Standard Motor Catalog

Section TR Technical Reference Guide

VOLTAGE AND FREQUENCY VARIATION All motors are designed to operate successfully with limited voltage and frequency variations. However, voltage variation with rated frequency must be limited to ±10% and frequency variation with rated voltage must be limited to ±5%. The combined variation of voltage and frequency must be limited to 10% (subject to the sum of absolute value with frequency variation not exceeding ±5% of rated value). Variations are expressed as deviation form motor nameplate values, not necessarily system nominal values.The allowable ±10% voltage variation is based upon the assumption 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: A. 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 insulation life. B. 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. C. Locked-rotor and breakdown torque will be proportional to the square of the voltage. Therefore, a decrease in voltage will result in a decrease in available torque. D. An increase of 10% in voltage will result in a reduction of slip for approximately 17%. A voltage reduction of 10% would increase slip by about 21%.

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

Figure 2: Performance variations with respect to Frequency Variation

TECHNICAL REFERENCE GUIDE

Figure 1: Performance variations with respect to Voltage Variation.

TR.5

Data subject to change without notice. 03/24 • www.wolongamerica.com