Comparison of the Different Circuits used for Three-phase AC Regulators
Comparison of the Different Circuits used for Three-phase AC Regulators:
Fig: 3 wire Delta load, 4-Wire star load & Control in delta
- The important points of comparison between the circuits are:
1. In two circuits of 3 wire ac regulator and 4 wire star load, the individual phase controllers control their own loads independently of the other.
2. In other circuits, the individual phase controllers affect the other phase loads also, and they have to be studied as complete three-phase circuits.
3. The peak voltages occur across thyristors at or near the fully off state. In case of two circuits of delta connected ac regulator and control in delta, the maximum thyristor voltage is the peak of the line voltage, whereas in the circuit of 4- wire star load, it is the peak of the phase voltage; in two circuits of 3 phase,3 wire ac regulator and 3 wire delta, the maximum thyristor voltage will be somewhere between the peak of the phase and line voltages depending on the leakage currents of the thyristors, the method of firing and the presence of voltage-sharing resistors across the thyristors.
4. All the five circuits can be used under phase control.
5. The range of phase angle required to achieve full output range from zero to maximum varies between the circuits.
6. The maximum current in the thyristor is decided from the fully on condition, and the size of the thyristors to be used should be chosen from this condition. The peak, mean and rms values of the thyristor currents are related to the rms value of the input (ac) current, which should be found by applying the full supply voltage to the load circuit. The load impedance per phase is equal in magnitude (Z) and angle (ϕ), which is taken as positive, as it is mostly inductive (R-L).
7. The difference between the two circuits of 3 phase 3 wire ac regulator and 4-wire star load is that in the second one, the neutral point is available, making it a 4-wire one.
8. The difference between the two circuits 3 phase,3 wire ac regulator and 3 wire delta load is that in the second one, the three-phase balanced loads are connected in delta, which can be converted into its equivalent star, making it identical to the first circuit. Also, the current in the thyristors in the second case are the line currents, which are higher than the phase currents, which are flowing in the thyristors in the first one.
9. The difference between the two circuits 3 phase, 3 wire ac regulator and control in delta is that, in the second one, the thyristors connected back to back, are in delta, with the load connected in the three lines. Also, the current in the thyristors in the second case are the phase currents, which are lower than the line currents, which are flowing in the thyristors in the first one.
- It may be mentioned that other types of circuits for three-phase ac regulator can be used, but either the circuits are not bidirectional, i.e. unidirectional, or if they are bidirectional, in one half, only diode connected back to back per phase, instead of thyristor, is used. In the second case, only in one half with the thyristor per phase, controlled output voltage, but in the other one, uncontrolled output voltage, same as input one is obtained.