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Here is a random collection of some relatively common misconceptions about three-phase squirrel cage motor performance characteristics.
Aquí tenemos una colección de algunos de los conceptos errados más comunes acerca de las características de desempeño de los motores eléctricos tipo jaula.
Although the earliest practical DC motor was built by Moritz Jacobi in 1834, it was over the next 40 years that men like Thomas Davenport, Emil Stohrer and George Westinghouse brought DC machines into industrial use. It’s inspiring to realize that work-ing DC motors have been around for over 160 years. For the past century, DC machines over 30 or 40 kW have been cooled in the same manner – by mounting a squirrel cage blower directly over the commutator.
We know that excessive temperature and moisture are the largest contributors to bearing and winding failures. Understanding the source of the increased temperature will help us to correct the problem and improve the machine’s life expectancy.
The most stressful time for electric motors is during starting. At starting, the motor current is the highest it will ever be. This is referred to as starting or locked rotor current. These different terms describe that when the shaft speed is zero, the current is maximum. Once the motor has been successfully started, the load current level is reached and the cooling circuit of the motor is able to dissipate the additional heat produced by the starting current. Restarting the motor before this additional heat has been dissipated means more heat in the form of kW•h will be added on top of that which is there. Each subsequent start before the additional heat has been dissipated will add more heat — raising the temperature until some component in the motor reaches its failure point. This article looks at safe starting practices for electric motors.
Thermistors, derived from the term thermally sensitive resistors, are a very accurate and cost effective method for measuring temperature. Thermistors are usually two-terminal semiconductor devices made from semi-conductor materials that have an electrical resistance that varies non-linearly with temperature. Some materials provide better stability while others have higher resistance ranges and are fabricated into smaller thermistors. Each specificthermistor has its own unique resistance versus temperature characteristic.
Whether an old or new design, lowering temperatures is based on the same principles. I've often commented on how fortunate we are to work on such a variety of electric motor designs. One day, you are working on a new design some designer has recently created, and the next day you are repairing a motor that could be in a museum. It's fascinating to see the different ways engineers have devised to do the same thing, and yet reassuring to see how many things remain unchanged even after a century of electric motors. One aspect of electric motors that could be placed in both categories is the way an electric motor is cooled. This article takes a look at how motors are cooled and how we can improve cooling for some of the special applications we encounter.
We often hear from members that a customer has reported that a motor that has been repaired is now running hot. We always ask how hot and the reply frequently is: "Well, I can't hold my hand on it!" Let's think about that answer for a minute. The typical human can tolerate about 60-65°C (140-150°F) depending on calluses, threshold of pain, how many people are watching, etc. ... Temperature is often the Number 1 enemy of the electric motor. Care must be taken in the design, application and maintenance of these machines to optimize their performance and life. All that said, it is unsafe to lay your hand on a motor to see if it is too hot; get a thermometer instead.
Frecuentemente escuchamos decir a nuestros miembros, que uno de sus clientes le ha informado que un motor que había sido reparado, ahora se calienta. Nosotros siempre les preguntamos ¿Qué tan caliente? y por lo general responden “Bueno, no puedo mantener mi mano sobre él”.
Vamos a pensar un minuto en esta respuesta. La mano del ser humano típico, puede soportar una temperatura entre 60-65°C (140-150°F), dependi-endo de las callosidades, el dolor que pueda tolerar, cuantas personas estén observando, etc. ... la temperatura es el enemigo número 1 de los motores eléctricos. Para optimizar la vida útil y el buen funcionamiento de los motores, se debe tener cuidado con el diseño, la aplicación y el mantenimiento de estas máquinas. Teniendo en cuenta todo lo anterior, no se considera seguro tocar con la mano la superfcie de un motor para saber si está muy caliente. En vez de esto, lo mejor es utilizar un termómetro.
This 40-page booklet provides great advice for obtaining the longest, most efficient and cost-effective operation from general and definite purpose electric motors.
This booklet covers topics such as:
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The Effect of Repair/Rewinding on Premium Efficiency/IE3 Motors
Tests prove Premium Efficiency/IE3 Motors can be rewound without degrading efficiency.
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Recommended Practice for the Repair of Rotating Electrical Apparatus
This is a must-have guide to the repair of rotating electrical machines. Its purpose is to establish recommended practices in each step of the rotating electrical apparatus rewinding and rebuilding processes.
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Revised May 2021
The EASA Technical Manual is the association's definitive and most complete publication. It's available FREE to members in an online format. Members can also download PDFs of the entire manual or individual sections.
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