Fluke 438-II/BASIC User Manual - Page 16

438-II Users Manual Derating Screen NEMA has guidelines for the application of induction motors to the characteristics of the power system. The NEMA standard MG 1 2014 recommends derating the permissible motor load if Voltage Unbalance or Voltage Harmonics exist in the power system. Note The Derating screen is not available when Variable Speed Drive is set to YES. Voltage Unbalance When the line voltages applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings will result. A small percentage of voltage unbalance will result in a much larger percentage of current unbalance. Consequently, the temperature rise of the motor that operates at a particular load and percentage voltage unbalance is greater than the motor that operates under the same conditions with balanced voltages. Percent voltage unbalance = 100x (maximum voltage deviation from average voltage)/average voltage If the voltages are unbalanced, the permissible motor load should be derated. The following derating curve is recommended by NEMA for motors with normal starting torque and is implemented in the NEMA Derating screen. 12