%Torque α %Voltage2
Given this relationship, a 60% reduction in the applied voltage results in approximately an 84% reduction in generated torque. In this example, 40% voltage is used.
(0.4)2 = 0.16, or 15% of Locked Rotor Torque is present
So now you look at a speed torque curve having a locked rotor torque compared to full running torque at 90%. Reduce your voltage to 40% as in the example, and now you have 16% of the actual starting torque. That is pretty low. This value may or may not be enough to start the motor.
This was an example based on math. Of course, the reduced voltage can be adjusted to provide a start up to 100%. The point is, if there is little margin of torque to play with because the starting torque level percent is so low to start with, the use of a soft starter is diminished.
The most common motor used is the NEMA Design B. Many traditional NEMA Design B motors are/were 150 – 250% or higher for percent locked rotor torque compared to full running torque.
It is now common to see motors with much lower initial percent starting torque. This needs to be taken into account when looking at a soft starter for an application.
We offer a free electronic tool to help determine whether an SMC soft starter would be a good fit for an application, so check it out.