Energy Efficient Motors

Description

Energy efficient motors use less electricity, run cooler, and often last longer than NEMA (National Electrical Manufacturers Association) B motors of the same size.

To effectively evaluate the benefits of high efficiency electric motors, we must define "efficiency". For an electric motor, efficiency is the ratio of mechanical power delivered by the motor (output) to the electrical power supplied to the motor (input).

Efficiency = (Mechanical Power Output / Electrical Power Input) x 100%

Thus, a motor that is 85 percent efficient converts 85 percent of the electrical energy input into mechanical energy. The remaining 15 percent of the electrical energy is dissipated as heat, evidenced by a rise in motor temperature. Energy efficient electric motors utilize improved motor design and high quality materials to reduce motor losses, therefore improving motor efficiency. The improved design results in less heat dissipation and reduced noise output.

Most electric motors manufactured prior to 1975 were designed and constructed to meet minimum performance levels as a trade-off for a low purchase price. Efficiency was maintained only at levels high enough to meet the temperature rise restrictions of the particular motor. In 1977, the (NEMA) recommended a procedure for labeling standard three-phase motors with an average nominal efficiency. These efficiencies represent an industry average for a large number of motors of the same design. Table 1 compares the current Standard full load nominal efficiencies for standard and energy efficient motors of various sizes. Note that these efficiencies are averages for three-phase Design B motors. (Design B motors account for 90 percent of all general purpose induction motors. See NEMA Specifications Publication MG-1-1.16 for classifications of induction motors.) Motors of other types (Design A, C, or D) have slightly different efficiencies, while single phase motors have substantially lower efficiencies. Energy efficient motors are only marketed with NEMA B speed-torque characteristics.

TABLE 1
Average Full Load Nominal Efficiencies Standard and Energy Efficient Motors
Rated hp
Standard Motor*
High-Efficiency Motor*
   1.0
75.5
82.6
   1.5
78.1
83.3
   2.0
80.5
83.8
   3.0
81.2
87.7
   5.0
82.8
88.6
   7.5
83.8
89.8
 10.0
85.2
90.1
 15.0
86.8
91.3
 20.0
87.8
91.9
 25.0
88.3
92.8
 30.0
89.1
92.7
 40.0
89.6
93.3
 50.0
90.5
93.8
 60.0
90.6
94.1
 75.0
91.2
94.4
100.0
91.8
94.7
125.0
92.4
95.3
150.0
92.9
95.5
200.0
94.0
95.4
 
*Design B, Four Pole, Three-Phase

Motor efficiency is a factor of a variety of mechanical and electrical imperfections within the motor. Resistance (I2R) losses in the stator windings and rotor bars can constitute up to a 15 percent loss in efficiency in three-phase motors. I2 R losses in single phase fractional horsepower motors may be as high as 30 percent. Magnetization losses in the stator and rotor cores cause about a 1 percent to 7 percent efficiency loss. Friction losses in the bearings and inefficiency in the cooling fans result in 0.5 percent to 1.5 percent loss in motor efficiency. Friction and magnetization losses are independent of motor load and relate solely to motor size and design. The remaining losses are referred to as stray load losses. Severely underloaded motors have lower efficiencies because the friction and windage and core losses remain constant and comprise an increasingly larger percentage of total motor power consumption. The figure below shows the various components of motor losses as a function of motor load.

The construction materials and mechanical and electrical design of a motor dictate its final efficiency. Energy efficient motors utilize high quality materials and employ optimized design to achieve higher efficiencies. Large diameter copper wire in the stator and more aluminum in the rotor reduce resistance losses of the energy efficient motor. Improved rotor configuration and optimized rotor-to-stator air gap result in reduced stray load losses. An optimized cooling fan design provides ample motor cooling with a minimum of windage loss. Thinner and higher quality steel laminations in the rotor and stator core allow the energy efficient motor to operate with substantially lower magnetization losses. High quality bearings result in reduced friction losses.

Cost-Savings Analysis

When considering energy efficient motors, two factors will affect the payback period: power cost and operating hours per year. Where electricity is inexpensive or operating time is low, it may take several years for the savings from installation of high efficiency motors to outweigh the difference in initial cost. On the other hand, where power costs and the operating hours per year are high, it may be possible to replace an existing standard efficiency motor with an energy efficient motor and realize a paycheck of less than one year (Table 2). Furthermore, the economic advantages of energy efficient motors over rewound motors often provide the opportunity for an upgrade to energy efficient motors when old motors burn out.

TABLE 2
Motor Choice Decision Matrix with Example of a 10-HP
AC-Polyphase Induction Motor
 
Standard Motor
High Efficiency Motor
   
A
B
C
 1. First Cost
$180
$224
$252
$279
 2. %Life = Annual Cost
$22.50
$28.00
$31.50
$34.88
 3. Electricity Required (kW)
8.78
8.52
8.43
8.38
 4. Hours Use/Year
4,000
4,000
4,000
4,000
 5. Efficiency
85.0
87.5
88.5
89.0
 6. kWh/Year**
35,120
34,080
33,720
32,520
 7. Cost/kWh (Energy + Demand)
$0.06
$0.06
$0.06
$0.06
 8. Annual Electric Cost
$2,107
$2,045
$2,023
$2,011
 9. Difference in Electricity Costs
-0-
$62.00
$84.00
$96.00
10. Total Annual Cost
$2,130
$2,073
$3,055
$2,046
11. Payback - Years***
-0-
0.71
0.86
1.03
Source: NEMA Publication MG-1.

Factors to Remember When Buying Energy Efficient Motors:

  • Not all high efficiency motors are created equal. NEMA requires that the average nominal efficiency test method be listed on the nameplate. IEWC 34-2 and JEC 37 test methods result in slightly higher efficiencies than the IEEE 112 method. Current NEMA codes require that the motor nameplate carry both the efficiency and test standards.

  • Motors perform best at full load. An underloaded motor, energy efficient or not, is less efficient than a fully loaded motor.

  • Energy efficient motors are most attractive economically when power costs and/or operating hours per year are high.