3x400V Mains Supply Soft Start and Data Monitor

[Mads Barnkob]

Construction of a 3 phase 32A 400VAC Soft Start Power Supply with safety features and a data monitor. All parts like enclosure, current transducers, voltage step down, contactors and wiring is reused from my 3 phase 13 A Lübcke variac build, that unfortunately had one of the auto transformers burn up.

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I wanted to reuse as many old components and parts, from my storage, as possible. Despite that the PMA Data Monitor KS4010 is quite old and slow, I figured it would still be good enough with its 6 analogue inputs and 125 ms sample rate. The only downside to that instrument, is the rather big size. Mostly due to the fact that it is hosting a 3½ floppy disk drive for saving measurements.

For the voltage measurements I used 400 VAC to 20 VAC step down transformers acquired on ebay and through a single diode and a resistor voltage divider getting it to a level that the data monitor can work with.

Back in 2015, I found the green and affordable AC RMS current transducers. They output a 0-5 VDC signal, which is within the range of the data monitor inputs.



Energizing the unit will light up the red lamp, indicating that it is not ready or is in soft start mode. For safety reasons, there is not a status light that can be trusted, other that all states indicate a powered and dangerous state. Using lamps to indicate powered and “dangerous” state, can lead to accidents, if the lamp is not lighting up when it should. Pressing the start button activates contactor Q3, that self-holds and turn on Q1. This also starts the count down timer that switches on Q2, which bridges out the soft start resistors.