Impact of voltage sags and continuous unbalance on variable-speed drives

The use of variable-speed drives (VSDs) is increasing fast. The operation and the output voltage quality of such devices can be significantly affected by power supply voltage sags and continuous unbalance, ultimately influencing the performance of the controlled motor. This paper presents an analysis of the impact of such power supply anomalies on VSDs with 3-phase diode rectifier and PWM voltage-source inverter, which are the most used in the low/medium power range. In VSD applications requiring ride-through capability to voltage sags, the investment in extra DC-bus capacitance can be cost-effective, particularly when the motor driven system stop leads to significant injuries, including loss of production. The extra DC-bus capacitance also improves the output voltage in VSDs fed from unbalanced power supplies.