Sizing Three-Phase Inverters for Single-Phase Power Applications

Although Hitachi does not offer inverters above 3 hp specifically sized and rated for single-phase operation, single-phase power can be safely used with larger 3-phase rated inverters, provided that care is taken to properly upsize and apply the inverter. As background, for a given power (kW/hp) and voltage, the ratio of current for a single-phase circuit will be 3 (1.732) times that of a three-phase circuit. This means that the input rectifier will see 1.732 times the current of the output devices. When powered by three-phase, these currents are nearly the same. This higher current would destroy the input of the drive if an oversized inverter were not used. Furthermore, full-wave rectified single-phase power has a much higher harmonic content than full-wave rectified three-phase power. This would introduce large ripple into the DC bus of the inverter, potentially causing other malfunctions. Larger size inverters have larger bus capacitors, thus more inherent filtering. So upsizing the drive ameliorates the ripple problem as well.