AC Drives and Soft Starter (2)

Soft Starters
The soft starter operates on a different premise. This principle is that by adjusting the voltage applied to the motor during starting, the current and torque characteristics can be limited and controlled.

For induction motors, the starting torque (LRT) is approximately proportional to the square of the starting current (LRA) drawn from the line. LRT ∞ I2. This starting current is proportional to the applied voltage (V). So the torque can also be considered to be approximately proportional
to the applied voltage. LRT ∞ V2.. By adjusting voltage during starting, the current drawn by the motor and the torque produced by the motor can be reduced and controlled.
By using six SCR’s in a back-to-back configuration as shown in figure 2, the soft starter is able to regulate the voltage applied to the motor during starting from 0 volts up to line voltage. Unlike the VFD, line frequency is always applied to the motor. Only the voltage changes.
Feedback from the motor to the logic circuit controlling the SCR firing is required to stabilize motor acceleration.

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