The efficiency of a motor depends on how well it converts electrical power into work. Efficient electric motors have good starting and running performance, and provide a high level of reliability. An electric motor’s efficiency is numerically calculated as the ratio of the output power of the motor shaft over the electrical power required to turn the motor shaft and can be calculated using power values measured in wattage or horsepower.
Optimizing Electric Motor Efficiency
An electric motor's rated efficiency is just one of several factors to consider in the management of electric motor operating costs. It is important that an electric motor be correctly selected for size and application, installed and operated appropriately, and periodically maintained. Incorrect operating voltage, overloading, lack of ventilation, and/or high temperatures can result in overheating which, in turn, typically results in low operation efficiency and/or motor failure.
For optimum efficiency, motors should be sized to operate with a load factor between 65% and 100%. Oversizing results in less efficient motor operation. For example, a motor operating at a 35% load is less efficient than a smaller motor that is matched to the same load.
Comparing the Efficiency of Electric Motors
When comparing 3-phase motor efficiencies, be sure to use a consistent measure of efficiency. Nominal efficiency is best. Nominal efficiency is an average value obtained through standardized testing of a population of motors. Minimum guaranteed efficiency, which is based on nominal efficiency, is slightly lower to take into account typical population variations. Minimum guaranteed efficiency is also less accurate, because the value is rounded. Other efficiency ratings, including apparent and calculated, should not be used. High efficiency 3-phase motors are designated as "Premium Efficiency" motors and are 2 to 4% more efficient than a standard motor.
Single-phase electric motors do not have a high efficiency standard but some manufacturers make and market higher efficiency single-phase motors in the 1/4 to 5 HP range. They can be up to 19% more efficient for a 1/4 HP motor to about 4% for a 5 HP single phase motor.
The initial cost of a motor is only about 1 to 3% of the lifetime cost of owning a motor; up to 96% is energy cost. Therefore spending a few dollars more to purchase a higher efficiency motor will return lower operating costs over the life of the motor.
Variable Speed Drives
Variable speed drives on electric motors can save considerable energy on farms. Known by a variety of names such as VSDs, variable frequency drives (VFDs), or adjustable speed drives, these drives reduce the power to electric motors until the power is needed. Variable speed drives will save energy on systems with varying loads such as a milking vacuum system. A standard motor has two speeds, full power or completely off. VSDs are capable of running at the needed power rating for the given task and adjusting speed to match the conditions. In the case of a milking vacuum pump, the VSD will run to supply the needed amount of pressure for the pump and will increase speed when conditions require. Utilizing VSDs can reduce electricity use by 60 percent sometimes more (See ATTRA publication, Dairy Farm Energy Efficiency).
Calculating the Payback of an Efficient Electric Motor
The U.S. Dept. of Energy offers the following advice on buying an energy-efficient electric motor.
The cost effectiveness of an energy-efficient motor in a specific situation depends on several factors, including motor price, efficiency rating, annual hours of use, energy rates, costs of installation and downtime, your farm’s payback criteria, and the availability of utility rebates.
Energy-efficient motors are cost effective when they operate more than 4000 hours a year, given a 2-year simple payback criterion. For example,with an energy cost of $0.04/kWh, a single point of efficiency gain for a continuously operating 50-hp motor with a 75% load factor saves 4079 kWh, or $163 annually. Thus, an energy-efficient motor that offers four points of efficiency gain can cost up to $1,304 more than a standard model and still meet a 2-year simple payback criterion. A utility rebate program would further enhance the benefits of an energy-efficient motor.
Electric motors are generally a low maintenance piece of equipment but there are a few simple things that can be done to ensure efficient and reliable operation.
- Keep dust and dirt from building up on motor enclosure –built up dirt acts like insulation, causing the motor to run at an elevated temperature. Over time this can cause deterioration of the insulation within the motor and cause it to fail prematurely. Motors operating at a higher temperature run less efficiently.
- Check for air being discharged from motor fans or cooling ports – Clean the openings if air flow is obstructed.
- Lubricate motor bearings if required per manufacturer’s recommendations – Do not over lubrication to prevent excess lubricate from attracting dust and dirt.
- Ensure correct motor shaft alignment – miss alignment with the driven device can reduce motor bearing life.
- Correct belt tension – loose belts will slip, reducing power transfer and shortening belt life; Over-tightened belts increase motor bearing loads and may shorten bearing life. Fans and other devices can be purchased with self-tightening devices to ensure belts are tensioned properly to minimize power loss.
- Replace safety guards to protect yourself, your family members, visitors, employees and livestock.
- Maintaining Irrigation Pumps, Motors, and Engines ATTRA publication includes descriptions and diagrams of recommended installation, checklists for maintenance tasks, and a troubleshooting guide. Each system component is treated separately and maintenance tasks are broken down by how frequently they need to be done.
- Select Energy Efficient Motors, Penn State University
- Dairy Farm Energy Efficiency ATTRA publication describing ways to reduce energy use on dairy farms, including description of VSDs.
- Motor Best Practices - U.S. Dept. of Energy Web site has fact sheets on about all aspects of motor efficiency.
- Motor Decisions Matter Website with resource on making rebuild versus replacement decision about motors.
Contributors to this Article
- Richard Beard, Associate Professor, Extension Agricultural Engineering, Utah State University
- Scott Sanford, Senior Outreach Specialist, University of Wisconsin - Madison