Φ Abstract — 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. Index Terms — DC-bus capacitance, power quality, PWM, ride-through capability, variable-speed drives, voltage-source inverters, voltage sags, voltage unbalance. I. INTRODUCTION HE use of variable-speed drives (VSDs) to control 3-phase squirrel-cage induction motors is increasing fast. VSDs with 3-phase diode rectifier and PWM voltage-source inverter (VSI), whose basic topology is shown in Fig. 1, are the most used in the low/medium power range (hereafter denoted as VSDs). Such type of VSDs is very sensitive to short interruptions and to voltage sags. They normally trip well within 1 second, sometimes even within 1 cycle. Therefore, even the shortest interruption can cause a loss of the load or the stop of the motor system [1]. Fig. 1. Basic topology of VSDs with 3-phase 6-pulse/full-wave diode rectifier and voltage-source inverter. VSD tripping can occur due to several factors, namely: (a) when the drive controller/protection detects a change in operation conditions and trip the drive to prevent damage to the power electronic components; (b) when a DC-bus voltage drop resulting from a voltage sag causes maloperation or tripping of the drive controller; (c) when the increased input AC currents during a voltage sag or the post-sag overcurrents charging the DC-bus capacitors cause an overcurrent trip or blowing of fuses protecting the power electronics components; F. J. T. E. Ferreira is with the Department of Electrical Engineering, Engineering Institute of Coimbra (ISEC), 3030-199 Coimbra, Portugal, and with the Institute of Systems and Robotics, University of Coimbra (ISR-UC), Polo II, 3030-290 Coimbra, Portugal (phone: +351-967-074- 930; fax: +351-239-406-672; e mail: fernandoferreira@ieee.org). A. T. de Almeida is with the Institute of Systems and Robotics, University of Coimbra (ISR-UC), Polo II, 3030-290 Coimbra, Portugal (e-mail: adealmeida@isr.uc.pt). G. Baoming is with the School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China (e-mail: bm-ge@263.net). Φ (d) when the process driven by the motor will not be able to tolerate the drop in speed or the torque variations due to a voltage sag. After a trip, some modern VSDs restart automatically immediately the moment the voltage comes back, or after a certain delay time. Others require a manual restart. The various automatic restart options are only relevant when the process tolerates a certain level of speed and torque variations. In fact, when VSDs are disconnected from the power supply for several seconds the driven process often becomes so much disrupted that VSD reconnection does not make much sense anymore. Caution has to be taken when changing minimum tolerated DC-bus voltage in the VSD since, when the voltage back to normal values, the overcurrents charging the DC-bus capacitors can be high enough to damage the rectifier diodes. In these situations can be preferable to stop the VSD and restart it with a current limiting resistor in the DC link. Additionally it is preferable to use fast response fuses rather than protection circuit breakers, since the latter devices have, in general, a slower time response, not fast enough to avoid diode damage. Voltage tolerance in most VSDs can be as sensitive as 80-85% of rated voltage for less than six cycles. However it is possible that a large fraction of the VSDs is not sensitive to sags at all. In general, the lower the rated power is, the higher the tolerance to sags will be. In Table I, a summary of a number of experimental data reported in [2] and [3] can be seen. It is possible to conclude that very short interruptions (0% of the input line-to-line voltage during up to 33 ms) can be handled by almost all 2.2-kW VSDs and by a large part of the 15-kW VSDs. However, VSDs can have severe difficulties with sags during 100 ms or more, especially if a slightly decrease in the motor speed means a serious disruption of sensitive mechanical processes. In fact, VSDs trip if the DC-bus voltage crosses a preset lower limit. The trip or malfunction can be due to the inverter controller not operating properly when the voltage gets too low, but it can also be due to the intervention of undervoltage equipment protection connected to the DC bus. Most likely, the protection will intervene before any equipment malfunction occurs. The DC-bus capacitance has only limited energy content (relative to the power consumption of the motor) and will not be able to supply the load much longer than a few cycles. An improved voltage tolerance of VSDs can be achieved by lowering the setting of the undervoltage protection of DC bus. However the protection should trip before any malfunctions occur and before components are damaged. Not only the undervoltage is a potential source of damage itself, but also the consequent overcurrent when the power supply voltage recovers, being the undervoltage protection critical in these situations to avoid damages in the power electronic devices. This paper presents an analysis of the impact of voltage sags and continuous unbalance on VSDs. Impact of Voltage Sags and Continuous Unbalance on Variable-Speed Drives Fernando J. T. E. Ferreira, Anibal T. de Almeida, and Ge Baoming T