Three Phase Inverters

A three phase inverter supplies a three phase load from a single DC source by operating three inverter legs with specific phase shifts. They are the backbone of industrial AC motor drives.

Core Concept

The 180 degree conduction mode keeps each switch ON for half a cycle. The gate signals for the three legs are phase shifted by 120 degrees. This produces a balanced three phase output voltage.

During any instant, three switches are conducting simultaneously. The phase voltages have a six step staircase waveform. The line voltages have positive, zero, and negative levels.

The 120 degree conduction mode ensures only two switches conduct at any time. This avoids cross conduction between top and bottom switches entirely. However, the device utilization is lower compared to 180 degree mode.

Key Formulas

Line voltage Vab = sqrt(2/3) * Vdc for 180 degree mode. Phase voltage Van = (sqrt(2) / 3) * Vdc. THD for line voltage in 180 degree mode is 31.08 percent.

Given: Vdc = 600 V, 180 degree conduction mode.
Formula: Vab = sqrt(2/3) * Vdc, Van = (sqrt(2)/3) * Vdc.
Steps:
1. Vab = 0.816 * 600 = 489.6 V.
2. Van = 0.471 * 600 = 282.8 V.
Final Answer: Line RMS = 489.6 V, Phase RMS = 282.8 V.

Exam Tip: The line voltage waveform in 180 degree mode is a quasi square wave while the phase voltage is a six step waveform. Triplen harmonics are absent in the line voltage.

Performance Parameters

• Total harmonic distortion for line voltage is 31 percent.
• Device utilization is higher in 180 degree mode.
• Isolation transformers are used to prevent DC injection.
• Common mode voltage is generated at the neutral point.
• Snubber circuits protect the switches from voltage spikes.

Quick Revision

• Six switches are controlled in a specific sequence.
• 180 degree mode provides higher output voltage.
• 120 degree mode naturally prevents shoot through faults.
• Star connected loads receive stepped phase voltages.
• Delta connected loads receive stepped line voltages.
• Exam trap: Calculating phase voltage using the single phase formula instead of the six step formula.