An inverter is a device that converts DC (direct current) power into AC (alternating current) power. Its output current’s size and direction are regulated by the input AC power’s voltage and phase. When fed with DC power, the inverter processes it to create an output current displaying various waveform types, thereby transforming DC into AC power.
Pure Sine Wave Inverter find wide application in home solar power systems, especially in conjunction with off-grid solar batteries. The output waveform of an inverter when supplied with AC power is determined by its operational principle. This article provides a comprehensive introduction and comparison of inverter waveforms.
1. Output Principles of Inverter Waveforms
The shape of an inverter’s output waveform is determined by various factors, including the circuit components’ characteristics, parameters, and the working principle of the inverter. The output waveform’s shape is controlled by the PWM (Pulse Width Modulation) converter’s output voltage, processed according to the input AC power signal, to generate a specific inverter waveform shape and frequency.
Moreover, the circuit topology of the inverter greatly influences the resulting output waveform. Different circuit topologies lead to varied inverter waveform shapes.
2. Basic Principles of Pulse Width Modulation (PWM)
PWM is a technique utilizing a digital output from a microprocessor to regulate an analog circuit. Regardless of the inverter waveform shape, the equivalence of the inverter waveform and the time axis ‘t’ results in the same effect (average output voltage). This is known as the principle of area equivalence.
PWM utilizes a rectangular pulse (square wave) form for control purposes. The duty cycle, representing the high-level proportion of the entire cycle, allows adjustments to the output voltage by modifying this cycle’s size. A higher duty cycle results in a larger output voltage, while a lower one diminishes it.
3. Generating Square Wave Alternating Current with an Inverter
Older inverter models predominantly generated square wave AC outputs, suitable for less demanding equipment. By controlling the on and off of semiconductor switches (MOS tubes) within the circuit, square wave AC is produced through a specific sequence of current direction changes.
4. Generating Pure Sine Wave Alternating Current with an Inverter
While square wave output is highly efficient, it might not be compatible with certain appliances. For applications needing smoother AC power, inverters producing pure sine wave alternating current are essential. By adjusting the duty cycle of PWM according to sinusoidal law, inverters generate a waveform resembling a sine wave.
SPWM (Sine Wave Pulse Width Modulation) arranges pulse widths and duty cycles to mimic a sinusoidal pattern. Dedicated integrated circuits, replacing complex analog circuitry, facilitate precise control and output of a perfect sine wave. This approach offers simplicity, efficacy, and high reliability in inverter control, widely used in contemporary systems.
5. Square Wave vs. Rectangular Wave vs. Modified Sine Wave vs. Pure Sine Wave
- Square Wave: Instantly switches between positive and negative voltage levels. It contains only odd harmonics and is widely used in digital circuits and logic control but may cause electromagnetic interference.
- Rectangular Wave: Alternates between two voltage levels within a cycle and finds applications in PWM control and audio signal synthesis.
- Modified Sine Wave: An improvement on square waves, closer to pure sine waves, reducing signal distortion and interference. It’s cost-effective but might not suit sensitive electronic equipment.
- Pure Sine Wave: Smooth and continuous, resembling an ideal sine function. It contains only a fundamental frequency, doesn’t cause interference, and is suitable for sensitive applications.
6. Converting Square Wave to Sine Wave
Several methods convert square waves to sine waves, such as using D/A conversion chips, function generation chips, or Wien bridge oscillation circuits. Additionally, the Fourier transform can decompose square waves into trigonometric functions, while specific circuits like RC filters can be employed to approximate sine waves.