Electric motors are simply devices that convert electrical energy into mechanical energy. Like all electromechanical equipment, motors consume some extra energy in order to make the conversion. Efficiency is a measure of how much total energy a motor uses in relation to the rated power delivered to the shaft.
A motor's nameplate rating is based on output horsepower, which is fixed for continuous operation at full load. The amount of input power needed to produce rated horsepower will vary from motor to motor, with more-efficient motors requiring less input wattage than less-efficient models to produce the same output. Electrical energy input is measured in watts, while output is given in horsepower. (This convention applies in the USA; output power for motors manufactured in other countries may be stated in watts or kilowatts.) One horsepower is equivalent to 746 watts.
There are several ways to express motor efficiency, but the basic concept and the numerical results are the same. For example:
Efficiency, % = 746 x Horsepower (output) x 100
= Watts (output) x 100
The ratio describes efficiency in terms of what can be observed from outside the motor, but it doesn't say anything about what is going on inside the motor, and it is what's happening inside that makes one motor more or less efficient than another. For example, we can rewrite the equation as:
Losses stands for all the energy "fees" the motor charges in order to make its electrical-to-mechanical energy conversion. Their magnitude varies from motor to motor and can even vary among motors of the same make, type and size. In general, however, older, standard-efficiency motors have higher losses than motors that meet more current energy standards.
NEMA Premium® motors, which become the required minimum efficiency starting in late 2010, will have lower losses still.
Types of Losses
Energy losses in electric motors fall into four categories:
•Magnetic core losses
•Friction and windage losses, and
•Stray load losses.
Power losses and stray load losses appear only when the motor is operating under load. They are therefore more important in terms of energy efficiency than magnetic core losses and friction and windage losses, which are present even under no-load conditions (when the motor is running, of course).
Power losses, also called I2R losses, are the most important of the four categories and can account for more than one-half of a motor’s total losses. Power losses appear as heat generated by resistance to current flowing in the stator windings and rotor conductor bars and end rings.
Stator lossesmake up about 66% of power losses, and it is here that motor manufacturers have achieved significant gains in efficiency. Since increasing the mass of stator windings lowers their electrical resistance (and therefore reduces I2R losses), highly efficient motors typically contain about 20% more copper than standard efficiency models of equivalent size and rating.
A typical NEMA Design B motor showing components that can be modified to increase the motor’s efficiency: (a) Stator windings; (b) Rotor length; (c) conductor bars and end rings; (d) air gap; (e) laminations; (f) bearings; (g) fan.
Rotor losses, another form of power losses, are also called slip losses because they are largely but not entirely dependent on the degree of slip the motor displays. Slip is the difference in rpm between the rotational speed of the magnetic field and the actual rpm of the rotor and shaft at a given load: Rotor losses are reduced by decreasing the degree of slip. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity (see below), and to a lesser extent by increasing the total flux across the air gap between rotor and stator.
Conductivity is an important characteristic of the rotor. Conductor bars in large motors are normally made from high-conductivity copper. Conductor bars in small-to-intermediate size motors, up to about 200 hp, depending on manufacturer, in the form of a die-cast aluminum “squirrel cage” that gives these motors their common name. Increasing the mass of the die-cast bars requires changes in the slots in the rotor laminations, through which the bars are cast, and that changes the rotor’s magnetic structure. Lowering rotor I2R losses in what are typically aluminum alloy squirrel cage motors is therefore not a simple task.
Conductor bars, end plates and fan in a typical squirrel cage motor. The steel rotor laminations have been removed by etching.
Copper has higher electrical conductivity than aluminum, and is an ideal conductor bar material, notwithstanding the fact that it is difficult to die cast. A process to produce die-cast copper rotors has been developed, and motors using this efficient technology are commercially available in several sizes. Tests have shown that these cast-copper-rotor motors exceed even NEMA Premium efficiencies. (There are several motors on the market that exceed NEMA Premium efficiencies utilizing more traditional cast aluminum rotors, but using more material in the stator and controlling other losses.)
Cross-section of a die-cast copper motor rotor. The blue area represents the surface of one of the rotor laminations, through which the copper has been cast.
The fact that high-efficiency motors tend to have less slip (run faster) than standard-efficiency motors must be taken into account in certain applications. For example, energy consumption by centrifugal loads such as fans and rotary compressors is proportional to the cube of rotational speed. If such loads are driven at the higher speed of a low-slip, high-efficiency motor directly replacing a standard motor, energy consumption can actually increase. This situation can sometimes be resolved by lowering rotational speed with a variable-speed drive, gears or pulleys. There are other parameters, such as torque or starting current, that can vary among motors of the same nominal horsepower. It is important to properly engineer the application of any motor to the intended task.
