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An Inside Look at How an Energy Efficient Motor is Built to Reduce Losses & Increase Sustainability

By John Malinowski

October 23, 2013

A motor’s energy efficiency is based on the losses inside the motor during power conversion from electrical to mechanical energy. Better stator and rotor lamination material, more active material (laminations and copper), lower-loss cooling fans, more copper in the stator slot, and increased stack length decrease stator resistance losses and improve sustainability. Replacing inefficient motors with NEM® Premium energy efficiency-labeled designs is a first step toward energy savings, but every component in the entire system should be analyzed--power distribution transformer, smart starter, soft starter, adjustable-speed drive, mechanical power transmission components, and driven load, as well as the motor.

When manufacturing plant owners and facility managers look for ways to reduce energy consumption, they often see lighting as the main opportunity, so it’s the first upgrade they make. In actuality, the percentage of total energy used in a plant for lighting is much smaller than the percentage used to power equipment and machinery.

In industrial settings, motor-driven systems use 60 to 90 percent of the total power. The purchase price of an electric motor is only about 2 percent of the life cost (see Figure 1). Nearly 98 percent of life cost comes from the cost of the electricity the motor uses. To put that in perspective, the purchase price of an electric motor is equal to the cost of electricity to operate that motor continuously for about one month.

Where Losses Occur

Energy efficiency is based on the losses inside the motor during power conversion from electrical to mechanical energy (see Figure 2). The major loss is stator resistance loss (stator I2R), which is the product of the square of the current multiplied by the resistance of the stator winding. The rotor also experiences I2R losses in the squirrel-cage rotor bars, called rotor resistance loss (rotor I2R). Core losses also occur, originating in the lamination steel. Core losses include hysteresis losses, which result from reorientation of the magnetic field within the motor’s lamination steel, and eddy current losses resultingfrom electrical currents produced between laminations due to the presence of a changing magnetic field.

These electrical currents occur in both stator and rotor cores, but primarily in the stator, as these losses are proportional to the frequency of the current. The frequency of current in the rotor bars is only a small fraction of the line frequency, as the rotor current frequency is proportional to slip (the difference between operating speed and synchronous speed). Both the stator and rotor laminations have an insulated coating to reduce shorting losses (eddy current) from adjacent laminations.

Friction losses are from the motor bearings and lubrication. Windage losses combine losses from the rotor spinning in air that creates drag and those from cooling fans used on the motor, along with friction losses in the bearings.

Stray load losses also are present. While these losses are not easily calculated, IEEE 112 method B provides a procedure for determining them based on test data. Typical loss breakdown for a 15-HP induction machine is shown in Figure 3.

What Makes an Energy-efficient Motor Efficient?

While a more energy-efficient motor could be designed to reduce all of these losses, doing so might not be practical or cost-effective. Reducing the stator resistance by using more copper in the stator slot will decrease losses, but the slot fills in today’s motors already have reached the limit of practical manufacturability; there isn’t much room for improvement. Higher-grade lamination steel, or thinner laminations, can be used to reduce core losses. The higher-grade steel often is supplied in thinner laminations to reduce hysteresis and eddy current losses.

Increased stack length (the length of stator and rotor steel) can help reduce flux density and lower losses. Switching from an energy-efficient motor to one labeled by NEMA as a Premium® efficiency motor typically adds about 20 percent more stack length. With the thinner laminations, this adds about 40 percent more laminations, which again affects manufacturability and cost.

Rotor resistance and losses can be reduced, but not without affecting the motor’s performance. Inrush current may increase while the starting torque may decrease, and that’s not desirable. Some engineers add oversized bearings on each end of the motor, but that actually increases losses and lowers efficiency. On the plus side, loss reduction in the stator and rotor leads to fewer losses that need to be dissipated from the motor, so a smaller fan may be used, resulting in reduced windage losses.

As motor manufacturers consider the redesign of motors to raise efficiency, their engineers must balance the trade-offs in performance. Redesign can lead to higher inrush current, reduced starting torque, and less slip. If new laminations are used, the trade-off is that the winding equipment must be redesigned. These types of engineering redesigns can cost many millions of dollars per frame size — costs that usually are passed along to customers.

What’s Next Then?

The U.S. Department of Energy (DOE) currently is conducting a technical study to see if raising the efficiency level of motors above the NEMA Premium level is feasible and justifiable. The DOE continues to work with industry on this study and will announce its proposal in late 2013.

Motor manufacturers are reaching the limitations of physics on increasing the efficiency levels of conventional AC squirrel-cage induction motors. For some ratings, there is room to improve efficiency by about 0.5 percent. Some have proposed the use of die-cast copper rotors to reduce rotor losses and raise efficiency, but copper is a much more expensive material than the aluminum commonly used today, and new lamination designs will be required. Some DOE-sponsored tests have shown that die-cast copper rotors do not produce the gains that would appear feasible on paper. This is because the high heat from casting damages the insulating coating on the rotor laminations, causing additional losses and counteracting some of the anticipated gain. Copper rotors could be produced in high volumes with a fabricated cage using a robot for assembly, but this may be economically feasible only on motors 250 HP and larger.

Permanent magnet (PM) motor designs can yield high energy efficiency, but the rare-earth magnets used in these designs are costly. Research is being conducted on other magnetic materials, but few have the flux levels and temperature resistance-to-demagnetization required. New domestic sources of rare-earth material are expected, and perhaps these will add price pressure to the current Chinese suppliers. However, most of these PM designs require an adjustable-speed drive for operation, as few PM motors are available with line start capability at this time.

Other technologies, such as switched reluctance and synchronous reluctance motors, are being developed, but both require a drive for operation with no across-the-line operation. Currently OEMs seem to be gearing both to specific applications.

Best Approach for Maximum Energy Savings

Replacing inefficient motors with NEMA Premium efficiency designs is a first step toward energy savings, but additional savings can be realized by going beyond replacing one component in an existing system. Every component in the entire system should be analyzed, including the power distribution transformer, smart starter, soft starter, adjustable-speed drive, motor, mechanical power transmission components, and driven load. A system in which a 96 percent-efficient motor is powering a 50 percent-efficient load is not a good design.

System analysis should begin with selection of the most efficient pump, fan, or compressor, using best practices. That higher-efficiency machine will require less horsepower for operation, thus saving energy. Switching from a worm gear speed reducer to a helical or bevel gear system could improve that component’s efficiency from 50 or 60 percent to 95 percent. This also would allow use of a smaller motor, saving considerably more energy than the increase from an energy-efficient to a NEMA Premium efficiency motor design would provide.

Early in 2013 the DOE started a study to set efficiency levels for pumps, fans, and air compressors. This study will continue for the next several years and likely result in efficiency standards for these components by 2020.

Recent surveys seem to indicate the number of installed Premium efficiency motors is in the minority. Less efficient motors are being replaced on failure.

View the original article on Green Manufacturer Here.