No end in sight for motor efficiency gains

By now, most engineers who work with induction motors know that new efficiency standards went into effect for them last year. The Energy Independence and Security Act (EISA) mandated upgrades to full-load efficiencies, and the Committee of European manufacturers of Electrical Machines and Electronic Power (CEMEP) also developed its efficiency classification for induction motors. EISA mandated full-load efficiency standards that apply to general-purpose three-phase ac industrial motors ranging from 1 to 500 hp. CEMEP developed an efficiency classification for 1.0 to 500 hp motors (0.75 to 375 kW).

But there are ways to improve efficiency levels above even those mandated by the new regulations. For example, many applications use variable frequency drives (VFDs) to adjust motor power based on application requirements, thus saving energy. In a fan application, for example, a VFD may be used to cut the speed on an 1,800 rpm motor during off-peak hours by 20%, which reduces the power consumption by 50%.

Still, connecting a VFD to a motor does not improve the motor efficiency - an 89% efficient motor with a VFD is still, at best, only 89% efficient. Further improvements in the motor system efficiency only come from making the motor itself more efficient.

How efficient is an ac induction motor?

To understand how it's possible to make a motor more efficient, consider the construction of an ordinary ac induction motor. It fundamentally consists of a stator with windings and a “squirrel cage” rotor. The rotor is a cylinder mounted on a shaft. The “squirrel cage” moniker comes from the use of longitudinal conductive bars (usually made of aluminum or copper) set into grooves and connected at both ends by shorting rings to form a hamster-wheel-like shape. The solid core of the rotor is built with stacks of electrical steel laminations. The rotor has a smaller number of slots than the stator. The number of slots must be a non-integral multiple of stator slots so as to prevent magnetic interlocking of rotor and stator teeth when the motor starts. Induction motor efficiency varies by design and power output. All induction motors carry an efficiency rating on their nameplates.

For induction motors, nameplate efficiency is expressed at the motor's rated point. For typical induction motor designs, the motor efficiency peaks at the motor's rated point and declines as the operating speed or load moves away from this point. For example, consider a 3 hp, 89.5% efficient 1,800 rpm motor in a variable torque application. When running at half speed, i.e., 900 rpm, a typical NEMA Premium motor's efficiency drops to less than 80%.

There are several ways to boost the efficiency of an ac induction motor. One is to use better, higher grade electrical steel laminations and coat them with improved oxides. Another is to use thinner laminations, though this approach requires more laminations to get the same output power. And both these approaches are costly.

Efficiency can also be improved by lengthening the lamination slots (where the wire inserts). Efficiency rises because the motor uses more copper wire. But use of lengthier slots results in a longer rotor/stator design. Thus a typical 3 hp 1,800 rpm premium efficient motor can be at least 2 inches longer than the standard design. There are ways to shorten the motor while maintaining high efficiency, but these typically boost the price by 20% to 30% or even more.

There are other approaches that boost motor efficiency, including: use of smaller cooling fans (less friction and windage); use of smaller rotor bearings (less friction, at the expense of a shaft that won't handle heavier loads); and the use of copper bar rotors.

Synchronous motors

For some applications, a synchronous motor may be a possibility. Tests indicate that synchronous motors can be more efficient than induction motors and maintain their efficiency throughout a broader operating range.

The synchronous motor is basically the same as the induction motor but with a special rotor construction that lets it rotate at the same speed, i.e., in synchronization, with the stator field. This contrasts to an induction motor, where the rotor lags/slips behind the rotating stator field. There are basically two types of synchronous motors: self excited (similar to the induction motor) and directly excited (as with permanent magnets).

The self-excited motor, also called a switched reluctance motor, has a solid-steel cast rotor that includes notches or teeth, termed salient poles. The notches let the rotor “lock in” and run at the same speed as the rotating magnetic field. While switched reluctance motors are simple, they are poor performers when driven via line power. To move the rotor from one position to the next, power must be sequentially switched to consecutive stator windings/phases in a manner analogous to that of a stepping motor.

The switched reluctance design operates at a higher power density level compared with a NEMA Premium induction motor handling the same amount of power. This higher density level comes through use of a small rotor air gap and rotor steel carrying high flux levels. Unfortunately the resulting construction can be costly because of the special steel involved, and noisy because the magnetic pulsations tend to cause vibrations.

Large switched reluctance motors are found in commercial washers, dryers, automotive test stands, vehicle traction, carpet yarn machines and other applications in which audible noise may not be of concern. Stepping motors are small versions of synchronous motors which can be found in many applications that involve position control.

