The Use of Active Elements to Reduce the Size and Weight of Passive Components in Adjustable Speed Drives

Ram Krishan Maheshwari

Research output: Book/ReportPh.D. thesis

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Adjustable speed drives are considered as workhorse of industry due to various applications in different kind of industries. According to a market survey, low voltage drives are most profitable in the drive industries. The drive consists of a machine and a converter, which processes grid power and converts into usable power for the machine. The converter consists of active and passive components.
The size, weight, and volume of the drive are decided by the passive components since they typically contribute to 75% of the weight and volume of the drive. Most popular topology for the low voltage drive is a six-pulse diode bridge rectifier followed by a two-level inverter. For these drives, the DC-link capacitor is the major contributor among the passive components.
The DC-link capacitance value can be reduced in the drive systems known as
small DC-link capacitor based drives. However, small DC-link capacitor based drive system shows instability while it is operated with high input line inductance at the operating points with high power. This thesis presents a simple, new active damping technique that can stabilize effectively the drive system at the unstable operating points offering greatly reduced input line current THD. The active damping terms introduced are linked directly to the DC-link voltage ripple, and damping voltage components are added to the input voltage of the drive machine. The stabilizing effect of the active damping terms is demonstrated for an induction machine drive system. To facilitate the design and analysis of the active damping terms, the effects of the active damping terms on the machine current and the DC-link voltage of the drive system are discussed. A design recommendation for the proposed active damping terms is given. Simulation and experimental results verifying the
effectiveness of the new active damping method are presented.
A Neutral-Point-Clamped (NPC) three-level inverter with small DC-link capacitors is selected as viable option for a two-level inverter in this thesis for low voltage applications. This inverter requires zero average neutral-point current for stable neutral-point potential. The small DC-link capacitors may not maintain the capacitor voltage balance even with zero neutral-point current due to nonlinearities present in the circuit. This requires a fast control of the neutral-point voltage. A simple carrier-based modulation strategy is proposed which allows modeling the neutral-point voltage dynamics as a continuous function of the power drawn from the converter. This model shows that the neutral-point current is proportional to the power drawn from the converter, and it enables the use of well established classical control theory for the neutral-point voltage controller design. A simple PI controller is designed for the neutral-point voltage balance based on this model. The design method for optimum performance is discussed. The implementation of the proposed modulation strategy and controller is very simple. The controller is implemented in a 7.5 kW induction machine based drive with only 14 μF DC-link capacitors. A fast and stable performance of the neutral-point voltage controller is achieved and verified by experiments.
The performances of the two-level and three-level inverter based drive systems
with small DC-link capacitors are compared. The shaft voltage, the common mode voltage, the conducted emissions, and the efficiency of the two systems are compared since they are major factors which affect the size of the passive components in the drives. Although the three-level inverter requires higher number of active components as compared to the two-level inverter, it has been found that the three-level inverter is better solution than the two-level inverter even with small DC-link capacitors if the passive components size is concerned.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages119
Publication statusPublished - 2012


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