Voltage Source Inverters

Voltage source inverters convert a stiff DC supply into an AC voltage with variable magnitude and frequency. They are heavily utilized in UPS systems and AC motor drives.

Core Concept

The single phase full bridge inverter uses four switches to alternate the load polarity. When S1 and S2 conduct, the load sees positive DC bus voltage. When S3 and S4 conduct, the load sees negative DC bus voltage.

Shoot through faults occur if both switches in a single leg turn ON simultaneously. A dead time of 2 microseconds is inserted between switching transitions. This guarantees safe operation of the 1200 V IGBT modules.

Square wave operation produces large low order harmonics. High frequency switching techniques eliminate these harmonics. An output LC filter smooths the voltage for the connected load.

Key Formulas

Fundamental RMS voltage V1rms = 0.9 * Vdc for square wave. Total harmonic distortion THD measures the harmonic content. Fourier series expansion provides the amplitude of individual harmonics.

Given: Vdc = 300 V, fundamental frequency f = 50 Hz.
Formula: V1max = (4 * Vdc) / pi, V1rms = V1max / sqrt(2).
Steps:
1. V1max = (4 * 300) / 3.1416 = 381.97 V.
2. V1rms = 381.97 / 1.414 = 270.1 V.
Final Answer: V1rms = 270.1 V.

Exam Tip: For a single phase full bridge inverter, the fundamental RMS voltage is 0.9 times Vdc. Do not confuse this with the half bridge formula which is 0.45 times Vdc.

Performance Parameters

• Input DC link must have a large capacitor for stiff voltage.
• Output voltage is independent of the load type.
• Current waveform depends entirely on the load impedance.
• Switching losses are significant at high frequencies.
• Third harmonic is zero in a three phase configuration.

Quick Revision

• VSI requires a constant DC input source.
• Freewheeling diodes carry the reactive load current.
• Dead time prevents dangerous short circuits across the leg.
• Square wave operation causes severe harmonic distortion.
• IGBTs are the preferred active switches.
• Exam trap: Forgetting that load power factor changes the diode conduction interval.