wvmarle:
That may be your problem. That capacitor tries to level the voltage, so every time you open the TRIAC it tries to charge, causing a current surge. The longer the cuts in the supply current (i.e. lower duty cycle) the more this may become an issue.Isn't it possible to install a MOSFET in the DC side (which, by the way, should be more like 320-340V: normal AC line is 230-240V nowadays, times 1.42 for the smoothing), so you can use that to regulate the current? I don't know if it makes sense to keep in sync with the AC power supply, though.
thanks for the reply. to give you more detail i am rebuilding an existing motor driver that was damaged its old technology and its no longer possible to get spare parts. the capacitor was part of the existing circuit after the triac so it has been designed for that and was at some point working with the capacitor. but i have also removed the capacitor from the circuit and the effect is the same.
i have also tried a new capacitor, still no luck.
i had the same idea about a DC side MOSFET, i had already bought the components to replace the AC side like the original design but a quick experiment on the DC side left me with a blown transistor, i think maybe the capacitor acts like an infinite load? and it sucked too many amps through the transistor. but i am not sure, i didn't pursue it further but maybe i should. i will post a schematic up in a sec