The directly excited synchronous motor also may be called an ECPM (electronically commutated permanent magnet), BLDC (brushless dc), or just a brushless permanent magnet motor. Its rotor includes permanent magnets. The magnets may mount on the rotor surface or be inserted within the rotor assembly (in which case the motor is called an interior permanent magnet motor). The permanent magnets are the salient poles of this design and therefore prevent slip. A microprocessor controls sequential switching of power on the stator windings at the proper time using solid-state switches, minimizing torque ripple.

Directly excited motors are commercially available worldwide with a variety of magnet materials (samarium cobalt, neodymium, and ferrite), with the rare-earth magnets being by far the most expensive. But in all cases, because the rotor's magnetic field needn't be induced electrically, as in an induction motor, the permanent magnet motor is inherently more energy efficient.

Synchronous ac motors are used in robotics, machine tools, packaging, handling equipment for the food and beverage industry, and a host of other applications in both small and large volumes.

Recent advances in brushless PM motor technology have resulted in motors that are substantially more efficient over a broad operating range compared with the more ubiquitous ac induction motor designs. One such higher-efficiency design was recently commercialized by NovaTorque. Lab tests of the NovaTorque motor have documented efficiencies far exceeding those mandated by NEMA Premium standards. In the 3 to 5 hp range, the NovaTorque motor hits peak efficiencies exceeding 92% (versus the NEMA Premium 89.5%) and maintains efficiency above 90% over a broad speed and load range.

Several design innovations are responsible for the NovaTorque motor's high performance. The NovaTorque design consists of 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. Notably, the NovaTorque design does not use rare-earth magnets. The mating of the conical geometry with IPM rotors lets the design use less costly and readily available ceramic ferrite magnets. This makes the motor an economical alternative to induction motors.

This innovative motor design also doesn't generate much heat, promoting reliability. 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 sit near the motor housing and create an effective thermal path for heat dissipation from the coils. Additionally, the use of grain-oriented steel in the axial stator reduces eddy current losses, thereby further boosting motor efficiency.

Efficiency saves

Historically, motors have been bought largely based on dollars-per-horsepower figures. Increasingly, however, informed buyers look closely at the true cost of ownership. This more expansive view includes the cost of powering the equipment over its useful life and the opportunity to substantially reduce those costs through more efficient systems. On average, the purchase price of the motor represents only 2% of the cost of owning and powering it. Various installation and maintenance costs represent 0.7%. Energy cost over the life of the motor represent 97.3%.

To illustrate the savings that can accumulate over the life of a product, consider a 3-hp 1,800 rpm motor running a typical fan, blower or compressor. Suppose the motor operates half the time at 100% speed with 100% output and half the time at 50% speed with 12% output. In this case the savings realized by using NovaTorque's brushless permanent magnet motor rather than a NEMA Premium induction motor would be approximately 10%. Even at a modest initial price premium, the payback is often less than a year.

Inside an efficient IPM motor

The field poles used in the NovaTorque stator use conical end-surfaces, instead of the perpendicular flat cross-section found in conventional induction motors. The rotor hubs have mating conical shapes that match the end-surfaces of the field poles. This approach maximizes the surface area available for transmitting magnetic flux, while minimizing the volume of materials. In fact, the rotor/stator surface area interface is twice the perpendicular cross-sectional area of the stator field pole. The larger surface area at the rotor/stator interface concentrates the magnetic flux density. This permits the use of low-cost ferrite magnets to get efficiency and performance that equals or exceeds that of motors that use rare-earth (neodymium) magnets.

Another notable feature of the design is its axial flux path. Flux flows straight (parallel to the shaft) through the axially-oriented field poles of the stator. Grain-oriented transformer-grade steel is used here to lower eddy current losses and improve efficiency.

The axial orientation of the stator field poles also allows the use of bobbin-wound coils. This type of coil is easier to wind and uses less conductor material than the coils in the slots of a typical, radially-oriented motor. This coil arrangement also creates a better thermal path, because one face of the coils is next to the external motor case, instead of being inside the lamination stack as is typical in an induction motor.

The rotor in the NovaTorque motor design consists of a pair of hubs mounted on opposite ends of the motor shaft. The rotor hubs use an interior permanent magnet (IPM) arrangement which provides another source of flux concentration. A further benefit of an IPM design, due to its saliency, is better sensorless motor control.