Improved motor starting capability of three phase UPS inverters

PWM inverters are increasingly being applied to general power system loads, both as uninterruptible power supplies (UPS) and in serving dedicated load from alternative energy sources. The size of motors that can be served from these inverters is limited by the inrush requirements of the motors and the limited overload capability of the inverters. This paper presents two methods to limit motor starting inrush currents within an appropriate range, in order to allow increased motor starting capability without exceeding the inverter overload limits. Both methods work by introducing voltage sags during the motor starting. Comparisons are made between the two methods, and are also compared with the existing method which has no current limitation. Both methods are also compared with acceptable sag and harmonic distortion limits. Conclusions are offered regarding the viability of these methods.