LCI Drives vs VSI Drives

An LCI drive is an old and different topology to a VSI AFE drive. The LCI drive has a long history and its advantages and limitations are well understood. It is simple and reliable, but requires careful attention to ac power system issues related to harmonic currents and reactive power. It also demands specially designed motors with low reactance, and must work with harmonic heating and air gap torque harmonics.

LCI drives have low input power factor, 0.5-0.92, especially at lower speeds, and often additional power factor correction equipment is required. VSI drives on the other hand operate at close to unity power factor - This additional reactive power demand by an LCI drive usually requires addition of capacitors to support voltage at the supply bus, which also makes it less efficient than a VSI drive.

Modern VSI designs now provide many advantages over their LCI predecessors, including low power system harmonics, low reactive power demand, and low torque pulsations in the motor air gap and shaft.

LCIs still develop considerable input current harmonic current.

VSI AFE's however, operate at unity power factor, or can be modulated to provide VARs to the power system (leading power factor). This characteristic can be used on isolated drives supplied by weak power systems that may require voltage support. The reactive power can be generated without external capacitors and is controllable within the rating of the source converter.

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