Magnetic core losses arise from hysteresis effects, eddy currents and magnetic saturation, all of which take effect in the steel laminations. Magnetic losses can account for up to 20% of total losses. With proper design, use of better materials and stringent quality control, these losses can be reduced considerably.
The most effective means to reduce hysteresis and saturation losses is to utilize steels containing up to 4% silicon for the laminations in place of lower-cost plain carbon steels. The better magnetic properties offered by silicon steels can reduce core losses by 10% to 25%. Reducing the laminations' thickness also helps: substituting 26-ga or 29-ga steel for the 24-ga steel found in standard-efficiency motors lowers core losses by between 15% and 25%. Lengthening the lamination stack, which reduces the flux density within the stack, also reduces core losses. Eddy current losses can be reduced by ensuring adequate insulation between laminations, thus minimizing the flow of current (and I2R losses) through the stack.
These are photos of three different efficiencies for the same horsepower rating. Top: standard-efficiency pre-1992 motor; lower left: EPAct-level (post 1992) motor; lower right: NEMA Premium-efficiency motor. Notice that the rotor and stator lengthen (and the amount of copper in the motor rises) as efficiency increases. (Courtesy: Toshiba)
Photo of a motor utilizing a cast copper rotor. Such motors typically exceed NEMA Premium efficiencies
Save Energy and Money
The Energy Independence and Security Act of 2007 (P.L. 110-140, usually called “EISA”) will go into effect on December 19, 2010. If you specify or purchase electric motors, the law will affect how you do your job. If you own or manage a business, it will or at least, it can improve your company’s bottom line by cutting your energy costs for years to come. EISA will/can do that because it raises the mandated nominal full-load efficiencies of many types of electric motors in commercial and industrial service. More-efficient motors save money.
Just a few paragraphs in EISA’s 300-plus pages deal with motors, but they’re important. Why? Because motors account for nearly 50 percent of total U.S. energy use and two-thirds of all electrical energy used in industrial settings. Also, and here’s the important statistic: the cost of operating an electric motor can run up to 25 times the motor’s purchase cost per year.
How Will EISA Affect Your Company?
EISA only affects new motor purchases made after December 19, 2010. Existing motors are now, and will be, grandfathered and can be rebuilt as usual.
However, rebuilding is usually a wasteful, costly choice. At best, and that isn’t always the case, it will restore the motor to its original efficiency.
Installing more efficient motors almost always saves enough money to pay off the new motor quickly. There are exceptions: motors that operate only intermittently (low duty cycle) will save less since they only accrue savings while running. Low utility rates (the national industrial average is around $0.07/kWh) will also stretch out the pay-back time. On the other hand, many utilities and some jurisdictions offer rebates or credits to companies that install more efficient motors, a practice that reduces up-front cost and shortens pay-back times considerably.
For all but large motors (which tend to be highly efficient), the replace vs. rewind decision is obvious. Think of a one-year payback as a 100% return on investment and upgrading to a NEMA Premium motor becomes an obvious choice.
Going One Step Farther:
A growing number of commercially available, general-purpose motors now exceed NEMA Premium efficiency standards. Some of these motors are equipped with die-cast copper rotors to reduce I2R losses. Better-than NEMA Premium motors are not specifically addressed by EISA; however, their extra-high efficiency make them wise choices for large energy/cost savings and rapid pay-back. Table 3 lists a few of these motors, chosen because they display at least 10% lower total electrical losses than an average NEMA Premium motor of the same size as defined by MotorMaster+. Readers are urged to consult the DOE software for manufacturers’ names and model numbers.
Whether you choose a NEMA Premium motor or one that exceeds NEMA Premium standards, the data are clear:
•Old, inefficient motors waste hundreds to thousands of dollars per year.
•Higher-efficiency motors yield larger savings and faster pay-backs.
•Develop an energy program that includes motors.
•Evaluate your motor inventory
Let us at ENERGY SAVINGS ENGINEERING help you evaluate and select your best and most energy efficient motor solutions for your new or retrofit installation. You will be surprised at the amounts of energy and moneys that can be saved. Remember that when investing in premium efficiency motors you are also investing in “Up-Time” (vs “Down-Time) on your operations. These motors will not only save you money but will last longer than any other type.