Quantcast Effect of Voltage Variations

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7.2.2 Effect of Voltage Variations. When the voltage at the terminals of utilization
equipment deviates from the value listed on the nameplate of the equipment, the performance
and the operating life of the equipment is affected. The effect may be minor or serious
depending on the characteristics of the equipment and the amount of the voltage deviation from
the nameplate rating. Generally, performance conforms to the utilization voltage limits, but it
may vary for specific components of voltage-sensitive equipment. In addition, closer voltage
control may be required for precise operations. Induction Motors. Motor voltages below the nameplate ratings result in reduced
starting torque, increased full-load temperature rise, and increased load current. Motor voltages
above nameplate ratings result in increased torque, increased starting current, and decreased
power factor. The increased starting torque will increase the accelerating forces on couplings and
driven equipment. Increased starting current causes greater voltage drop in the supply circuit and
increases the voltage dip on lamps and other equipment. In general, voltages slightly above
nameplate ratings have less detrimental effect on motor performance than voltages slightly below
nameplate ratings. Synchronous Motors. Synchronous motors are affected by variations in voltage in
the same manner as induction motors, except that their speed remains constant (unless the
frequency changes). Additionally, their maximum or pullout torque varies directly with their
voltage, if the field voltage remains constant. If the field voltage varies with the line voltage, as
in the case of a static rectifier source, then the maximum or pullout torque varies as the square of
the voltage. Incandescent Lamps. The light output and life of incandescent filament lamps is
critically affected by voltage. The light output decreases with lower voltages but the life of the
lamp increases. The reverse is true for higher voltages. Fluorescent Lamps. Fluorescent lamps, unlike incandescent lamps, operate
satisfactorily over a range of +/- 10 percent of the ballast nameplate voltage rating. Light output
varies approximately in direct proportion to the applied voltage. Thus a one percent increase in
applied voltage will increase the light output by one percent and, conversely, a decrease of one
percent in the applied voltage will reduce the light output by one percent. The life of fluorescent
lamps is affected less by voltage variation than that of incandescent lamps. The voltage-sensitive
component of the fluorescent fixture is the ballast, a small reactor or transformer which supplies
the starting and operating voltages to the lamp and limits the lamp current to design values.
These ballasts may overheat when subjected to above normal voltage and operating temperature. High-Intensity-Discharge Lamps (Mercury, Sodium, and Metal Halide). Mercury
lamps using the conventional unregulated ballast will have a 30 percent decrease in light output
for a 10 percent decrease in terminal voltage. If a constant wattage ballast is used, the decrease
in light output for a 10 percent decrease in terminal voltage will be about 2 percent. The


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