AC Drives and Soft Starter (4)

Operation of Soft Starters

Timing of when to turn on the SCR’s is the key to controlling the voltage output of a soft starter. During the starting sequence the logic of the soft starter determines when to turn on the SCR’s. It does not turn on the SCR’s at the point that the voltage goes from negative to positive, but waits for some time after that. This is known as “phasing back” the SCR’s. The point that the SCR’s are turned on is set or programmed by what is called either initial torque, initial current or current limit setting.

The input voltage to the soft starter is the same as the VFD shown in figure 3. The result of phasing back the SCR’s is a nonsinusoidal reduced voltage at the terminals of the motor which is shown in figures 7. Since the motor is inductive and the current lags the voltage, the SCR stays turned on and conducts until the current goes to zero. This is after the voltage has gone negative. If compared to the full voltage waveform in figure 3, it can be seen that the peak voltage is the same as the full voltage wave. However the current does not increase to the same level as when full voltage is applied due to the inductive nature of motors.
When this voltage is applied to a motor, the output current looks like figure 8. As the frequency of the voltage is the same as the line frequency, the frequency of the current is also the same. As the SCR’s are phased on to full conduction, the gaps in current fill in until the wave form looks the same as applying the motor directly across the line.


One Response to “AC Drives and Soft Starter (4)”

  1. izu Says:

    where can i get a circuit on i can build a softstarter with a closed loop.

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