EC ferrite permanent magnet motors leapfrog NEMA Premium standards.
There is a tipping point when an established technology hits the wall of its technical limitations. With the imposition of NEMA Premium (IE3-level) efficiency standards, AC induction motors in the 3 to 20 HP range have reached the point of looming obsolescence. As energy-efficiency regulations tighten globally, there is little or no room to find further energy efficiency from these machines, given the power loss inherent in their core design. After decades as the workhorse motors for HVAC and other air-moving industries, companies are now having to design new equipment around motors that are no longer up for new jobs, given their inefficiency at different rated speeds, excessive heat and lower power density, and their relative size, weight and bulk.
The rise of permanent magnet motors
Since its emergence in the 1960s, permanent magnet (PM) motor technology has already far surpassed the energy efficiency and operational flexibility of AC induction motors. And as PM motor power output has increased and its footprint grows smaller, they have been implemented in a rising number of applications.
The key to increased efficiency is the permanent magnets attached to or imbedded in the motor rotor. In operation, the magnets couple the rotor with the magnetic field generated by the current being sent to the stator. This “piggyback” magnetic field produces significantly more power and torque than an induction engine using the same amount of current. The result: a variable-speed motor with far greater energy efficiency and a new level of performance and flexibility, providing a higher energy density, broader and variable RPM ranges while generating considerably less heat.
Over the past decade, engineers have developed new materials and motor geometries to further increase the output and energy efficiency in heavier-duty motors. Now they have further surpassed the efficiency and torque of induction with a new machine: the ultra-efficient, electronically commutated (EC) permanent ferrous magnet motor. In particular, a new conical air gap applied to axial flux motors has combined the efficiency improvements resulting from their axial flux geometry with the improved flux concentration of the conical air gap. The result is a high-efficiency motor which can effectively utilize ferrite magnets and provide the magnetic field strength needed to support the higher output integral to horsepower motors.
A new type of magnet — and a new rotor and stator geometry
Over the course of the last decade, engineers and metallurgists have contributed to the development of ferrite-based magnet alloys and composites that could match the high magnetic anisotropy, i.e., attraction and resistance power, of rare earth magnets, using readily available base metals at a fraction of the price of rare earth magnets. The first ferrite EC permanent motors, primarily rated at less than 1 HP, focused on residential HVAC applications. Today, there is an installed base of several million of these lower-power but higher-efficiency units at work. As the strength of ferrite magnets advanced, the next step was to develop larger ferrite EC motors for the commercial market that could significantly surpass the output and energy efficiency of AC induction motors of the same power and at a competitive cost.
The answer lay in the motor geometry. An axial, rather than radial design, would allow a motor to have a very compact bobbin-type electrical winding with no end turns, so the wound conductor in the motor would be used far more effectively, with a lower winding resistance and lower resistive (I²R) losses, especially at low speeds, where resistive losses soar in induction motors.
A new conical rotor/stator geometry was introduced to amplify the magnetic flux created by the newer ferrite permanent magnets on the rotor. Twin conical-shaped rotor hubs maximized the surface area and air gap mating of the stator and rotor, focusing and concentrating the power of the magnetic flux for greater output and efficiency and cooler operation. The axial design and conical geometry enabled the use of grain-oriented steel for the stator poles. This made the flux transmission through the field poles far more efficient than with other motor designs; the core losses were as much as 75% lower than in a radial motor.
The design also reduced the amount of expensive copper in the motor windings as compared to that of induction and other PM motors, which used cylindrical air gaps, sharply decreasing energy losses and excessive heat from the core.
Baseline: Optimum energy efficiency and application flexibility
The latest generation of these new, electronically commutated ferrite permanent magnet motors was introduced in 2017. In the lab and prototypes, the combination of enhanced ferrite magnets and the conical design had been tested across a wide range of application criteria.
