Brian DelBarba

Considerations in specifying large-frame motors

Most small motors are built to common dimensional and performance criteria provided in NEMA MG-1, and are readily interchangeable. Up to approximately 250 hp (the actual limit varies with enclosure type and synchronous rpm), NEMA provides the market with horsepower-to-frame assignments. Key concepts Efficiency helps determine whether a motor is repaired or replaced Enclosures are available to meet most ambient needs Bearing selection depends on how the motor is loaded Sections: Efficiency Enclosures Noise Installation Voltage Bearings Protection Sidebars: Motor enclosure acronyms Issues to consider when a motor fails However, when that old 500-hp, medium-voltage motor used to drive the plant's air compressor dies, the plant engineer is left with some difficult and generally expensive decisions to make. As manufacturers become increasingly efficient, new motor prices become more competitive compared to repair costs. Efficiency Since larger motors are often run more hours per year than smaller models, consuming far more power, energy efficiency is important.