Modified Sine Wave Inverters

Modified sine wave inverters

Modified sine wave inverters have a non-square waveform that is a useful approximation of a sine wave for power translation purposes. Most inexpensive consumer power inverters produce a modified sine wave rather than a pure sine wave.

Modified sine wave inverter has a non-square waveform that is a useful approximation of a sine wave for power translation purposes. Most inexpensive consumer power inverters produce a modified sine wave rather than a pure sine wave.

The ratio from on to off time can be adjusted to vary the root mean square voltage while maintaining a constant frequency. The generated gate pulses are given to each switch in accordance with the developed pattern to obtain the desired output. Harmonic spectrum in the output depends on the width of the pulses and the modulation frequency. When operating induction motors, voltage harmonics are usually not of concern. However the harmonic distortion in the current waveform introduces additional heating and can produce pulsating torques.

Numerous items of electric equipment will operate quite well on modified sine wave power inverter devices, especially loads that are resistive in nature such as traditional incandescent light bulbs.

A common modified sine wave inverter topology found in consumer power inverters is as follows:

An on-board micro controller rapidly switches on and off power MOSFETs at high frequency like ~50 kHz. The MOSFETs directly pull from a low voltage DC source (such as a battery). This signal then goes through step-up transformers (generally many smaller transformers are placed in parallel to reduce the overall size of the inverter) to produce a higher voltage signal. The output of the step-up transformers then gets filtered by capacitors to produce a high voltage DC supply. Finally, this DC supply is pulsed with additional power MOSFETs by the microcontroller to produce the final modified sine wave signal.

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