Three-Phase AC Voltage Control Circuit

The present invention relates improved firing angle control circuits of the thyristors in a voltage control circuit containing reverse parallel connections of a thyristor and a diode or reverse parallel connections of thyristors in at least two phases of the three-phase AC circuit.

The prior art three-phase AC voltage control circuit will be illustrated.

FIG. 1 is a prior art circuit diagram and

FIG. 2 shows voltage waveforms in various parts in the circuit.

In FIG. 1, the references R, S and T respectively designate line terminals of a three-phase AC power source; L designates a load; S.sub.1 to S.sub.3 and D.sub.1 to D.sub.3 respectively designate thyristors and diodes for controlling the output voltage; S.sub.1 and D.sub.1 are connected in reverse parallel connection; S.sub.2 and D.sub.2 are connected in reverse parallel connection and S.sub.3 and D.sub.3 are connected in reverse parallel connection, and they are respectively connected in the R, S and T phases between the line terminals of the three-phase AC power source and the load to form the main circuit.

The references G.sub.R, G.sub.S, G.sub.T respectively designate firing angle control circuits for the thyristors S.sub.1, S.sub.2, S.sub.3 in the R phase, the S phase and the T phase. The internal circuits of G.sub.S and G.sub.T are the same as the internal circuit of G.sub.R. The references T.sub.1 to T.sub.3 respectively designate power source transformers for the firing angle control circuits and the terminals of the primary windings of the transformers are respectively connected to the power source terminals R, S, T and the terminals of the secondary windings of the transformers are respectively connected to the firming angle control circuits G.sub.R, G.sub.S, G.sub.T.

A voltage having 30.degree. of phase lag from that of the phase voltage of the power source corresponding to the firing angle control circuit, is applied to each power source transformer. That is, a voltage having the same phase as the phase of the voltage between R-T is applied to the firing angle control circuit G.sub.R in the R phase; a voltage having the same phase as the phase of the voltage between S-R is applied to the firing angle control circuit G.sub.S in the S phase and a voltage having the same phase as the phase of voltage between T-S is applied to the firing angle control circuit G.sub.T in the T phase.

The firing angle control circuit G.sub.R in the R phase comprises a diode for rectification D.sub.4, a resistor R.sub.1, a zener diode D.sub.5, a phase control variable timing resistor R.sub.2, a timing capacitor C.sub.1, a resistor R.sub.3, a unijunction transistor TR.sub.1, a pulse transformer PT.sub.1 and a resistor R.sub.4.

FIG. 2(1) shows the line voltages V.sub.RS, V.sub.ST, V.sub.TR and the phase voltage V.sub.R in the R phase of the power source.

The AC voltage V.sub.1R having 30.degree. of phase lag from the phase voltage V.sub.R in the R phase in FIG. 2(2) is applied from the terminal of the secondary winding of the transformer T.sub.1 to the firing angle control circuit G.sub.R in the R phase.

The AC voltage V.sub.1R is converted to the half-wave rectified wave V.sub.2R through the diode D.sub.4 as shown in FIG. 2(3) and it is further converted to the trapezoidal wave V.sub.3R by being clipped by the zener diode D.sub.5 and it is applied to the timing circuit having the resistor R.sub.2 and the capacitor C.sub.1 and the unijunction transistor TR.sub.1.

The timing capacitor C.sub.1 is charged through the timing resistor R.sub.2 and the charged voltage V.sub.4R is shown in FIG. 2(4). At the time when the voltage having the trapezoidal waveform V.sub.3R begins to rise (at the point of .theta. = 30.degree. of phase in FIG. 2), the charging to C.sub.1 is initiated. When the charged voltage V.sub.4R in C.sub.1 reached to the breakover voltage of the unijunction transistor TR.sub.1 .eta..multidot.V.sub.3R (.eta.: stand-off ratio of TR.sub.1), the charge in C.sub.1 is discharged through the unijunction transistor TR.sub.1 and the primary winding of the pulse transformer PT.sub.1 to generate the pulse output V.sub.5R to the secondary winding of the pulse transformer PT.sub.1 as shown in FIG. 2(5).

The pulse output V.sub.5R of the pulse transformer PT.sub.1 is applied through the resistor R.sub.4 between G-K of the thyristor S.sub.1 in the R phase whereby the thyristor S.sub.1 in the R phase is fired by the pulse output V.sub.5R and the voltage V.sub.6R between A-K of the thyristor S.sub.1 becomes substantially zero as shown in FIG. 2(6).

The firing angle .alpha. of the thyristor S.sub.1 can be controlled according to be FIG. 2(a) to (d) by controlling the charging speed of the timing capacitor C.sub.1 by adjusting the variable resistor R.sub.2.

The operations of the firing angle control circuits G.sub.S, G.sub.T in the S phase and T phase are the same as the operations of the firing angle control circuit G.sub.R in the R phase.

The voltage V.sub.6R between A-K of the thyristor S.sub.1 in the R phase and the output voltage V.sub.ORS between R-S to each firing angle are as shown in FIG. 2(6), (7) in the case of equal firing angles of the thyristors S.sub.1 to S.sub.3 in the R, S and T phases.

In FIG. 2, the charging of the timing capacitor of the firing angle control circuit is initiated from the point of the phase .theta. = 30.degree. and the time for charging the timing capacitor to the breakover voltage of the unijunction transistor is at least about 30.degree. and accordingly, the maximum firing angle .alpha. is about 150.degree. in the circuit of FIG. 1.

However, in the voltage control circuit containing a reverse parallel connection of the thyristor and the diode in each phase of the three-phase AC circuit, the control range of the firing angle .alpha. must be 0.degree. to 210.degree. in order to control the output voltage from zero to the full voltage.

Accordingly, the maximum output voltage in the circuit of FIG. 1 cannot be equal to the voltage of the power source as shown in FIG. 2(d).

It is an object of the present invention to provide firing angle control circuits of the thyristors for controlling the output voltage from zero to the full voltage in a voltage control circuit containing a reverse parallel connection of a thyristor and a diode in each phase of the three-phase AC ci620000000000000000000000000000000000000000000000000000000000000000