By V Cook
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Extra resources for Analysis of Distance Protection
Rewound motors can exhibit severe efficiency losses, especially if they were rewound more than 15 years ago or have been rewound several times. Rewind losses of 5 percent or more are possible. When should a energy-efficient motor be purchased in lieu of rewinding a failed standard-efficiency motor? This decision is quite complicated as it depends on such variables as the rewind cost, expected rewind loss, energy-efficient motor purchase price, motor horsepower and efficiency, load factor, annual operating hours, electricity price, and simple payback criteria.
2 If the motor is at the end of a long feeder, reconfiguration may be necessary. The system voltage can also be modified by: • Adjusting the transformer tap settings • Installing automatic tap-changing equipment if system loads vary considerably over the course of a day Installing power factor correction capacitors that raise the system voltage while correcting for power factor Over Voltage As the voltage is increased, the magnetizing current increases by an exponential function. 22 At about 10 to 15 percent over voltage both efficiency and • Since motor efficiency and operating life are degraded by voltage variations, only motors with compatible voltage nameplate ratings should be specified for a system.
However, if a motor core has been damaged or the rewind shop is careless, significant losses can occur. • Motors less than 100 hp in size and more than 15 years old (especially previously rewound motors) often have efficiencies significantly lower then current models. It is usually best to replace them. • If the rewind cost exceeds 65 percent of a new energy-efficient motor price, buy the new motor. Increased reliability and efficiency should quickly recover the price premium. 03/kWh or more, and a motor is operated for at least 4,000 hour per year, an energy-efficient motor is a worthwhile 26 investment.