Modified Sine Wave Inverters for Easy Use


Inverters are devices that are designed to do one task, i.e. switch DC to AC. This switching is done with the help of electrical components or a combination of electrical and mechanical components in an Inverter. Its origins can be traced back to the late 1800s when engineers and scientists soon realized that Alternating Current was much more advantageous for use and that it was leaps and bounds ahead of Direct Current in terms of efficiency. The most basic method of conversion is the square wave method; inverters of this kind simply draw power from the DC source and step it up using a boost converter and then using a line frequency transformer to convert the DC wave into an AC wave output.

Modified Sine Wave

The most common types of inverters in the world are the sine wave and modified sine wave inverter.  A modified sine wave whose output is the sum of two square waves, one the waves happens to be phase shifted 90 degrees relative to the other. The resulting waveform is of three equal levels, which are zero, peak positive, zero, peak negative and then zero volts. The sequence is repeated and the resulting waveform closely resembles that of a sine waveform. Modern inexpensive inverters use a modified sine wave over a pure sine wave.

The waveform also resembles a square wave but with a pause during the polarity reversal. For peak positive, negative and zero volts a switching state is developed to produce the wave. The RMS voltage ratio does not maintain a similar relationship as that of a sine wave. In order to achieve the similarity a method called Pulse Width Modulation (PWM) is used. The generated gate pulses are given to each switch in accordance with the developed pattern to obtain the output.

The Harmonics are usually of no concern when used in an induction motor; however, harmonic distortion in the current waveform introduces unwanted heating and can result in the production of pulsating torque.  Traditionally resistive loads such as an incandescent light bulb operate well on a modified sine wave. Items with a Switch Mode Power Supply (SMPS) operate without any problems. Problems arise when the item has a mains transformer which causes overheating depending on its marginal rating.

Most AC motors run with an efficiency reduction of about 20% due to the harmonics, which can be easily filtered using a series LC filter. Majority of inverters used today utilizes a MOSFET, which is rapidly switched on and off at a high frequency like 50Hz using a microcontroller. The process involves stepping up the DC power source using multiple mini transformers and then filtered by capacitors to produce a high voltage DC; which is then pulsed with additional MOSFETs to produce the final modified sine wave.  

This very simple device can be purchased online, all one needs to do is search for inverter online India, and a host of websites that sell inverters will pop up. The internet has now enabled us to easily find the best inverter online, at the click of a button.