Effects of Voltage Sags on AC Motor Drives

Voltage sags are normally described by magnitude variation and duration. In addition to these quantities, sags are also characterized by unbalance (asymmetry), nonsinusoidal waveshapes, and phase angle shift (phase jump). These factors are important for determining the behavior of ac motor drives during sags. Voltage unbalance and phase angle shifts cause large unbalanced source currents to, excessive voltage ripple in the dc-link, and reduced dc-link average voltage. The response of the motor and drive to these varies considerably. Experimental results clearly show the loaddependent behavior of a typical drive. The ability of the drive to ride-through a voltage sag is dependent upon the energy storage capacity of the dc-link capacitor, the speed and inertia of the load, the power consumed by the load, and the trip point settings of the drive. The control system of the drive has a great impact on the behavior of the drive during the sag and after recovery. The trip point settings of many drives can by field-adjusted and greatly improve many nuisance trips resulting from minor voltage sags.