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Core
Loss
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"Waster of Energy and Destroyer of Motors"
Not all power applied to an electric motor is converted to work. Principal sources of waste include winding (I²R) loss, windage, friction, stray load loss and loss in stator, rotor and armature cores. Studies have shown that, depending on load, core loss is the first or second leading cause of energy waste in rewound motors, and can account for 25% or more of motor inefficiency.
Lexseco determined that the most important indication of core steel condition is the watts of electrical energy lost per pound (kilogram) of core steel when the core is excited to operating conditions. Common electrical grades of core steel have inherent Epstein test rated watts per pound (kilogram) losses ranging from 1 to 2 watts per pound (kilogram) depending upon application. This value is increased by as much as a factor of 1.5 to 2 once this steel is punched and assembled into motor cores. This increase may be attributed to a combination of the following: Lamination punching or stamping burrs, Lamination thickness, Lamination clamping pressure, Type of insulation coating used on the steel, Heat treatment process used on punched laminations, Lamination assembly method, Silicon content and hardness of the steel, and Heavy welds across the back of the core stacking.
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Watts per pound (kilogram) core losses may be divided into hysteresis and eddy current losses. All cores experience some inherent loss. Increased loss results from physical damage or overheating during use or burnout of old windings. Core loss, dissipated in the form of heat, further degrades the core, causing greater core loss and more heat – a vicious cycle of rising operational inefficiency leading to premature motor failure. In DC armatures, core loss can cause commutator sparking and spotting, impeding motor performance. A significant percentage of motors have core loss exceeding statistical acceptability. Some special types, such as hermetic refrigeration and traction motors, suffer especially high losses. Moreover, government efficiency mandates make detecting sources of energy loss increasingly important. The critical importance of core testing has been acknowledged by preeminent technical authorities, such as the Engineering Committee of the Electrical Apparatus Service Association (EASA). EASA’s Guidelines for Maintaining Motor Efficiency During Rebuilding require motor repairers to "Conduct a stator core test before and after stripping [the winding]." Core Testing reveals repairable problems. Testing before stripping avoids wasting time and money on a core which should be replaced, and verifies that stripping did not damage the core. |
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