Rectifier Currents

If the inductor in an inductor-input filter were infinitely large, the current through it would remain constant. If the commutation reactance effect is not considered, the current through each tube of a single-phase rectifier would be a square wave, as shown by I₁ and I₂ of Fig. 49(a). The peak value of this current wave is the same as the d-c output of the rectifier, and the rms value is 0.707I_dc. With finite values of inductance, an appreciable amount of ripple current flows through the inductor and effectively modulates I₁ and I₂, thus producing a larger rms inductor current like the first wave of Table I.

Capacitor-input filters draw current from the rectifier only during certain portions of the cycle, as shown in Fig. 49(b). For a given average direct current, the peak and rms values of these current waves are much higher than for inductor-input filters. Values for the single-phase rectifiers are given in Fig. 52. If an L-C filter stage follows the input capacitor, the inductor rms current is the output direct current plus the ripple current in quadrature.

Polyphase rectifiers are ordinarily of the choke-input type, because they are used mostly for larger power, and therefore any appreciable amount of series resistance cannot be tolerated. For this reason, the low IR drop tubes, such as mercury-vapor rectifiers, are commonly used. Such tubes do not possess sufficient internal drop to restrict the peak currents drawn by capacitor-input filters to the proper values.

In a shunt-tuned power supply filter such as shown in Fig. 93, the current drawn from the rectifier is likely to be peaked because two capacitors C₁ and C₂ are in series, without intervening resistance or inductance. This peak quickly subsides because of the influence of inductor L, but an oscillation may take place on top of the tube current wave as shown in Fig. 95.

Fig. 95. Anode current with shunt-tuned filter.

The rectifier tube must be rated to withstand this peak current. At the end of commutation the voltage jumps suddenly from zero to V (Fig. 87). Peak rectifier current may be as much as

[54]

L_S is half the transformer leakage inductance, and ω = 2π · frequency of oscillation determined by L_s in series with C₁ and C₂. This peak current is superposed on I_dc. It flows through the anode transformer and tube, but the current in choke L (Fig. 93) is determined by ripple voltage amplitude and choke reactance. Series resistance R_s reduces this peak current to the value

[55]

It is obtained by applying a step function voltage to the series R_SL_SC circuit. The criterion for oscillations is

[56]

where C is the capacitance of C₁ and C₂ in series. Many rectifier tubes have peak current ratings which must not be exceeded by such currents.

Currents shown in Table VII and Figs. 49 and 95 are reflected back into the a-c power supply line, except that alternate current waves are of reverse polarity. Small rectifiers have little effect on the power system, but large rectifiers may produce excessive interference in nearby telephone lines because of the large harmonic currents inherent in rectifier loads. High values of commutation reactance reduce these line current harmonics, but, since good regulation requires low commutation reactance, there is a limit to the control possible by this means. A-c line filters are used to attenuate the line current harmonics. A large rectifier, with three-phase series resonant circuits designed to eliminate the eleventh, thirteenth, seventeenth, and nineteenth harmonics of a 60-cycle system, is shown in Fig. 96.

Fig. 96. A-c line filter for large power rectifier.

Smaller rectifiers sometimes have filter sections such as those in Fig. 97;

Fig. 97. A-c line filter for medium-sized power rectifier.

these are rarely used in large installations because of the excessive voltage regulation introduced by the line inductors. Filters designed to keep r-f currents out of the a-c lines are often used with high-voltage rectifiers. Even if the anode transformer has low radio influence, commutation may cause r-f currents to flow in the supply lines unless there is a filter.