Solid-state soft starting is the most commonly used technique for starting induction motors. It provides a smooth build-up of current and torque; the maximum current and acceleration time are easily adjusted. The only problem with this technique over conventional starters is that the mains currents during run-up aren’t sinusoidal, which can lead to interference with other equipment on the same supply.
The typical arrangement comprises three pairs of back-to-back thyristors (or Triacs) connected in series with the three supply lines as illustrated in the figure below:
Each thyristor is fired once per half-cycle, the firing synchronized with the utility supply and the firing angle being a variable so that each pair conducts for a varying proportion of a cycle. The typical current waveforms are demonstrated in the figure below, and you can see they aren’t sinusoidal, however the motor will work well with them.
We have multiplicity of control approaches that can be employed with varying degree of complexity which depends on the cost of the system. The low-cost open-loop systems simply alter the firing angle linearly with time, so that the voltage applied to the motor increases as it accelerates. The ramp ‘ramp-time’ can be set by trial and error to given an acceptable start, that is, one which the maximum allowable current from the supply is not exceeded at any stage. This method works well when the load remains the same, but requires resetting each time the load changes. Loads with high static friction are a challenge because nothing happens for the first part of the ramp, during which the motor torque is not enough to move the load. When the load finally moves, its acceleration is often too rapid. The more advanced open-loop versions allow the level of current at the start of the ramp to be selected, and this is useful in problematic situations like one described above. More complex systems – typically with on-board digital controllers, provide for tighter control over the acceleration profile by incorporating an inner current-control loop. After an initial ramping up to the start level [over the first few cycles], the current is held constant at the desired level throughout the accelerating speed, the firing angle of the thyristors being continually adjusted to compensate for the changing effective impedance of the motor. By keeping the current at the maximum value which the supply can tolerate, the run-up time is minimized. As with the open-loop systems the velocity-time profile is not necessarily ideal, because with constant current the motor torque exhibits a very sharp rise as the pull-out slip is reached, resulting in a sudden surge in speed. We also have some systems that include a motor model which estimates speed and allows the controller to follow a ramp or other speed-time profile.
Also Read: Direct-on-line (DOL) Starter
