Line harmonics on systems using reduced DC-link capacitors

As in all industrial sectors, drives for AC motors are constantly pushed for improvement. Drives using a reduced capacitance for DC-link filtering have gather attention in recent years as a way not only to increase the drives life-time but also to improve the line harmonics while, due to the increased computing capabilities, maintaining a reasonably good torque control performance. Several publications indicate that a drive using reduced DClink capacitance improves the line current and voltage harmonics. This article will demonstrate that this is not necessarily true when a large number of drives is used. Systems requiring several drives (such as any normal industrial installation) will not only not see any harmonic benefit but the total harmonic distortion will be worse than the one obtained with standard DC-link capacitors drives. The demonstration will be done through equations, simulation of an industrial installation and experimental results using five 9 [kW] drives running in parallel.