Soft Starter With SCRs: Why Does Output Voltage Exceed Input at No Load or Open Output?

I had connected a soft starter to a very low rating motor and I found that in case the output current is very low, it trips. (Soft starter has SCRs)


I do know that we should not connect contactors at output terminals of drives.


Today, I pulsed gates of diac SCRs(connected anti parallel to each other) and found that output voltage was more than input voltage at no load (open output terminals). Why did this happen? Was it a kind of ferranti effect? (Input was 415 V, output was 609V)Is it because of this reason that soft starter trips as soon as output terminals get opened by mistake or say output current is very low. Is it the same reason that we dont connect contactor at output terminals of a drive? 

Hi Nand-What is the make and model of the soft starter?  I have not heard of this happening, but I can look into it. There's not enough information given for me to comment on the issue with the output voltage being greater than the input.  An SCR by itself cannot increase the voltage, but if t is connected to an inductive load (like a motor winding), there can be spikes that are higher than the input voltage when the SCR turns off.-Rick