Vaggelis,
Welcome!
I love engineering challenges! I love designing, thinking, researching, creating, developing, etc. It's great fun.
However, in the USA, if you personally modify a UL rated controller, (or even add anything to it), this opens up a "can of worms", and exposes yourself to liability in a lawsuit, if someone were to ever get hurt. The reason for the failure doesn't even have to be related to what work you did! The plaintiff's lawyer only has to raise doubt in the mind of the jury, to be awarded a judgement, and get a big settlement. I'm not sure about the legal climate in Österreich, but that's the way things are here. If we want the equipment modified, we have to go through the original equipment manufacturer, and have them perform the engineering. Even if they do exactly the same circuit design that we would do, it has to be done through them to maintain the UL rating, and protect ourselves from liability.
Therefore, when you talk to anyone from the USA about such issues, you will receive less than enthusiastic responses. I just wanted you to know why that is the case.
I am not familir with STHAL equipment at all. Is the hoist motor a 3 phase ac motor? Does it produce any voltage when it's being rotated or overhauled by a load, such as during empty car up? If so, then you could hang a phase monitor on the motor leads.
Like this-
http://www.transcat.com/PDF/KnoppK3.pdfOr this-
http://www.transcat.com/PDF/KnoppK3.pdfCheck with Sthal, check with your insurance company, check with a lawyer-
But above all, be safe!