Energy-efficiency Improvement in Electric Motors

2013-06-10_152620When considering energy-efficiency improvements to a factory’s motor systems, a systems approach incorporating pumps, compressors, and fans must be used in order to attain optimal savings and performance. In the following, considerations with respect to energy use and energy saving opportunities for a motor system are presented and in some cases illustrated by case studies. Pumping, fan and compressed air systems are discussed in addition to the electric motors. Motor management plan : A motor management plan is an essential part of a plant’s energy management strategy. Having a motor management plan in place can help companies realize long-term motor system energy savings and will ensure that motor failures are handled in a quick and cost effective manner. The key elements for a sound motor management plan are :

  1. Creation of a motor survey and tracking program.
  2. Development of guidelines for proactive repair/replace decisions.
  3. Preparation for motor failure by creating a spares inventory.
  4. Development of a purchasing specification.
  5. Development of a repair specification.
  6. Development and implementation of a predictive and preventive maintenance program.

Maintenance : The purposes of motor maintenance are to prolong motor life and to foresee a motor failure. Motor maintenance measures can therefore be categorized as either preventative or predictive. Preventative measures, include voltage imbalance minimization, load consideration, motor alignment, lubrication and motor ventilation. Some of them aim to prevent increased motor temperature which leads to increased winding resistance, shortened motor life, and increased energy consumption. The purpose of predictive motor maintenance is to observe ongoing motor temperature, vibration, and other operating data to identify when it becomes necessary to overhaul or replace a motor before failure occurs. The savings from motor maintenance program could range from 2 to 30% of total motor system energy use.

Energy-efficient motors : Energy-efficient motors reduce energy losses through improved design, better materials, tighter tolerances, and improved manufacturing techniques. With proper installation, energy-efficient motors can also stay cooler, may help reduce facility heating loads, and have higher service factors, longer bearing life, longer insulation life, and less vibration. The choice of installing a premium efficiency motor strongly depends on motor operating conditions and the life cycle costs associated with the investment. In general, premium efficiency motors are most economically attractive when replacing motors with annual operation exceeding 2,000 hours/year. Sometimes, even replacing an operating motor with a premium efficiency model may have a low payback period. It has been found that the upgrade to high-efficiency motors, as compared to motors that achieve the minimum efficiency can have paybacks of less than 15 months for 50 hp motors.

Rewinding of motors : In some cases, it may be cost-effective to rewind an existing energy-efficient motor, instead of purchasing a new motor. As a rule of thumb, when rewinding costs exceed 60% of the costs of a new motor, purchasing the new motor may be a better choice. When repairing or rewinding a motor, it is important to choose a motor service center that follows best practice motor rewinding standards in order to minimize potential efficiency losses. When best rewinding practices are implemented, efficiency losses are typically less than 1%. Software tools are available that can help identify attractive applications of premium efficiency motors based on the specific conditions at a given plant.

Proper motor sizing : It is a persistent myth that oversized motors, especially motors operating below 50% of rated load, are not efficient and should be immediately replaced with appropriately sized energyefficient units. In actuality, several pieces of information are required to complete an accurate assessment of energy savings. They are the load on the motor, the operating efficiency of the motor at that load point, the full-load speed (in revolutions per minute [rpm]) of the motor to be replaced, and the full-load speed of the downsized replacement motor. The efficiency of both standard and energy-efficient motors typically peaks near 75% of full load and is relatively flat down to the 50% load point. Motors in the larger size ranges can operate with reasonably high efficiency at loads down to 25% of rated load. There are two additional trends: larger motors exhibit both higher full- and partial-load efficiency values, and the efficiency decline below the 50% load point occurs more rapidly for the smaller size motors. Software packages are available that can aid in proper motor selection.

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