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Which of the following correctly describes the output of a three-phase full-wave rectifier that rectifies three-phase AC with a bridge of six diodes?

  1. AIts ripple is smaller than with single-phase full-wave rectification, and the ripple frequency is six times the supply frequency
  2. BBecause there are more phases, its ripple is larger than with single-phase full-wave rectification, so a large smoothing capacitor is always required
  3. CThe output is not DC but comes out as three-phase AC at the same frequency as the supply
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Answer: A. Its ripple is smaller than with single-phase full-wave rectification, and the ripple frequency is six times the supply frequency

In a three-phase full-wave rectifier, the difference between the highest and the lowest of the three phases (a line voltage) always appears at the output. The output therefore consists of six humps near the peak of the line voltage per cycle, and the ripple frequency is six times the supply frequency (300 Hz for 50 Hz). Because the ripple is small, an output close to DC is obtained even without a large smoothing capacitor, and the circuit is widely used in industrial motor drives and similar equipment. The average output voltage is about 1.35 times the RMS line voltage.

  • A. Correct. Six humps per cycle make the ripple small and its frequency high.
  • B. The more phases, the smaller the ripple and the smaller the smoothing capacitor needed.
  • C. The diode bridge picks out the difference between the highest and the lowest phase, so the output is a unidirectional voltage (DC).
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More Power Electronics Circuits questions

  1. A sinusoidal AC voltage of 100 V RMS is full-wave rectified with an ideal diode bridge. Without a smoothing capacitor, which of the following is closest to the average output voltage?
  2. Estimate on the assumption that the capacitor supplies all of the load current for one full ripple period. When 50 Hz AC is full-wave rectified, smoothed with a 10,000 µF capacitor, and a constant 2 A flows to the load, which of the following is the approximate peak-to-peak output voltage ripple?
  3. In phase control with an AC supply and thyristors, which of the following correctly describes how the average output voltage changes when the firing angle (control angle) α is increased?
  4. A 200 V RMS single-phase AC supply feeds a resistive load through a single-phase full-wave rectifier using thyristors, with phase control. Which of the following is closest to the average output voltage at a firing angle of 60°?
  5. In a thyristor phase-control circuit connected to an AC supply, which of the following correctly explains why the thyristor turns off without any action to turn it off from the gate?

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