Motor Efficiency

Electric motors can be a significant source of electrical energy use for medium to large commercial or industrial users.  Motors are typically used to run fans, pumps or process machinery.  Significant savings can be achieved by upgrading motors in a facility. 

Usually larger motors with long runtimes are the best candidates for energy efficiency.  The most efficient motors on the market today are designated as "NEMA Premium".  In general, it is best to purchase a new, NEMA premium motor when installing a new motor or instead of rewinding an existing motor. It can also be cost effective to replace existing motors that have long runtimes and low efficiency.  Even if an efficiency gain between two motors is only a few percent, if the motor has long runtimes, then the energy savings realized from a small gain in efficiency can pay off relatively quickly.

What Is Motor Efficiency?
Efficiency is the ability of a motor to convert electrical input to mechanical work. A motor’s efficiency is usually expressed as a percentage, such as 91.8%.

Areas of Efficiency Loss

• Mechanical Losses such as friction or windage, (air resistance caused by rotation of fans or rotors) occurs in the bearings, fans, and brushes, and are independent of the load put on the motor.

• Iron Losses caused by the constant reorientation of the magnetic field within the core of stator magnet and rotor, result in small electrical currents which produce heat and do not contribute to power output.

• Stray Load Losses resulting from various minor loss sources such as non-uniform current distribution in the stator and rotor conductors, air gap, and leakage flux.

• Electrical Losses occur as electrical currents pass through the windings of the motor. The largest sources of this type of loss can be found at the stator and rotor.

Methods of Improving Efficiency

• Copper and Iron: Increased amounts of copper and iron within the rotor and stator help with the effects of heat and reduce resistance losses. A premium efficiency motor uses purer copper as well as up to 20% to 60% more in its construction.

• Improved Materials: High quality steel and thinner laminations use less additives and reduce core losses improving overall motor efficiency.

• Motor Design: Increased rotor and stator length helps maintain the balance of slip and stray load losses. Stagger-Stacked laminations help to improve the displacement of heat producing cooler functioning motors. Larger, higher quality ball bearings can reduce friction losses. More exacting manufacturing processes also reduce losses.

Savings Analysis
A savings analysis starts by determining how much energy is saved by changing an existing motor with a new one, then considering the price of the new motor, disposal fees, and so on. A simplified equation to determine energy savings is:

Savings = A x B x 0.746 x C x D x [100/E – 100/F]

Where:

A = Motor Hp
B = Load Factor (% of load/100)
C = Operating Hours (annually)
D = Energy Rate ($/kWh)
E = Current Motor Efficiency Rating (%)
F = New Motor Efficiency Rating (%)

Simple Payback for a motor is the amount of time required for the resulting energy savings in dollars to pay off the initial investment of purchasing a premium motor. Paybacks for most applications happen within two or three years, depending on the size of the motor. The amount of time for a motor to pay for itself directly relates to the number of hours that it operates: longer hours of operation equal faster payback. See the table for example savings and paybacks for several motor sizes.

* Calculations prepared using MotorMaster+4.0, and assume average efficiencies and average purchase (35% dealer discount) and rebuilding costs for standard and NEMA premium motors, 6,000 h/y operation at 75% of full load, $0.129/kWh utility rate.

Replace vs. Repair?
In general, it is economically more beneficial to repair larger horsepower motors rather than replace them. If the repair costs are more than 50% of the cost of the new motor, replacement is recommended. Based on improvements in motor stock and technological trends over time, replacing the existing motor with a new motor will increase energy efficiency.

Timing: Depending on the duty and the availability of parts or replacement motor, replacement or repair could essentially take the same amount of time. Assuming that parts are available, the repair turn around time on a 50 hp motor is approximately 3 working days.

Reliability: Although a quality rewind may maintain the efficiency of a motor, standard motors also create more heat than premium efficiency motors, causing for the propensity for further failure. Motor failures occur most often immediately after installation, declining as the motor is broken in, and then reoccur as the motor reaches old age.

RCEA can assist in obtaining rebates for motor efficiency projects. Call our Small Business Direct Install office for more information at (707) 269-1700.

Adjustable Speed Drives
ASDs allow a motor to operate at different speeds with about the same performance as its base, or optimal, speed. ASDs offer two economic benefits: enhanced production process control and increased savings by matching motor speed with load. ASDs can be designed into new motor systems or can be installed during system retrofits.

Useful links:

• Introduction to Premium Efficiency Motors: basic information on motor efficiency, design, and energy calculations, courtesy of the Copper Development Association.
• Motor Decisions Matter: a national public-awareness website sponsored by the U.S. Department of Energy and industry groups.
• MotorMaster+: a free online motor selection and management tool to assist in the adoption of efficient motors. Provided by the U.S. Department of Energy. 
• Motor Fact Sheet: an informational flyer provided by PG&E (PDF 205 KB)