EE 318 Chapter 4: Three-Phase Controlled Rectifiers

Three-Phase Controlled Rectifiers

• Three-phase controlled rectifiers have a wide range of applications, including electrochemical processes, motor drives, traction equipment, and controlled power supplies.
• Features of three-phase controlled rectifiers:
  • Operate from 3-phase AC supply voltage
  • Provide higher DC output voltage and power
  • Provide lower ripples in the output voltage and current
  • Require lower values of capacitance for filtering
  • Are extensively used in high-power variable-speed industrial DC drives

Review of 3-Phase Supplies

• Three-phase supplies usually represent the voltages generated by three coils (physically displaced by 120° from each other) in an AC rotating machine (alternator).
• The three-phase supply waveforms are 120° displaced from each other in time.
• Assuming a maximum phase voltage of Vm, the phase sequence is A-B-C, and B lags A by 120°, C lags B by 120°, and A lags C by 120°.

Controlled Three-Phase Half-Wave Rectifiers

• A 3-phase half-wave converter combines three single-phase half-wave controlled rectifiers in one single circuit feeding a common load.
• The 3-phase input supply is applied through a star-connected supply transformer.
• The thyristor will conduct (ON state) when the anode-to-cathode voltage is positive, and a firing current pulse is applied to the gate terminal.
• Delaying the firing pulse by an angle α controls the load voltage.

Operation of Controlled Three-Phase Half-Wave Rectifiers

• When thyristor T1 is triggered, the phase voltage Van appears across the load when T1 conducts.
• When thyristor T2 is triggered, T1 becomes reverse-biased and turns off, and the load current flows through thyristor T2 and the supply phase winding 'b − n'.
• When thyristor T3 is triggered, T2 becomes reverse-biased and turns off, and the load current flows through thyristor T3 and the supply phase winding 'c − n'.

Circuits with RL Load

• There is no discontinuous conduction mode for three-phase controlled rectifiers if L >> R.

• The average output voltage can be calculated using the following equation:

  Vdc = (3Vm / 2p) * [sin(wt + α) + sin(wt - α)]

• The average output voltage and current can be calculated as follows:

  Vdc = (3Vm / 2) * cos α Idc = (3Vm / 2) * cos α

Quiz on three-phase controlled rectifiers, their applications, and uses in electrochemical processes and motor drives.