ADJUSTABLE SPEED AC DRIVES BEHAVIOR DUE TO VOLTAGE SAG

Many adjustable-speed drives are equally sensitive to voltage sags as process control equipment discussed in the previous section. Tripping of adjustable-speed drives can occur due to several phenomena:

• The drive controller or protection will detect the sudden change in operating conditions and trip the drive to prevent damage to the power electronic components.

• The drop in de bus voltage which results from the sag will cause mal-operation or tripping of the drive controller or of the PWM inverter.
• The increased ac currents during the sag or the post-sag over currents charging the de capacitor will cause an overcurrent trip or blowing of fuses protecting the power electronics components.

• The process driven by the motor will not be able to tolerate the drop in speed or the torque variations due to the sag.

After a trip some drives restart immediately when the voltage comes back; some restart after a certain delay time and others only after a manual restart. The various automatic restart options are only relevant when the process tolerates a certain level of speed and torque variations. The effect of the voltage sag on the de bus voltage is the main cause of equipment tripping.