Modified Sine-Wave Inverter Enhanced

Aug 1, 2006 12:00 PM
By James H. Hahn, Associate Professor Emeritus, University of Missouri-Rolla Engineering Education Center, St. Louis

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With the increasing popularity of alternate power sources, such as solar and wind, the need for static inverters to convert dc energy stored in batteries to conventional ac form has increased substantially. Most use the same basic concept: a dc source of relatively low voltage and reasonably good stability is converted by a high-frequency oscillator and stepup transformer to a dc voltage with magnitude corresponding to the peak of the desired ac voltage. A power stage at the output then generates an ac voltage from the higher-voltage dc. Conceptually, the operation is illustrated in Fig. 1.

Current State of the Art

There are basically two kinds of dc-ac inverters on the market today. One category is the “pure sine-wave” inverter, which produces sine waves with total harmonic distortion (THD) in the range of 3% (-30 dB). These are typically used when there is a need for clean, near-sine-wave outputs for medical, instrument and other critical applications.

Some, for example, are used in boats and RVs as the main source of electricity, and some feed energy back into the utility power grid. Waveforms approaching sine waves, with minimal distortion, are required in any case. These inverters are available in sizes up to several thousand watts and typical costs are in the range of $0.50 per watt (visit invertersrus.com for an example). Early techniques for designing these true sine-wave inverters incorporated significant linear technology, reducing their efficiency and contributing to their higher cost.

More recent designs used pulse-width modulation (PWM) to produce a pulsed waveform that can be filtered relatively easily to achieve a good approximation to a sine wave (for example, see U.S. patent numbers 4,742,441; 4,600,984; 6,980,450; and 4,466,052). The significant advantage of the PWM approach is that switching techniques are used in the power stages, resulting in relatively high efficiency.

However, PWM, with the pulse width made to vary according to the amplitude of a sine wave, requires significant control circuitry and high-speed switching. This is because the frequency of the PWM signal has to be much higher than that of the sine wave to be synthesized if the PWM signal is to be filtered effectively. So the PWM approach introduces significant complexities and switching losses.

The second category consists of relatively inexpensive units, producing modified sine-wave outputs, which could logically be called “modified square waves” instead. They are basically square waves with some dead spots between positive and negative half-cycles. Switching techniques rather than linear circuits are used in the power stage, because switching techniques are more efficient and thus less expensive. These inverters require no high-frequency switching, as the switching takes place at line frequency. Their costs are generally in the range of $0.10 per watt (for an example, see www.invertersrus.com/inverters.html).

The typical modified sine-wave inverter has a waveform as shown in Fig. 2. It is evident that if the waveform is to be considered a sine wave or a modified sine wave, it is a sine wave with significant distortion.

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