In solid state soft starters, how to fix the intial firing angle of thyristor for the given intial torque requirement?

The initial firing angle of the SCRs is preset by the Soft Starter manufacturer. With a closed loop system such as the controlled current or torque starters, this is then controlled by the feedback loop.

With a TVR soft starter, it is influenced by the initial start voltage setting.

Ravi , I thing what you are getting at is how to set the "start Voltage"or Voltage Pedestal according to your Torque requirements .

You can calculate it but it is not straightforward unless you have done it before...... and anyway cannot be done to 100% accuracy unless you know exactly what the characteristics are of the driven Machine ............remember that Motor Nameplate Data only has to be accurate within a certain percentage.

You are usually much better setting this up on site .....adjust the minimum (start) Volts or Amps so that when the start signal is given there is just a fraction of a second delay before shaft rotation .

Hi Rav,

Does your soft starters have potentiometer adjustments for initial torque and start ramp time?

If it does, one technique is to set the initial torque to the lowest possible setting and the start ramp time to its maximum setting. When you call for a start, increase the initial torque setting until the motor starts drive the connected load. Once the motor starts to drive the load, you have found the optimum setting for initial torque.

You should then observe the starting time (time to reach full speed) and adjust the start ramp time setting accordingly.

Short starting times often result in high starting current and high mechanical stress. Long starting times can however cause excessive motor heating and other problems. It is therefore necessary to find the best compromise for starting time on a case by case basis.

I hope this helps.

Regards,
GGOSS