Grid-Controlled Rectifiers

The basic a-c grid control circuit described in the last section may be extended to more than one tube and may control large amounts of power. Any of the rectifier circuits of Table VII may be used with grid control of output voltage, which supplants the older practice of using induction regulators in the supply lines.

Smooth control of rectifier d-c voltage under load conditions is possible through the use of thyratrons or ignitrons with phase-shift control of the grid or igniter. Stable control of filtered output is possible only with choke input filters. In Chapter 4 the regulation of a rectifier is shown to be lowest if the input choke has inductance greater than critical value. With grid control, if the filter choke inductance is great enough, the tube conducts even after the anode reaches zero. The tendency of current to stop at voltage zero builds up voltage across the filter choke in such a direction that cathode potential is less than zero after the anode reaches zero. Thus conduction in the tube is maintained until the next tube fires. If the choke inductance is less than critical, tube current wave is discontinuous, regulation is poor, transient surges and oscillations in the output voltage occur, and control is unstable.

For a single-phase full-wave rectifier with grid control, the direct voltage output decreases as shown by curve I in Fig. 186. Critical value of inductance increases with firing angle and so does ripple voltage as shown by curves II and III in this figure. For a three-phase full-wave rectifier, the direct output voltage is approximately 41 per cent greater than the single-phase values shown in Fig. 186, and the critical value of choke reactance less filter capacitor reactance is approximately one-tenth of the single-phase values over the range of 20° to 90° firing angle.

Fig. 186. Output voltage, ripple, and current continuity in single-phase full-wave grid-controlled rectifier.

At 90° firing angle, the d-c output always is zero. Voltage across the choke reverses in sign but does not increase in magnitude even with the maximum angle of 90°. Therefore the maximum voltage from choke to ground is not changed, and the design of a reactor for this type of rectifier is the same as for a rectifier without grid control, except for the value of inductance.

Choke-input filters can be used to maintain continuous current flow in single-phase half-wave rectifiers. Although the output voltage is reduced, as mentioned in Chapter 3, this combination is occasionally useful.⁽¹⁾

Grid-controlled rectifiers have more irregular current wave forms and therefore more pronounced a-c line harmonics than ordinary rectifiers.⁽²⁾

Two methods of providing phase shift control of a constant amplitude a-c grid voltage for grid-controlled rectifiers are shown in Fig. 187.

Fig. 187. Resistance-inductance phase shift circuits.

In (a) a small value of resistance R effectively connects the upper grid circuit terminal to the left-hand terminal of the supply transformer, and a large value of R shifts it nearer to the right-hand terminal of the supply transformer. If the supply transformer is center-tapped, the vector diagram of Fig. 188 shows the phase position of the grid voltage E_g in solid lines for a small value of R, and in dotted lines for a large value of R. Varying the rheostat R thus varies the rectifier output from full voltage to a low voltage.

In Fig. 187(b) resistor R is fixed and inductance L is varied by means of direct current flowing in one of its windings.

Fig. 188. Vector diagram for Fig. 187.

The vector diagram of Fig. 188 still applies; the solid lines are for high inductance and the dotted lines for low inductance. Direct current for varying the inductance may be obtained through a thyratron or a vacuum tube, especially when rectifier output voltage is automatically controlled. The reactor is usually of the saturable type.

The widest range of inductance is obtained with zero direct current at the higher inductance. In some vacuum-tube circuits, the minimum direct current is not zero, and a bias winding is added to the center leg to cancel the d-c ampere-turns with minimum current in the main d-c winding. Saturable reactors have many uses besides that described here, and are discussed more fully in Chapter 9.

(1)	For general calculation of discontinuous waves, see "Voltage and Current Relations for Controlled Rectification with Inductive and Generative Loads," by K. P. Puchlowski, Trans. AIEE, 64, 255 (May, 1945). For theory of controlled rectifiers, see "Critical Inductance and Control Rectifiers," by W. P. Overbeck, Proc. I.R.E., 27, 655 (October, 1939).
(2)	See "Harmonics in A-C Circuits of Grid-Controlled Rectifiers and Inverters," by R. D. Evans and H. N. Muller, Jr.. Trans. AIEE, 58, 861 (1939).