C. Pulse Width Modulated Inverter Model.
There are several circuit topologies and control methods used
to convert a dc input into a 3-phase ac output. A common circuit-topology
is a voltage-source inverter which is shown in Fig. PWM-1.
The inverter is fed by a dc voltage and has three phase-legs each consisting of two transistors and two diodes (labeled with subscripts a, b,c). A common inverter control method covered in both beginning undergraduate and graduate power electronics or machines courses is sine-triangle pulse width modulation (STPWM) control. With STPWM control, the switches of the inverter are controlled based on a comparion of a sinusoidal control signal and a triangular switching signal. The sinusoidal control waveform establishes the desired fundamental frequency of the inverter output, while the triangular waveform establishes the switching frequency of the inverter. The ratio between the frequencies of the triangle wave and the sinusoid is referred to as the modulation frequency ratio. The switches of the phase legs are controlled based on the following comparison:
A graphical representation of the switch control is shown in Fig. PWM-2. In theory, the switches in each leg are never both on or off simultaneously; therefore, the voltages Vag, Vbg, and Vcg fluctuate between the input voltage (Vdc) and zero. By controlling the switches in this manner, the line-line inverter output voltages are ac, with a fundamental frequency corresponding to the frequency of the sinusoidal control voltage. In most instances the magnitude of the triangle wave is held fixed. The amplitude of the inverter output voltages is therefore controlled by adjusting the amplitude of the sinusoidal control voltages. The ratio of the amplitude of the sinusoidal waveforms relative to the amplitude of the triangle wave is the amplitude modulation ratio. In systems in which the inverter sources inductive loads, the inverter must source power in all four quadrants. The diodes provide paths for current when a transistor is gated on but cannot conduct the polarity of the load current. For example, if the load current is negative at the instant the the upper transistor is gated on, the diode in parallel with the upper transistor will conduct until the load current becomes positive at which time the upper transistor will begin to conduct.
An ACSL/GM model of a sine-triangle PWM controlled, voltage-source inverter
has been created to facilitate undergraduate and graduate education of
both basic and advanced concepts. Fig. PWM-3 shows the Graphic
Modeller simulation of the PWM connected to an inductive load. It
is assumed that all switches are ideal, and thus have zero voltage across
them when on and zero current through them when off.
To model a system in ACSL/GM, the dynamic equations of the system must
be established. In the case of a PWM inverter sourcing an inductive
load, the differential equations describing the system can be expressed
From Fig. PWM-1, using Kirchoffs voltage laws , the phase voltages can be expressed as:
The ACSL/GM model is useful for teaching both basic and advanced STPWM
theory. For basic concepts such as the control method or the
conduction of the switches, the model can be used to generate waveforms
which highlight specific points of interest. For example, plots of
the phase-a current, the switching signal of the upper transistor,
along with the upper transistor and diode currents are shown in Fig.
PWM-4. Therein, it can be seen that if the transistor is gated on
while the load current is negative, the upper diode will conduct current
(lower right) until either the lower transistor is gated on, or the load
current becomes positive, at which time the upper transistor will conduct
(upper right). Through multiple studies, the length of time the diode
or transistor conducts (conduction angle) can be observed as a function
of the power factor of the load. As the power factor increases (more
resistive), the diode-conduction angle will decrease while the transistor
conduction angle will increase.
Another use of the model is to determine the amplitude of the fundamental frequency of the line-line voltage as a function of the amplitude modulation ratio. The determination of such a function requires a means of transforming the simulated time-domain line-line voltage to the frequency domain. Built-in algorithms for performing such transformations, such as an FFT, are not available within the Graphic Modeler run-time environment; however, ACSL provides a direct interface to Matlab or ACSL-Math from the run-time command line. ACSL-Math is a mathematical software package provided with the full version of ACSL/GM. Thus variables are easily loaded into either mathematical package for post processing of simulation results. To illustrate this utility, the STPWM inverter (with Vdc = 350 volts, Rs = 0.5 ohm, and Ls = 70 mH ) was simulated repeatedly with increasing values of the amplitude modulation ratio. The frequency modulation ratio was held constant at mf = 15. The resulting line-line voltages were input to Matlab where an FFT was performed to determine the amplitude of the fundamental frequency (60 Hz) component. A plot of the amplitude of the 60 Hz component versus the amplitude modulation ratio is shown in Fig. PWM-5. Therein it is seen that the line-line voltage increases linearly for ma less than or equal to one. For ma > 1.0 (overmodulation), the line-line voltage continues to increase nonlinearly until eventually any increase in amplitude modulation ratio has no effect on the line-line voltage. Therein STPWM degenerates into square-wave inverter operation.
An important aspect of STPWM design is the minimization of the
system harmonics. The harmonic performance of the system is
readily obtained from the inverter model using the same simulation postprocessing
described above. An example frequency response of the output line-line
voltage of an inverter was calculated using an FFT and is shown in Fig.
PWM-6. By performing repeated studies, comparisons of harmonic performance
can be made with regard to the modulation frequency (synchronous or asynchronous)
or the operating region of the inverter. In addition to providing
design information, such exercises are an excellent means of encouraging
students to use tools such as the FFT in their engineering analysis.
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