Figure 8 2 shows the schematic for a current source

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Unformatted text preview: ure 8-2 shows the schematic for a current source inverter for a permanent magnet synchronous motor (PMSM). Note that for a generator, the direction of the current is reversed. Figure 8-2 Schematic for a current source inverter for a permanent magnet synchronous motor Of the voltage source inverters, this class may be further divided into two main groups: pulse width modulated and six-step inverters. The six-step inverter normally uses six solid-state power transistors or gate-turn-off thyristors (GTO’s). The solid-state switches are controlled to produce a six-step voltage waveform for each phase. Changing the conducting time for each of the six switches results in a change in frequency of the output wave. The output waveform resembles a stepped ‘stair case’ thus requiring filtering to reduce harmonics and smoothen the waveform. The pulse width modulated (PWM) inverter accurately controls the on/off times of a high frequency switching component sourced with a constant voltage. This method is called pulse width modulation since the ‘width’ or the ‘on’ duration of the switch is controlled. The switching frequency is constant and depending on the inverter design is typically in the range of 10 –20 kHz. Higher switching frequencies generally result in smaller, hence less costly, inductive components but at the expense of lower switching component efficiency. Figure 8-4 Generation of a pulse width modulated (PWM) waveform (Source: http://deec.ist.utl.pt/~ineit/imdrives/index.html) Voltage source inverters operate from a stable DC voltage (usually stabilised by a capacitor bank) and are those most commonly used in the wind energy industry because of their less complex control circuitry and lower cost. Figure 8-3 Schematic for a voltage source inverter for a permanent magnet synchronous motor ............................................................................................................................................................................... Figure 8-4 illustrates the basis behind generating a PWM waveform by comparing a reference waveform, of the desired frequency, to a linear slope wave such as a saw tooth. When the reference sine wave’s amplitude is greater than the saw tooth wave the semiconductors start conducting creating the PWM wave. Inverters may take their switching signal (reference wave) from an external source such as the grid or the switching may be self-governed. If the switching signal is taken from the grid, the inverter is known generically as line-commutated. Inverters used in stand-alone power systems or autonomous applications, must self-generate the reference waveform, and are termed self-commutated inverters. 8.2 Grid Connection of Asynchronous Generators (Source: http://deec.ist.utl.pt/~ineit/imdrives/index.html) 8-3 Asynchronous generators may be directly or indirectly grid connected. Direct grid connection has the significant advantage of simplicity, reliability and low cost. By connecting the generator ‘across the line’ to 8-4 the grid, the rotor’s rotational speed will be fundamentally governed by the number of poles in the generator and...
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This note was uploaded on 06/09/2011 for the course PV 5053 taught by Professor Aasd during the Three '11 term at University of New South Wales.

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