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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 energy-efficient 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.

Adjustable speed drives (ASDs) :

Several terms are used in practice to describe a motor system that permits a mechanical load to be driven at variable speeds, including adjustable speed drives (ASDs), variable speed drives (VSDs), adjustable frequency drives (AFDs), and variable frequency drives (VFDs). Generally, the use of different terms is interchangeable. Adjustable-speed drives better match speed to load requirements for motor operations, and therefore ensure that motor energy use is optimized to a given application. As the energy use of motors is approximately proportional to the cube of the flow rate, relatively small reductions in flow, which are proportional to pump speed, already yield significant energy savings. However, this equation applies to dynamic systems only. Systems that solely consist of lifting (static head systems) will accrue no benefits from (but will often actually become more inefficient) ASDs because they are independent of flow rate. Similarly, systems with more static head will accrue fewer benefits than systems that are largely dynamic (friction) systems. More careful calculations must be performed to determine actual benefits, if any, for these systems. Adjustable-speed drive systems are offered by many suppliers and are available worldwide. Researchers have estimated savings achieved with ASDs in a wide array of applications; typical energy savings have been shown to vary between 7% and 60% with estimated simple payback periods ranging from 0.8 to 2.8 years.

Power factor correction :

Power factor is the ratio of working power to apparent power. It measures how effectively electrical power is being used. A high power factor signals efficient utilization of electrical power, while a low power factor indicates poor utilization of electrical power. Inductive loads like transformers, electric motors, and HID lighting may cause a low power factor. The power factor can be corrected by minimizing idling of electric motors (a motor that is turned off consumes no energy), replacing motors with premium-efficient motors, and installing capacitors in the AC circuit to reduce the magnitude of reactive power in the system.

Minimizing voltage unbalances :

A voltage unbalance degrades the performance and shortens the life of three-phase motors. A voltage unbalance causes a current unbalance, which will result in torque pulsations, increased vibration and mechanical stress, increased losses, and motor overheating, which can reduce the life of a motor’s winding insulation. Voltage unbalances may be caused by faulty operation of power factor correction equipment, an unbalanced transformer bank, or an open circuit. A rule of thumb is that the voltage unbalance at the motor terminals should not exceed 1% although even a 1% unbalance will reduce motor efficiency at part load operation. A 2.5% unbalance will reduce motor efficiency at full load operation. By regularly monitoring the voltages at the motor terminal and through regular thermographic inspections of motors, voltage unbalances may be identified. It is also recommended to verify that single-phase loads are uniformly distributed and to install ground fault indicators as required. Another indicator for voltage unbalance is a 120 Hz vibration, which should prompt an immediate check of voltage balance. The typical payback period for voltage controller installation on lightly loaded motors in the U.S. is 2.6 years.

Case studies :

Some case studies of the energy-efficiency improvement opportunities in electric motors in the textile industry are given below.

Downsizing of motors in finishing processes :

EMT has reported that they replaced a 30hp motor with 20hp motor on a rope scouring machine at the finishing department of their plant after the analysis of process requirements. This resulted in 12 MWh/year in electricity savings. The same company conducted another project on downsizing of motors on press machines at the finishing department. This resulted in electricity savings of 33 MWh/year.

Downsizing of motors in spinning processes :

In another plant in India, four 4.5kW under-loaded motors were replaced with 2.2kW motors in bobbin winding machines and soft package machines. The average electricity savings achieved was about 5.5 MWh/year/motor with an average investment cost of US$1000 per motor.

Replacement of old inefficient motors by energy-efficient motors in viscose production plants :

A viscose production plant in India replaced 39 of its old inefficient motors with energy-efficient motors. This resulted in average energy savings of 7 MWh/year/motor. The investment cost of this retrofit was on average US$1500 per motor replaced (the investment cost of motor replacement varies by motor size).

Use of Cogged ‘v’ belts in place of ordinary ‘v’ belts

A textile plant replaced ordinary ‘v’ belts with cogged ‘v’ belts for various machines and achieved an energy savings of 37.5 MWh/year. The investment cost of this measure was low and was about US$260.

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