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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.