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Induction Motors and Energy Use It is estimated that as much as 60 percent of total power generation capacity of the United States is consumed by electric motors. An EPA study completed in 1996 found that the 1 billion electric motors in the United States consume 1700 billion kWh per year. Of this total consumption, more than 80 percent is utilized by motors greater than 20 horse power; these large motors represent less than 1 percent of all motors in use.1 As energy costs continue to rise, industry has great incentive to reduce operational costs by optimizing energy consumption. Obvious candidates for this optimization are large inductive motor systems, as these are the greatest single segment of total power budget for most industries. Optimization opportunities include carefully matching motor horsepower ratings to the requirements of the job, replacing older, low-efficiency motors with modern high-efficiency designs, and regulating motor operating speeds to control flow or process rates rather than utilizing dampers or diverter valves on constant speed systems. The savings potential of such efforts can be substantial. An optimization program implemented by Minnesota Mining and Manufacturing (3M) involving 1000 electric motor systems resulted in a 41% net reduction in electrical consumption. This equates to 939,400 kWh annually, $77,554.00 in fiscal 2002.2 Another area that can offer substantial savings is insuring that all motors in service are healthy. AC induction motors are subject to many different failure modes. Broken rotor bars, cracked end rings, high resistance joints (bad welds) excessive air gap eccentricity, and shorted stator windings are typical problems, particularly on large motors subjected to multiple start/stop cycles. Motor Basics Electric motors are effectively magnetic devices. As every youngster has discovered, if you have two magnets and hold them next to one another the opposite poles will attract, pulling the magnets together, and like poles will repel, pushing the magnets apart. This basic principle is utilized in all electric motors. In motors, the first magnet is the stator, the second is the rotor. As the names suggest, the stator is the stationery portion of the motor, secured in the exterior enclosure or frame, and the rotor is the central, rotating part. When a rotating magnetic field is created in the either of these elements, and the rotor will be forced to move, keeping the magnetic poles aligned. These magnetic fields are created by passing electrical current through wire coils assembled into the motor. Dependent on the design of the motor the coils may be only on the rotor (the ‘armature’ in DC motors) with permanent magnets on the stator, a combination of stator and rotor windings (DC and AC motors), or a wound stator with an inductive rotor (AC motors). High power AC motors are typically inductive in design. An inductive rotor is constructed by laminating iron disks together to form the central core of the rotor. Slots around the circumference of the core receive conductors, termed ‘rotor bars’ which are shorted together at either end of the rotor. This is accomplished by ‘end rings’ which may be welded to the ends of the rotor bars, or the end rings and rotor bars may be injection molded around the core, effectively forming a single, homogenous structure. When the rotor is assembled inside of the stator, current flow through the stator windings generates corresponding ‘induced’ current flow through nearly aligned, adjacent rotor bars. This current path is completed by the end rings, creating a closed circuit. The induced current flow generates magnetic force in the rotor, compelling it to rotate to keep the stator and rotor fields aligned. The greater the misalignment, the greater the induced current and therefore the higher the electromotive force (emf) and torque generated. CONTINUED....... Click here to download the entire White Paper in PDF format. Click here to download and save the example program files.
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