The key benchmark was energy efficiency. With a 5-20% reduction in electricity costs compared to the most efficient NEMA Premium induction motors—and 40 percent or more versus conventional induction motors with the same horsepower—the motors are estimated to deliver payback in 18 months or less in applications such as HVAC. Particularly striking was the motor’s ability to maintain this level of efficiency—92-95%—over the broadest operating range, from 600–3,600 RMP, regardless of motor horsepower: a level of consistent performance unmatched by either AC induction or many comparable rare earth permanent magnet motors. This opens the door for multiple new commercial applications.
Consistent efficiency at any speed makes the EC ferrite PM motor ideal for variable-speed applications. Compatible with readily available variable frequency drives (VFDs) from most leading manufacturers, the motors can be rated to exactly the speed a manufacturer specifies. Rather than adapting new equipment to accommodate the capabilities of an induction motor, an OEM can adapt the motor and optimize its speed for the desired performance of the new equipment.
Choose any speed: Customizing motors to optimize equipment performance
Examples of the value of this across-the-board efficiency at variable speeds and the level of customization it makes possible are found in HVAC and fan manufacturing, data centers and pumping operations.
Several fan manufacturers have turned to plenum fan blades at different inclinations designed to increase air flow with less energy. Many efficient fan designs are most effective at motor speeds between 1,800–3,000 RPM, with an energy sweet spot of 2,400 RPM. However, AC induction motors within this range are limited by their pole count and line voltage to just two speeds: 1,800 or 3,600 RPM. Running the fan at the optimal speed requires either overdriving a four-pole, 1,800 RPM motor, or under-driving a two-pole, 3,600 RPM motor. In either case, this results in a significant loss in motor efficiency. New EC ferrite PM motors are available with a standard nameplate speed of 2,400 RPM, specifically designed to support the new plenum fan designs.
Industrial fans and fan banks often require retrofits as motors age. The motors are packaged in standard NEMA frame sizes and mounting dimensions, enabling simple substitution in retrofit operations. EC ferrite motors operated with variable-speed drives provide a huge increase in efficiency.
Data center HVAC systems often require motors to operate at low static pressures, and thus need high-torque, low-speed motors, operating at 600 RPM (or even lower). Induction motors have very low efficiency at these slower speeds, resulting in higher operating costs. The induction motor solution is a higher-power, higher-speed motor: typically a six-pole, 1,200 RPM-rated motor to support the requirement of high torque at low speed. The EC ferrite motor solution is a high-efficiency motor specifically designed to support lower speeds. These motors will have a much lower current requirement than the induction motor solution, which enables the use of a smaller drive, and a correspondingly lower load on the overall electrical infrastructure. This can result not only in lower operating costs, but a lower overall system installed cost. This means a more energy-efficient data center is possible with lower capex—a huge benefit over HVAC systems with AC induction motors.
NEMA Premium standard is the highest possible efficiency threshold of AC induction motors in the 180 and 210 frame sizes. But the IE3 standards are moving up to IE4, driven by increasingly stringent regulatory requirements and manufacturers’ demands for lower energy costs and higher performance. AC induction motors cannot meet that standard without adding materials, resulting in significant cost increases. Ultra-efficient EC ferrite PM motors already do. In fact, they also already exceed the proposed efficiency standards for the IE5 standards in development. As a result, OEM equipment designed to incorporate an EC ferrite PM motor today will remain energy-compliant for years to come.
As these motors come online, initially in 180 and 210 frame sizes in 2017 and 250 in 2018, operators will continue to apply ferrite PM technology to larger, higher-horsepower motors for heavier-duty applications. The technology’s efficiency at variable speeds and its ability to provide continuous torque (especially at very low speeds) make it an optimum solution for energy-demanding applications such as belt conveyors, escalators and moving walkways, food and industrial processing equipment, and high-powered inertia servos.
With higher efficiency and lower costs, EC ferrite permanent magnet motors are clearing a direct path to the future of electric motors.