Concluding this series on three-phase systems, in this month's column I'll look at two-wire control of these systems.

Two-Wire Control of Three Phases. You can connect two SCRs in just two of the three secondary lines and use fast-cycle control. You buy only two SCRs and rate them for the line current and line voltage. This arrangement rules out both phase-angle control and load balancing because of you cannot use a neutral conductor.

SCRs Controlling in the Primary. The general rule here is to use phase-angle control to avoid the high inrush problems of fast cycling noted in earlier parts of this series. Two preferred connection would be:

  • SCRs in series, with each of three-delta-connected primary windings. They would be rated for line voltage and phase current. This may impair the benefits of circulation of third harmonic currents in the primary.

  • SCRs in series, with each line of a wye-connected primary that has a neutral conductor. They would be rated for line voltage divided by the square root of three (line voltage/?) and line current.

In both cases, you would use soft-start to avoid excessive current inrush. You would use current limit if the load were temperature dependent.

These two types of connection provide three predictable low-impedance voltage sources on all three primary windings. With other configurations, the balance of these voltages can be upset by unbalance or partial loss of load. You can use SCRs in series with the three lines, but make sure that the secondary loads are well balanced.

Overvoltage Caused by Loss of One Heater when SCRs are Used in the Primary. This overvoltage condition is in addition to the ever-present threats of lightening strikes, line voltage surges, spikes and transients.

Some experimental work and measurement has been done around this topic. It reports cases of repetitive overvoltage that can harm SCR devices and destroy metal oxide varistors ( MOVs).

Apart from avoidance of certain primary/secondary/load configurations, solutions include use of higher-voltage-rated SCR devices (for example, 1.5 times the normal rating that you would use plus higher-joule-rated MOVs). These are threatened because of the repetitive nature of the overvoltages as opposed to the short duration of the expected -- if that's the word -- random disturbances.

Among the many experimental revelations, the delta primary and wye secondary with neutral configuration look like the least harmful. Removing the secondary neutral threatens the SCRs and MOVs and competes for the most harmful configuration.

Checking the integrity of the heaters, fuses and wiring is a good precaution. If burnout or loss of a heater is a possibility, you might consider a heater-fail alarm or shutdown circuit. You would like to know anyway if you have lost a heater in case it harms your processing.

Figure 1. The Scott-connected transformer can be made out of one center-tapped transformer and another having a tap 0.866 from one end.

Scott-Connected Transformer

The Scott-connected transformer can be made out of one center-tapped transformer and another having a tap 0.866 from one end. They connect to each other and the three-phase supply is connected as shown in figure 1. The vector geometry works out to produce two secondary outputs that are mutually 90oapart in phase.

This creates a three-phase to two-phase conversion and preserves a balanced load at the input. It is a handy way to serve a high powered two-zone process without grossly unbalancing the supply. You would use one SCR in each of the three input lines.

The two transformers can be arranged to share one three-limb iron core. So, you can call the result a Scott T-connected transformer. The T refers to the T junction at the center-tap.