page_title: Rectifier
section_title: Three-phase bridge rectifier controlled
hierarchical_section_title: Rectifier / Rectifier circuits / Three-phase rectifiers / Three-phase bridge rectifier controlled
caption_attribution_description: English: Voltage and current waveforms from a three-phase Graetz bridge rectifier with diodes, or with thyristors at alpha=0, ignoring the effect of overlap.
context_page_description: A rectifier is an electrical device that converts alternating current, which periodically reverses direction, to direct current, which flows in only one direction.
  The process is known as rectification, since it "straightens" the direction of current. Physically, rectifiers take a number of forms, including vacuum tube diodes, wet chemical cells, mercury-arc valves, stacks of copper and selenium oxide plates, semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches. Historically, even synchronous electromechanical switches and motors have been used. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena to serve as a point-contact rectifier or "crystal detector".
  Rectifiers have many uses, but are often found serving as components of DC power supplies and high-voltage direct current power transmission systems. Rectification may serve in roles other than to generate direct current for use as a source of power. As noted, detectors of radio signals serve as rectifiers. In gas heating systems flame rectification is used to detect presence of a flame.
context_section_description: The controlled three-phase bridge rectifier uses thyristors in place of diodes. The output voltage is reduced by the factor cos(α):
  Or, expressed in terms of the line to line input voltage:
  Where:
  VLLpeak, the peak value of the line to line input voltages,
  Vₚₑₐₖ, the peak value of the phase (line to neutral) input voltages,
  α, firing angle of the thyristor (0 if diodes are used to perform rectification)
  The above equations are only valid when no current is drawn from the AC supply or in the theoretical case when the AC supply connections have no inductance.  In practice, the supply inductance causes a reduction of DC output voltage with increasing load, typically in the range 10–20% at full load.
  The effect of supply inductance is to slow down the transfer process (called commutation) from one phase to the next.  As result of this is that at each transition between a pair of devices, there is a period of overlap during which three (rather than two) devices in the bridge are conducting simultaneously.  The overlap angle is usually referred to by the symbol μ (or u), and may be 20 30° at full load.
  With supply inductance taken into account, the output voltage of the rectifier is reduced to:
  The overlap angle μ is directly related to the DC current, and the above equation may be re-expressed as:
  Where:
  Lc, the commutating inductance per phase
  Id, the direct current
caption: Three-phase Graetz bridge rectifier at alpha=0° without overlap
exif: {}