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I started this week on my steam generator build.
Regarding the heating element and its control I have decided to use a 6KW 380VAC 3 phase element. I decided against a single phase 5.5KW element because I feel that it's on the upper acceptable limit for current loading.
I had originally decided to control the heating element with a SSR with 4-20mA from my PLC (I have spare 4-20mA modules in stock). This is fine for single phase but not so fine with 3 phase because it is not readily available for inexpensive solutions. YES you can buy some expensive Thyristor control for 3 phase and 4-20mA control.
So the question is ... without spending serious money, how can you effect PLC heating control with 3 phase.
Some of the guys I have spoken to have suggested 3 x SSR's in series for 4-20mA control of each phase.
Another has suggested a temperature controller (panel door mounted) which has 3 setpoints where you can drop idividual phase at temperature setpoint and then use a single SSR with 4-20mA on the one phase for fine temperature control.