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 mercury arc will be extinguished at about 20 percent under-voltage. The lamp life is related inversely to the number of starts. If low-voltage conditions require repeated starting, the lamp life will, therefore, be reduced. Excessively high voltage raises the arc temperature which could damage the glass enclosure if the temperature approaches the glass softening point. Sodium and metal halide lamps have similar characteristics to mercury lamps, although the starting and operating voltages may be different.


The reactive power output of capacitors varies with the square of the impressed voltage. A drop of 10 percent in the supply voltage, therefore, reduces the reactive power output by 19 percent.

Solenoid-Operated Devices

The pull of alternating current solenoids varies approximately as the square of the voltage. In general, solenoids are designed to operate satisfactorily on 10 percent over-voltage and 15 percent under-voltage.

Solid-State Equipment

All solid-state devices are very sensitive to change in voltage and temperatures with respect to time. All solid-state circuits, therefore, incorporate adequate regulation circuitry which then makes the devices very stable.

Original article can be seen here.