Q. Basic aspects of electromechanical energy converters?
Whereas detailed differences and particularly challenging problems emerge among various machine types, this section briefly touches on the interrelated problems that are common to all machine types, such as losses and efficiency, ventilation and cooling, machine ratings, magnetic saturation, leakage and harmonic fluxes, and machine applications. Various standards, developed by IEEE (Institute of Electrical and Electronics Engineers), NEMA (National Electrical Manufacturers Association), ANSI (American National Standards Institute), and IEC (International Electrotechnical Commission), deal with machine ratings, insulation and allowable temperature rise, testing methods, losses, and efficiency determination.
Besides the stator and rotor iron-core losses, friction and windage losses (which are generally functions of machine speed, and are usually assumed to be practically constant for small speed variations) are included in no-load rotational losses, which are effectively constant. Besides copper losses (stator and rotor winding I2R losses), stray-load losses (which arise from various causes that are not usually accounted for, and are usually taken to be about 0.5 to 5% of the machine output) are included for the determination of efficiency (= output/input).
Much of the considerable progress made over the years in electric machinery is due to the improvements in the quality and characteristics of steel and insulating materials, as well as to innovative cooling methods. Modern large turbo alternators have direct water cooling (cooling water circulated through hollow passages in their conductors, being in direct contact with the copper conductors) in the stator (and the rotor in a few cases) and hydrogen cooling (with hydrogen under 1 to 5 atmospheres of pressure) in the rotor. With hydrogen under pressure, sealing the bearings appropriately needs particular attention for turbogenerators. For hydroelectric generators, on the other hand, designing the thrust bearings for vertical mounting becomes a prominent issue.