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The motor itself can also be more energy efficient

Brushless PM motor technology more efficient over broad operating range, author says; use of less costly, readily available ferrite magnets

February 01, 2013
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By Shams Shaikh

NEMA Premium efficiency induction motors are considered by many to be the standard for integral horsepower, general-purpose, AC induction motors. While these motors raise the bar for energy efficiency, continuing improvement is both possible and cost-effective. The latest design innovations in motor-driven systems not only further improve equipment efficiency and performance, but bring benefits to bear such as higher power density, lower operating temperatures and longer life.

The U.S. Department of Energy says efficiency improvements can save as much as 60% of energy costs in some applications. Industry and business have gotten the message and want to expend energy efficiently as a means to reduced power bills and carbon emissions.

In the U.S., the Energy Independence and Security Act (EISA) mandates upgrades to full-load efficiencies. EISA mandated full-load efficiency standards that apply to general-purpose three-phase AC industrial 1.0 – 500 horsepower (hp) motors.

Within the European Union, The Committee of European Manufacturers of Electrical Machines and Electronic Power (CEMEP) developed its efficiency classification for 1.0 – 500 hp motors (0.75 – 375 kW).

Today’s installed process-industry equipment base presents many opportunities for additional energy-efficiency efforts. This includes line-powered equipment, such as fans, pumps, compressors, mixers, grinders, conveyors, packaging and labeling equipment, feeders, stackers and converting equipment. It also includes battery-operated mobile equipment such as electric vehicles, lift trucks and golf carts.

In many applications, efficiency can be improved using a variable frequency drive (VFD), which adjusts motor speed delivered based on load-demand requirements, thus saving energy. In a fan application, for example, a VFD can decrease speed on an 1800-rpm motor during off-peak hours by 20%, thereby reducing power usage by 50%.

While a VFD saves energy in many applications, connecting it to a motor doesn’t improve the motor’s efficiency — an 89% efficient motor with a VFD is still, at best, only 89% efficient. To further improve motor systems, a more efficient motor must be used.

How efficient is an AC induction motor?

The simplest and most common motor applied to commercial and industrial applications is the AC induction motor. Construction consists of a stator with windings and a “squirrel-cage” rotor. Induction motor efficiency varies by design and power output. An efficiency rating is provided on the motor’s nameplate.

For induction motors, nameplate efficiency is expressed at the motor’s rated point. And for typical designs, the motor’s efficiency is highest near the motor’s rated point and declines as the operating speed or load moves away from this point. For example, when operating a 3-hp, 89.5% efficient 1800-rpm motor in a variable-torque application at half speed, i.e., 900 rpm, a typical NEMA Premium motor’s efficiency drops to less than 80%.

AC induction motor efficiency can be improved several ways. One is to use better, higher-grade electrical steel laminations along with improved oxide coatings. Another is to use thinner laminations. This, however, leads to more laminations being used to obtain the same output power hp. Both of these approaches are costly.

Efficiency can also be improved by increasing the size of the lamination slots (where the wire is inserted). This allows for more copper wire, resulting in a longer rotor/stator design for a NEMA Premium efficient motor. A typical 3-hp 1800-rpm premium efficient motor can be 2 inches or longer than the prior standard design. Making the motor shorter increases prices at least 20% to 30%.

Considering synchronous motors

For some applications, a synchronous motor may be considered. Some believe these motors produce higher efficiencies than induction motors and maintain efficiency across a broader operating range.

The synchronous motor has a rotor construction that allows it to rotate at the same speed, i.e., in synchronization, with the stator field — as opposed to an induction motor where the rotor lags/slips behind the rotating stator field. Two basic types of synchronous motors are 1) self-excited (similar to the induction motor), and 2) directly excited (as with permanent magnets).

The self-excited motor, also called a switched reluctance motor, includes notches or teeth, termed salient poles, on the rotor. The notches allow the rotor to “lock in” and run at the same speed as the rotating magnetic field.

While the principle of a switched reluctance motor is simple, it is a poor performer when driven via line power. Sequential switching of power on the stator windings/phases results in the rotor moving from one position to the next. For comparable power levels, the switched reluctance design operates at a high power density level, and to achieve this, the steel is operated at high flux levels with a small rotor air gap. Thus, this costly motor can also be noisy.

The directly excited motor (which may be called ECPM, BLDC or brushless PM) has a rotor that includes permanent magnets mounted on the rotor’s surface or inserted within the rotor assembly (interior permanent magnet). The permanent magnets are the salient poles in this design, and therefore prevent slip. A microprocessor controls sequential power switching on the stator windings at the proper time with solid-state devices, minimizing torque ripple. These motors are commercially available worldwide with a variety of magnet materials, including samarium cobalt, neodymium and ferrite, with rare-earth magnets being by far the most expensive.

Because the rotor’s magnetic field does not have to be induced electrically, as in an induction motor, the permanent magnet motor is inherently more energy efficient.

Further possible efficiency improvements

Recent advances in brushless PM motor technology resulted in motors substantially more efficient over a broad operating range when compared to the more ubiquitous AC induction motor. One such higher-efficiency design was recently commercialized by NovaTorque.

Lab testing of the NovaTorque motor validates and documents efficiency results exceeding those mandated by NEMA Premium standards. In the 3-hp to 5-hp range, the NovaTorque motor achieves peak efficiencies exceeding 92% (versus NEMA Premium 89.5%) and maintains efficiency above 90% over a very broad speed and load range.

Design innovations responsible for the motor’s high performance include two conical hubs mounted on the rotor shaft, at opposite ends of the axial stator, which also has a conical end-surface.

The rotor hubs contain an interior permanent magnet (IPM) configuration in which the magnets are both mechanically and adhesively restrained within the hub structure. As mentioned earlier, rare earth magnets are expensive, and in recent years have become scarce. The performance advantages obtained through the mating of the conical geometry with the IPM rotors allows use of less costly and readily available ceramic ferrite magnets. Hence, NovaTorque says it can price the solution as an economical alternative to induction motors.

This motor also delivers higher reliability through cooler operation. The axial design maintains flux flow parallel to the shaft, allowing coils to be bobbin-wound around stator pole pieces. The outer surfaces of the coils are located near the motor housing and create an effective thermal path for heat dissipation from the coils. Additionally, use of grain-oriented steel in the axial stator reduces eddy current losses, thereby further increasing motor efficiency.

Efficiency saves money

Informed buyers look at the true cost of equipment ownership. The opportunity to substantially reduce those costs through more efficient systems can’t be ignored. For the average motor system, it has been said that the purchase cost represents only 2% of the cost of owning and powering that device. Energy costs, on the other hand, can easily take up more than 90% of total costs.

To illustrate possible accumulated savings over the life of a solution, take a 3-hp 1800-rpm motor operating in a typical fan, blower or compressor application. If the motor is operated half the time at 100% speed with 100% output and half the time at 50% speed with 12.5% output, the savings realized by using NovaTorque’s brushless permanent magnet motor, versus a NEMA Premium induction motor would be approximately 10%. Even at a modest initial price premium, the payback on that premium is often less than a year.

The trend toward better matching of motors to loads, increased use of variable frequency drives and moves to higher efficiency induction motors go a long way. Innovations in component efficiency, such as those highlighted here, can drive even higher levels of system efficiency and cost containment.

Shams Shaikh is an applications engineer with NovaTorque, Inc., Fremont, Calif.

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