What is lci drive

The thyristors are commutated by external voltage. On the line side it is the grid voltage and on the motor side it is the machine voltage. This is the reason why LCI can only work with synchronous machine. The thyristors can inherently switch very high current and allow easy series connection.

As the power goes up more thyristors are put in series and the motor nominal voltage increases correspondingly. LCI has therefore ideal scalability in voltage. For motors with low power and high voltage the product is usually not cost competitive, but the higher the power the better for LCI.

The low parts count small number of semiconductors and the simplicity of the circuitry provides superior reliability. On top, the current limiting reactor in the dc link provides the drive the ability of being short circuit proof. The topology is inherently suitable for 4-quadrant operation and allows regenerative braking without extra cost or complexity the direction of current does not change, but the voltages are reversed.

The thyristors are switching relatively slowly 50 or 60 Hz on the line side and usually similar frequency on the motor side and have low switching losses.

High speed drives are feasible with limitation to approx. The converter can be either air cooled or liquid cooled. High power drives with continuous duty are typically liquid cooled to minimize the losses into ambient.

On the other hand as soft starter the converter can achieve very high power with air cooling benefiting from transient thermal behavior.

One drawback of LCI is lower input power factor around 0. This is usually compensated by installing an input filter. This filter has two functions: eliminate absorb current harmonics that would otherwise be injected into the grid and compensate the reactive power so that overall power factor seen from the grid is almost unity 1. The history of VSI started much later when the semiconductors were ready. First medium voltage VSI drives were developed in s and marketed since They all have capacitors in dc link providing them the voltage source character therefore the name VSI.

The most common topology that every main manufacturer has in his portfolio is the neutral point clamped NPC. In low voltage drives 2-level VSI is very common. In medium voltage this would be insufficient to achieve a good output waveform close to a sine wave, i. Therefore, 3-level NPC is used as a standard platform. This means that the semiconductors can be switched on and off by the gate unit. 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. 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. 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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