Electric Motor Voltage Imbalance problems

 Failing to take voltage imbalance into consideration when making a motor selection.

Any plant that utilizes a significant number of motors or utilizes large motors, should monitor the voltage unbalances that occur within their plant.  Daily and weekly voltage variations within a plant can exceed acceptable ranges.  Measurements taken at just one point in time can very often me misleading.

Whenever the long-range average of the three phase voltages exceeds the range values in Table 1-8, the system should be considered out of compliance and balance. 

          Table 1-8

Nominal Voltage	Allowable Voltage Range
120 V(Line to Neutral)	114 V to 126 V
240 V (Line to Line)	228 V to 252 V
480 V(Line to Line)	456 V - 504 V

Voltage unbalance occurs when unequal voltages exist on the motor leads.   Voltage unbalance is defined as 100 times the maximum deviation of the line voltage from the average voltage, divided by the average voltage.  NEMA warns that you should not operate a motor with an imbalance exceeding 1%.   Those over 1% will require derating of the installed motor.

An unbalance in phase voltages will also cause the line currents to be out of balance.  These unbalanced currents will then cause torque pulsations, vibrations, increased mechanical stress and overheating.  This all causes a large increase in motor losses and a severe reduction in service life.

For example. An unbalance of only 3.5% can increase motor losses by approximately 20 percent.   Some plants are known to operate with unbalances of close to 5%.  This would indicate a serious problem.  Figure 1-4 displays derating factors.

With a well-designed plant electrical distribution system, the amount of unbalance at the service entrance should be the about the same as at the Motor Control Center.  Keep in mind that the differential voltage drops between the service entrance and the load centers, by single-phase loads, which are not uniformly allocated among the phases and cause the differences in voltage balance by open delta or open wye transformation.

Fig. 1-4