The most important feature to look for in a voltage monitor is sensitivity to unbalanced voltages. Sensitivity to unbalanced voltages is essential for reliable phase loss protection. The results of a side-by-side test conducted by ETL Laboratories in 1989, indicates that unless a voltage monitor is capable of detecting moderately unbalanced voltages, it may not sense phase loss faults when regenerated voltages are present.
Regenerated voltages occur whenever the supply voltage to one phase of a rotating motor is lost. When regenerated voltages are present, the voltages will appear to be only about 8 to 10% unbalanced. However, during phase loss, the current draw in the remaining phases increases to over 150% of normal full load current. This extra current creates the captive heat that causes the winding insulation to fail.
Voltage averaging types of line monitor designs are not designed to detect unbalanced voltages.
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