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ELECTRIC MOTORS  209
















        EFFECT OF VOLTAGE VARIATION                           EFFECT OF VOLTAGE UNBALANCE
        ON INDUCTION MOTOR CHARACTERISTICS                    ON MOTOR PERFORMANCE



                                                              When the line voltages applied to a polyphase induction motor are not equal,
                                                              unbalanced currents in the stator windings will result. A small percentage
                                                              voltage unbalance will result in a much larger percentage current unbalance.
                                                              Consequently, the temperature rise of the motor operating at a particular
                                                              load and percentage voltage unbalance will be greater than for the motor
                                                              operating under the same conditions with balanced voltages.
                                                              Should voltages be unbalanced, the rated horsepower of the motor should
                                                              be multiplied by the factor shown in the graph below to reduce the possibility
                                                              of damage to the motor. Operation of the motor at above a 5 percent voltage
                                                              unbalance condition is not recommended.
                                                              Alternating current, polyphase motors normally are designed to operate suc-
                                                              cessfully under running conditions at rated load when the voltage unbalance
                                                              at the motor terminals does not exceed 1 percent. Performance will not nec-
                                                              essarily be the same as when the motor is operating with a balanced voltage
                                                              at the motor terminals.
                                                              MEDIUM MOTOR DERATING FACTOR DUE
                                                              TO UNBALANCED VOLTAGE












        POWER SUPPLY AND MOTOR VOLTAGES
                                                                                  Figure 2
            NOMINAL POWER        MOTOR UTILIZATION
            SYSTEM VOLTAGE,      (NAMEPLATE) VOLTAGE,
            VOLTS                VOLTS
                   120                    115                 Example:  With voltages of 460, 467, and 450, the average is 459, the
                   208                    200                         maximum deviation from the average is 9, and the
                   240                    230
                   480                    460
                   600                    575
                  2400                   2300                 Reference: NEMA Standards MG 1-14.35.
                  4160                   4000
                  6900                   6600
        Reference: NEMA Standards MG-10.


                                                     FIGURE 12-3
                      © 1997, Electrical Apparatus Service Association, Inc., St. Louis, MO. Reprinted with permission.
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