New and increasingly stringent efficiency standards have made motor efficiency a priority among motor manufacturers and designers of motor-driven systems. While the Energy Independence and Security Act of 2010 mandated upgrades to full-load efficiencies in the U.S., future legislation is under consideration to require higher efficiencies even at partial loads and speeds. As a result, motor technology must continue improving to meet the efficiency standards proposed by energy-governing agencies. In the existing economic climate, however, end users are also seeking more immediate solutions to boost motor system efficiency and decrease power costs.
Drive review and caveat
In some applications, significant energy savings are achieved by using variable frequency drives (VFDs). The VFD allows motor speed to be controlled to match specific operating requirements. In general, the motor is operated at a lower power level to conserve energy. For example, a VFD may be used to reduce the speed on a conveyor or fan motor by 20% during nonpeak hours, which reduces power consumption by 50%.
However, simply applying a VFD to a motor doesn’t improve its efficiency. An 89% efficient motor will, at best, be 89% efficient when operated with a VFD. In fact, system efficiency will decrease slightly, because most VFDs are only 95% to 97% (not 100%) efficient. For most applications, reduced energy consumption at the motor compensates for this drop in system efficiency. However, to further enhance system performance, improvements must be made to the motor itself. A number of strategies exist to boost motor efficiency. Let’s consider an ordinary induction motor.
Beyond nameplate efficiency
All induction motors carry an efficiency rating on their nameplates. The “nameplate efficiency” is defined for the motor’s rated point and varies by design and power output. Most NEMA Premium efficient induction motors provide nameplate efficiencies close to the levels defined under the NEMA MG-1 standard.
Typically, motor efficiency peaks at the motor’s rated point and decreases as operating speed or load diverges from the rated point. For example, a 5-hp NEMA Premium fan motor may be 89.5% efficient when operating at 1,800 rpm. If the fan speed is reduced by half, the motor’s efficiency falls below 80%.
That said, induction-motor efficiency can
be increased with several design elements. One approach is to use thinner laminations in the rotor stack, although this requires more laminations to achieve the same output power. Another method is to use higher-grade electrical steel laminations with improved insulation. Both of these approaches are costly.
Efficiency can also be improved by using more copper wire in the motor. Typically, the length of the rotor and stator are extended, resulting in a longer motor. For example, a traditional 3-hp, 1,800-rpm Premium efficient motor may be at least two inches longer than the standard design. Maintaining the high efficiency with a shorter design is possible, but often increases the motor price by 30% or more.
Other approaches to increasing motor efficiency include using copper instead of steel for rotor construction, employing smaller bearings for less friction (though load capacity decreases), and using smaller cooling fans for less friction.
Considering synchronous motors
Synchronous motors are an alternative to induction motors where higher motor efficiency is required over a broad operating range. For applications in which the motor is frequently operated at speeds and loads diverging from the rated point, the broad high-efficiency band of a synchronous motor provides significant energy savings when compared to a traditional induction motor.
In short, synchronous motors employ a special rotor construction that allows its rotation in synchronization with the stator field. In contrast, the rotor lags or “slips” behind the rotating stator field in a traditional induction motor.
Two types of synchronous motors exist — switched reluctance and permanent magnet. Switched-reluctance types have salient poles in the rotor, formed by notches or “teeth” in a solid-steel cast rotor. These notches let the rotor “lock in” and run at the same speed as the rotating magnetic field. Switched reluctance motors may be smaller than NEMA Premium induction motors capable of comparable output due to a smaller rotor air gap, and higher flux levels carried through the rotor steel. Drawbacks of this design include higher levels of vibration caused by increased magnetic pulsations, and higher cost due to the special rotor steel.
Permanent-magnet motors are also known a
s ECPM (electronically commutated permanent magnet) or brushless dc (BLDC) permanent magnet motors. As the name implies, its rotor includes permanent magnets — either mounted on the rotor surface or inserted within the rotor assembly. For the latter, the motor is called an interior permanent magnet motor.
Permanent magnets perform the same function as salient poles and therefore prevent slip. Because the rotor’s magnetic field is not electrically induced, as in an induction motor, permanent magnet motors are inherently more energy efficient. Rare earth magnets (neodymium, samarium cobalt) provide the highest magnetic flux, allowing for compact permanent magnet motor designs with excellent efficiency levels. However, these rare earth magnets are very expensive.
Advances in brushless permanent magnet motor technology have resulted in motors that are substantially more efficient over a broad operating range than ubiquitous ac induction motor designs, yet are cost-effective.
Calculating lifetime costs
Increasingly, end users are looking at the true cost of ownership of an electric motor, rather than simply considering the dollar-per-hp figure. This more expansive view includes the cost of power consumed by the equipment over its useful life, and factors in the opportunity to significantly reduce those costs through more efficient systems. Typically, the purchase price and maintenance costs represent only 3% of the total cost of owning and powering the motor — while energy costs over the motor life represents the other 97%.
To illustrate the potential savings over the motor’s lifetime, consider a simplified variable-speed application, with a 3-hp, 1,800 rpm motor running a typical conveyor belt or fan. Suppose the motor operates half the time at 100% speed with 100% load, and half the time at 50% speed with 25% load. In this case, the cost of powering the brushless permanent magnet motor described in the sidebar (see “Case in point: Conical motor design”) would be reduced by approximately 10% compared to a NEMA Premium induction motor. The payback period in such cases is often less than one year.
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Case in point: Conical motor design
One higher-efficiency motor eschewing rare-earth magnets boasts efficiencies far exceeding those mandated by NEMA Premium standards. In the 3 to 5-hp range, the motor hits peak efficiencies exceeding 92% (versus the NEMA Premium 89.5%), and maintains efficiencies above 90% over a broad range of speeds and loads.
In this motor, two conical hubs are mounted on the rotor shaft at opposite ends of the axial stator with a complementary conical end-surface. This rotor-stator geometry increases the surface area for magnetic field interaction to beyond that of traditional rotor-stator interfaces. Therefore, the motor is able to utilize readily available ferrite magnets instead of costlier rare-earth magnets. The rotor hubs contain an interior permanent magnet (IPM) configuration, in which the magnets are mechanically restrained within the hub.
This motor, designed and manufactured by NovaTorque Inc., Fremont, Calif., also boosts reliability by minimizing the impact of heat generated within the motor. The motor is powered by a VFD, which produces an ac power signal to drive the motor using a sinusoidal waveform.
The axial design maintains flux flow parallel to the shaft, allowing bobbin-wound coils around stator pole pieces. The coil outer surfaces sit near the motor housing and create an effective thermal path for heat dissipation from the coils. Finally, grain-oriented steel in the axial stator reduces eddy-current losses to further boost motor efficiency.