SCR into stepdown xfrmr won't go to full power 1

[thickfilm1]

Need help/insight on SCR problems.
We have 240 3-ph center delta xfrmr feeding 2 phases to create 1-ph 240 to an SCR. It is set to phase angle as it feeds a 10/1 step down buck boost delta xfrmr which feeds heater filaments.
40 amp input.
When SCR set to full we see 156 v and 17 amps.
Same SCR direct.to heaters is 238 and 40 amps as it should be.
What is holding SCR power output back into xfrmr ?3 diff. SCR 's - same problem.
Is it that we are using 2 legs of 3 phase to get 1-ph ?
Is it the stepdown xfrmr ?
How to diagnose and fix ?
Goal is to vary heat of 24 volt heaters with SCR.

 

[Skogsgurra]

Is the SCR controller fed from directly from the two-phase supply or is there an extra supply wire for the control part?
If the latter, there's a phase shift between power and control that could result in limited control range.

Inductive loads are notoriously difficult to control with SCRs and Triacs. A snubber (R=100-330 ohms and C=.1-2.2 uF) across the load helps getting above holding current. Try an incandescent lamp first. That will always get current above holding current.

Gunnar Englund

http://www.gke.org

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