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Energy-efficiency Improvement in Electric Motors

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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