The purchase price of an electric motor is generally thought to end up as a small fraction (2–3%) of the motor’s total lifetime cost, with the majority of costs resulting from the electricity used by the motor. This makes motor efficiency a key parameter for the cost of operation. This one-page reference reviews concepts important to motor efficiency, focusing on three-phase, “squirrel-cage” induction motors. This type of motor is commonly used to drive rotating equipment, such as pumps, blowers, fans and compressors, as well as other mechanical equipment, in the chemical process industries (CPI).
Induction motors
In an induction motor, an iron rotor assembly on a shaft rotates within a stator, a stationary housing containing copper wires. The induced magnetic field of the stator winding induces a current in the rotor. If the stator windings and stator slots are designed correctly, applying alternating current to the stator will generate a rotating magnetic field. When electrical current is applied, the rotating magnetic field turns the motor shaft. Typically, three-phase alternating-current (a.c.) electric power is supplied to the stator so that the three phases are electrically separated from each other by 120 deg. For more information about induction motors, see Ref. 1.
Motor efficiency
The efficiency of an electric motor is a measure of the effectiveness with which the input electric energy is converted to output mechanical energy. The National Electrical Manufacturers Association (NEMA; Arlington, Va.; www.nema.org) defines energy efficiency as the ratio of a motor’s useful power output to its total power input and is usually expressed in percentage (Equation 1) [2].
η = (0.7457 × hp × Load)/Pi (1)
• η = efficiency as operated in %
• hp = nameplate rated horsepower
• Load = output as a % of rated power
• Pi = Three-phase power, kW
Higher efficiency is achieved with improved rotor-stator design and higher-quality materials.
Motor losses
There are several categories of electrical losses, which can reduce the efficiency of the motor (Figure 1). Three are described briefly here.
FIGURE 1. Electric motor efficiency is a ratio of power input to output. Motor designers seek to minimize energy losses, which can result from several sources (Diagram adapted from U.S. Department of Energy)
Ohmic losses. As input power is applied to the motor, the engine components exhibit resistance, which results in electrical loss that dissipates as heat. Combining Ohm’s Law (voltage = current × resistance) with the Power Law (power = voltage × current) shows that the power lost equals product of resistance and current squared. The major component of Ohmic loss (also known as I2R loss) is the stator, but I2R loss also occurs in the rotor bars of the motor. These losses can be reduced by increasing the conductor’s cross-sectional area, improving the motor winding technique, and using materials with higher electrical conductivities.
Mechanical loss. Mechanical losses are the result of friction from the rotating shaft and can occur in motor bearings, for example. Frictional losses depend on the motor’s speed. Maintaining proper bearing lubrication helps reduce frictional losses.
Iron loss. This category includes losses from hysteresis, resulting from reorientation of the magnetic field within the motor’s lamination steel, and eddy current losses resulting from electrical currents produced between laminations due to the presence of a changing magnetic field [3].
Motor efficiency standards
Two main standards address electric motor efficiency and categorize motors into efficiency classes: the International Electrotechnical Commission (IEC; Geneva, Switzerland; www.iec.ch) standard 60034-30-1 [4], used widely in Europe and Asia and elsewhere, and the National Electrical Manufacturers Association (NEMA; Arlington, Va.; www.nema.org) standard MG-1 [5], more common in North America. The IEC and NEMA standards provide for the global harmonization of energy-efficiency classes of electric motors.
For example, when classifying motors according to their efficiency, IEC has four classes: IE1 (standard efficiency); IE2 (high efficiency); IE3 (premium efficiency); and IE4 (super-premium efficiency). IE4 motors are capable of 97% efficiency. In future revisions of the standard, an even higher efficiency class (IE5) is planned.
Premium efficiency motors are particularly cost effective when annual operation exceeds 2,000 hours, utility rates are high, motor repair costs are a significant fraction of the price of a replacement motor, or electric-utility motor rebates or other conservation incentives are available.
References
1. Jenkins, S., Alternating-Current Induction Motors, Chem. Eng., July 2022, p. 20.
2. U.S. Department of Energy, Energy Efficiency and Renewable Energy (EERE) Office, When to Purchase Premium Efficiency Motors, Motor Systems Tip Sheet #1, 2012. https://www.energy.gov/eere/iedo/motor-systems
3. OSWOS, Losses in Electric Motors, Training Video, Berlin, Germany, www.oswos.com
3. International Electrotechnical Commission (IEC), International Standard IEC 60034-30-1, Efficiency Classes of Line-Operated AC motors (IE code), IEC, Geneva, Switzerland, www.iec.ch, 2014.
5. NEMA MG1, https://www.nema.org/standards/view/motors-and-generators