Circuit Solutions for Utilizing Capabilities of a Multilevel Self-Excited Voltage-Source Inverter in Variable Modes of a Variable-Frequency Drive

Abstract

In the conventional circuits of multilevel self-excited voltage-source inverters with clamping diodes, the inverter levels take power supply either from a single uncontrollable rectifier loaded on a string of capacitive dividers or from several uncontrollable rectifiers for each level. A drawback pertinent to the circuits of variable-frequency drives is that during operation in a variable-frequency mode at low rotation speeds, only a part of possible voltage levels is engaged in the operation, which results in a significantly degraded quality of the inverter output network. Under such conditions, an adverse effect of pulse-width modulation on the motor stator windings manifests itself at low rotation speeds. It should be noted that this adverse effect is little less than in case of supplying power to the motor from a usual two-level self-excited voltage-source inverter. A somewhat lesser negative effect is only due to a smaller amplitude of voltage pulses at the inverter output. At the same time, the voltage pulse derivatives still do not differ from those observed in the case of using a two-level voltagesource inverter. The drive motor durability decreases significantly due to parasitic currents flowing between the winding turns and between the winding and motor frame. This negative impact causes accelerated aging of the motor insulation and its premature breakdown. This problem is often solved by designing and using a special frequency-adjustable motor. Such a motor differs from general-purpose motor series in using special high-strength insulation and non-conducting non-metallic bearings. As a result, the electric drive system, which is in itself far from being cheap, becomes significantly more costly due to an expensive motor to be controlled by it. The article presents a relatively simple version of improving the power circuit arrangement for a four-level self-excited voltage-source inverter by using controlled rectifiers instead of uncontrolled ones for supplying power to the inverter levels. This measure makes it possible to obtain a better spectrum of the inverter output voltage. In this case, the total harmonic distortion factor of the line-to-line voltage decreases by almost a factor of 5. The solution can be recommended for use if the electric drive operation at low speeds makes a significant part of the production cycle.