Motors, Energy Efficiency
Motors and the loads they drive represent some of the largest users of electricity in commercial and institutional facilities. Because motors are such high users of energy, they present a tremendous opportunity for maintenance and engineering managers to reduce energy use and cost through improved motor efficiency.
One such opportunity occurs when managers must make a decision on whether to repair or replace an aging motor. If the determination is to replace the unit, managers have several options that can have a tremendous impact on system operation and overall energy efficiency.
Efficiency On The Rise
Much has happened recently that gives managers tools to improve motor efficiency. The federal government has developed energy standards that manufacturers must meet for the types of motors commonly found in a facility’s energy-using systems. Replacing standard-efficiency motors with high-efficiency motors will reduce the energy requirements for that motor by about 2-8 percent. While that might not seem like a major improvement, depending on the horsepower of the motor and the number of hours it operates annually, the energy savings can be significant.
The development of high-efficiency motors was only the first step. Today, premium-efficiency motors meet even higher energy-efficiency standards and can produce even greater savings. A side benefit for both energy-efficient and premium-efficiency motors is that to meet the energy standards, they require higher-quality components and more exacting manufacturing processes, resulting in a higher quality motor.
Manufacturers have developed reliable electronic controls that can reduce the operating speed of electric motors to match the load requirements for use in applications ranging from small pumps to large centrifugal chillers.
All of these energy-efficiency improvements come at a cost, however. The typical high-efficiency motor typically costs 10-15 percent more than the standard-efficiency motor it replaces. Premium-efficiency motors cost even more.
To help offset this increased cost and provide managers with the incentive to upgrade to more efficient motors, some utilities offer rebates and other incentives that can be as high as $50 per horsepower (hp). Programs are flexible, and many utilities offer technical assistance. Managers can contact their local utility to see if a particular application qualifies for a rebate, as well as procedures to followed to qualify.
Not all motors installed in facilities are candidates for replacement. Some provide a faster payback on the replacement investment than others. Identifying motors to replace and determining the time to replace them requires careful consideration by managers. The most comprehensive means of accomplishing this goal is a motor-management program.
The purpose of such a program is to get the highest rate of return on the motor inventory while minimizing downtime. These programs, which originated with industrial applications, provide managers with a tool to not only maintain their electric motor systems but also to maximize their operating efficiencies.
When examining different motor-management programs, managers should look for one that targets a specific type of facility and maintenance operation. Overly complex programs soon get set aside and ignored. Those that do not have the features needed for a particular application will fall short of expectations.
Comprehensive programs include such tasks as providing training, identifying critical components, implementing a facility-wide maintenance program, and performing ongoing monitoring of motor systems. Most programs are computerized and designed to give managers schedules for maintenance and replacement that meet the needs of their operations.
Programs typically include an inventory of all motors installed in a facility, and they can track motor-related maintenance activities, schedule preventive and predictive maintenance activities, and help managers maintain an inventory of spare parts and motors.
One key element of such a program is the inventory of motors installed in a facility. For each motor installation, technicians need to gather nameplate data, such as the motor’s horsepower, frame size, voltage, and number of phases. They also will need additional information, including the age of the motor, its maintenance history, and the size of the load it is driving.
Next, they will need to test each motor or load combination to determine if the proper horsepower motor has been installed for that particular load. Most motors installed in commercial and institutional facilities are induction motors, which operate at their highest efficiencies at or near their rated horsepower.
Operating these motors under part-load condition results in a significant drop in efficiency. The leading cause of in efficient motor operation is a mismatch between motors and loads.
Once technicians have completed the inventory, managers should divide it into two categories — motors that are candidates for immediate replacement and those that require replacement only when they fail. Candidates for immediate replacement include motors that have reached or passed their rated service lives and are driving critical loads, as well as those significantly oversized for the application and have an annual operating time exceeding 4,000 hours. Managers should leave in place motors that are not badly oversized, operate for less than 4,000 hours per year, and are in good operating condition until they fail or need a major overhaul.
Managers should consider these to be general guidelines, and exceptions will occur. For example, if a failed motor only operates for 2,000 hours per year, is a standard-efficiency motor, and has a cost to repair or rebuild that exceeds 50 percent of the replacement cost for a premium-efficiency motor, it usually is worthwhile to replace the motor rather than repair it.
In all cases, managers should evaluate their options based on the particular application. The amount of money they can save will depend not only on an improvement in operating efficiency but also on local utility rates and the annual number of hours of operation for that particular motor.
Motors Replacement Selection
Most motors made for use in facilities must meet the high efficiency standards established by the National Electrical Manufacturers Association (NEMA). In reality, many motors available before the energy-efficiency standards went into to place already met or exceeded those standards. While those standards were a starting point, some motor manufacturers felt they could achieve even higher levels of efficiency.
To promote higher efficiencies, NEMA developed the premium-efficiency standard for single-speed, polyphase motors of up to 500 horsepower. For example, a 5 hp, high-efficiency motor operating at 75 percent rated load has an operating efficiency of about 88 percent. A premium-efficiency motor of the same rating and load has an operating efficiency of nearly 91 percent.
This increase in operating efficiency might not seem significant, given the number of hours the average motor operates. But this savings typically translates into a paypack of less than three years.
When seeking a premium-efficiency replacement motor, managers must make certain it is certified by NEMA as premium efficiency. Some manufacturers might offer motors that meet the energy-efficiency requirements to be labeled as premium but fail to meet other requirements of the NEMA specification.