Voltage & Frequency Variation

All motors are designed to operate within limited voltage and frequency variations: voltage variation at rated frequency must be within ±10%, and frequency variations at rated voltage must be ±5%. The combined variation of voltage and frequency must be limited to the arithmetic sum of 10%. Variations are expressed as deviation from motor nameplate values, not necessarily system nominal values. The allowable ±10% voltage variation is based upon the assumptions that horsepower will not exceed nameplate rating and that motor temperature may increase. For instance, a 230-Volt motor operating at 207-Volts (90% of rated) loses any service factor indicated on the nameplate, and could run hotter than at rated voltage.

The following conditions are likely to occur with variations in voltage: An increase or decrease in voltage may result in increased heating at rated horsepower load. Under extended operation this may accelerate insulation deterioration and shorten motor life. An increase in voltage will usually result in a noticeable decrease in power factor. Conversely, a decrease in voltage will result in an increase in power factor. Locked-rotor and breakdown torque will be proportional to the square of the voltage. Therefore, a decrease in voltage will result in decreased torque availability. An increase of 10% in voltage will result in a reduction of slip of approximately 17%. A voltage reduction of 10% would increase slip by about 21%.

The following conditions occur with variations in frequency: Frequency greater than rated frequency normally improves power factor but decreases locked-rotor and maximum torque. This condition also increases speed, and the accompanying friction and windage losses. Conversely, a decrease in frequency will usually lower power factor and speed while increasing locked-rotor maximum torque and locked rotor current.