Energy Saving during Induction Motor Starting under Loaded Conditions

Lightly-loaded induction motors are normally working far from their best efficiency condition, moreover if they are supplied directly from the power network. The direct-on-line starting of induction motors may also cause undesirable high inrush current. The high spike of current can bring about troublesome tripping of protective equipment, an improper start of the motor, as well as voltage dips in the power line. The high torque related to high current may cause undesired abrupt acceleration of the motor which could damage the connected mechanical loads. Reduced voltage has been long proposed to improve efficiency under light load conditions. It has also been adopted to reduce the severity of current surge during motor starting, by increasing the supply voltage gradually. However, under low voltage conditions, the torque generated may not be sufficient to drive the load. An alternative method can be applied by maintaining the flux in the air gap through a reduction of the supply frequency in proportion to the input voltage to avoid machine saturation. This paper is intended for instructional purposes and describes the application of the variable-voltage variable-frequency method during an induction motor starting under loaded conditions. The results show the benefit of applying the supply frequency reduction along with the voltage reduction to improve the low-torque value at low-voltage conditions, which also means energy usage improvement.