Three-phase, Three-wire AC Regulator with Balanced Resistive Load
Three-phase, Three-wire AC Regulator with Balanced Resistive Load:
- The resistance connected in all three phases are equal. Two thyristors connected back to back are used per phase, thus needing a total of six thyristors.
- The numbering scheme, which is same as that used in a three-phase full-wave bridge converter or inverter. The thyristors are fired in sequence starting from 1 in ascending order, with the angle between the triggering of thyristors 1 & 2 being 60° (one-sixth of the time period (T) of a complete cycle).
- The line frequency is 50 Hz, with T = 1/ f=20 ms. The thyristors are fired or triggered after a delay of α from the natural commutation point. The natural commutation point is the starting of a cycle with period, (6/60T=°) of output voltage waveform, if six thyristors are replaced by diodes.
- The output voltage is similar to phase-controlled waveform for a converter, with the difference that it is an AC waveform in this case.
- The current flow is bidirectional, with the current in one direction in the positive half, and then, in other (opposite) direction in the negative half. So, two thyristors connected back to back are needed in each phase.
- The turning off of a thyristor occurs, if its current falls to zero.
- To turn the thyristor on, the anode voltage must be higher that the cathode voltage, and also, a triggering signal must be applied at its gate.
- The procedure for obtaining the expression of the rms value of the output voltage per phase for balanced star-connected resistive load, which depends on range of firing angle.
- If Es is the rms value of the input voltage per phase, and assuming the voltage, EAN as the reference, the instantaneous input voltages per phase are,
- For ,immediately before triggering of thyristor 1, two thyristors (5 & 6) conduct. Once thyristor 1 is triggered, three thyristors (1, 5 & 6) conduct.
- A thyristor turns off, when the current through it goes to zero. The conditions alternate between two and three conducting thyristors.
- At any time only two thyristors conduct for . Although two thyristors conduct at any time for , there are periods, when no thyristors are on. For α ≥ 150◦, there is no period for which two thyristors are on, and the output voltage becomes zero at α= 150°. The range of delay angle is .
- The expressions of the rms value of the output voltage per phase for balanced star-connected resistive load are as follows. Here,