Three-phase Delta-connected AC Regulator with Balanced Resistive Load
Three-phase Delta-connected AC Regulator with Balanced Resistive Load:
Fig: Delta connected three-phase ac regulator Fig: Waveforms for three-phase delta-connected ac regulator
- Two thyristors connected back to back are used per phase, thus needing a total of six thyristors. The numbering scheme should be looked upon.
- One phase of the balanced circuit is similar to that used for single-phase ac regulator. Since the phase current in a balanced three-phase system is only (1/√3 ) of the line current, the current rating of the thyristors would be lower than that if the thyristors are placed in the line.
- Assuming the line voltage as the reference, the instantaneous input line voltages are,
- The rms value of the line voltage in this case is ES. The rms value of the output phase voltage is obtained as
- When, the maximum value of the output voltage is obtained, and the control range of delay angle is
- It can be observed that the line currents depend on the delay angle, and may be discontinuous. The rms value of line and phase currents in this case can be determined by numerical solution or Fourier analysis.
- If In is the rms value of the nth harmonic component of a phase current, the rms value of the phase current is obtained from
- For delta connection, the triplen harmonic components (i.e., those of order, n = 3m, where m is an odd integer) of the phase currents flow around the delta, and would not appear in the line. This is due to the fact that these harmonic currents are like the zero sequence component, being in phase in all three phases of the load. So, the rms value of the line current is,
Therefore, the rms value of the line current would not follow the normal relationship of a three-phase system such that