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Rectifier Filter Charts

From the preceding sections, it can be seen that various properties of rectifier filters, such as ripple, regulation, and transients, may impose conflicting conditions on rectifier design. To save time in what otherwise would be a laborious cut-and-try process, charts are used. In Fig. 100 the more usual filter properties are presented on a single chart to assist in arriving at the best filter directly. This chart primarily satisfies ripple and regulation equations 46 and 53 for a choke-input filter.

Fig. 100. Choke-input filter chart.

Fig. 100. (Continued)

Abscissa values of the right-hand scale are bleeder conductance in milliamperes per volt, and of the left-hand scale, filter capacitance in microfarads. Ordinates of the lower vertical scale are inductance in henrys. Lines representing various amounts of ripple in the load are plotted in quadrant I, labeled both in db and rms per cent ripple. In quadrant II, lines are drawn representing different types of rectifiers and supply line frequencies. A similar set of lines is shown in quadrant IV.

Two orthogonal sets of lines are drawn in quadrant III. Those sloping downward to the right represent resonant frequency of the filter L and C, and also load resistance RL. The other set of lines is labeled √(L/C), which may be regarded as the filter impedance. It can be shown that the transient properties of the filter are dependent upon the ratio of √(L/C) to RL.

The L scale requires a correction to compensate for the fact that ripple is not exactly a linear function of L but rather of XL - Xc. The curves in the lower part of quadrant IV give the amount of correction to be added when the correction is greater than 1 per cent.

Instructions for Using Chart

1.  Assume suitable value of bleeder resistance or bleeder current I1 in millamperes per volt of Edc. This is also steady-state peak ripple current in milliamperes.

2.  Trace upward on assumed bleeder ordinate to intersect desired value of load ripple, and from here trace horizontally to the left to diagonal line for rectifier and supply frequency used. Directly under, read value of C.

3.  Trace downward on same assumed bleeder ordinate to intersect diagonal line below for rectifier and supply frequency, and read value of L.

4.  From desired ripple value, determine correction for L on graph at lower right, and add indicated correction to value of L.

5.  Using corrected value of L and next standard value of C, find intersection in third quadrant, and read maximum resonant frequency fr.

6.  Using same values of L and C as in 5, read value of ratio √(L/C).

7.  Under intersection of √(L/C) with load resistance RL read values of the four transients illustrated in Fig. 101 (in per cent).

Example (shown dotted). Three-phase full-wave 60-cycle rectifier; Edc = 3,000 v; I2 = 1 amp; I2 = 96 ma; load ripple = -50 db; balanced line.

Solution:

Bleeder ma/volt = 0.032.

C = 4.5 μf (use 5 μf).

Scale value of L = 0.78 h; corrected value = 0.82 h.

Resonant frequency = 75 cycles.

Load resistance RL = 3,000 ohms.

im = 7I2 = 7 amp; ΔED = 12 per cent; ΔER = 15 per cent; ΔES = 80 per cent.

Fig. 101. Four transient conditions in choke-input filter circuit and curves.

In polyphase rectifiers the possibility exists of enough phase unbalance to impress a voltage on the filter having a frequency lower than the normal fundamental ripple frequency. If the filter L and C resonate near the unbalance frequency, then excessive ripple may be expected. Conversely, the L and C should have a resonant frequency lower than the unbalance frequency to avoid this trouble. Quadrant III of the chart has a series of lines labeled fr, and the intersection of L and C thereon indicates this resonant frequency. It should be no higher than the value given in the small table on the chart if excessive ripple is to be avoided. This table is based on 2 per cent maximum unbalance in the phase voltages.

For most practical rectifier filters, transient conditions fall within the left-hand portion of the third quadrant. The other conditions sometimes help in the solution of problems in which L and C are incidental, e.g., the leakage inductance and distributed capacitance of a plate transformer.

Although the chart applies directly to single-stage, untuned filters with constant choke inductance, it can be used for other types with modifications:

(a)   Shunt-Tuned Choke per Fig. 93. Figure 100 can be used directly for capacitance C, but, for a given amount of ripple, divide the chart values of inductance by 3 in order to obtain the actual henrys needed in the choke.

(b)   Swinging Choke. If at light load the filter choke swings to S times the full-load value of henrys, multiply the capacitance obtained from the chart by the ratio S to find the capacitance needed (Cn). The value of L obtained by projecting the bleeder current downwards is the maximum or swinging value. It must be divided by S to obtain the full-load value. Transient conditions then may be approximated by using capacitance Cn and the full-load value of henrys.

(c)   Two-Stage Filters. In a filter with two identical stages, Fig. 84(b), the chart can be used if it is recognized that the ripple is that on the load side of the first choke. For example, if the filter consists of two stages both equal to that in the example given for the single-stage filter, the ripple would not be -100 db but - 75 db, because of the fact that the rectifier output has (per Table VII) only 4 per cent ripple, which is - 25 db.

The regulation in a two-stage filter, as far as capacitor effect is concerned, depends upon the inductance of the first choke as in the single-stage filter. Therefore the chart applies directly to the inductance and capacitance of one stage. The peak ripple current likewise depends upon the inductance of the first choke, regardless of the location of the bleeder resistor. Transients, however, are more complicated, owing to the fact that the two stages interact under transient conditions.(1)



(1) See Proc. I.R.E., 22, 213 (February, 1934).



Last Update: 2011-02-17