We are now developing a 3-phase anti-parallel SCR controlled soft starter for our own use.
We have 3 zero crossing circuits to detect the zero cross of L1, L2 & L3 and 3 SCR firing circuits to trigger the 3 sets of anti-parallel SCRs.
The idea is to fire these 3 sets SCRs at an angle just before zero cross and gradually increase the firing angle and eventually fully turn on all the 3 sets SCRs.
The problem is when we started to fire these SCRs, the motor started to ramp up slowly at CW direction at very low speed, after a short while, the motor started to jerk and then the motor started to ramp up at opposite CCW direction smoothly until it reached full voltage (SCRs fully on).
Does anyone have any idea what is the cause of this problem and how to overcome this problem?
All comments are welcome!
Cheers!
