A frequent misconception is that a solid-state starter is the solution to every motor starting problem. After all, since it can ramp the motor voltage gradually from zero to 100% at a user-selected rate, can't it limit the inrush current to any desired value and smoothly bring the motor up to speed just by selecting a long enough acceleration time? The answer is no, because this thinking overlooks two important factors in the motor starting process: torque and heat.
Figure 1 represents a typical motor torque-speed curve at full voltage superimposed on the torque-speed curve of a centrifugal compressor. The motor torque is higher than the load torque all the way from rest to the normal operating speed of the system, which is the point where the two curves cross. The difference between motor and load torque results in the acceleration of the system according to the equation TM - TL = Ja. If this margin drops to zero at any point prior to reaching the desired operating speed, the system will stop accelerating and remain at that speed.