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what is the working principle of soft-starter & how it
diff. from VFD ?????????
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Answer Posted / alfred salazar
The VFD works on the principle that the AC line voltage is converted to a DC voltage. This DC voltage is then inverted back to a pulsed DC whose RMS value simulates an AC voltage. The output frequency of this AC voltage normally varies for 0 up to the AC input line frequency. On certain applications the frequency may actually go above the line frequency. Though high performance current regulated AC drives capable of
operating in “torque mode” are available, the more prevalent volts per hertz drive is addressed. While Soft-starter, For induction motors, the starting torque is approximately proportional to the square of the starting current drawn from the line. This starting current is proportional to the applied voltage. So the torque can also be considered to be approximately proportional to the applied voltage. By adjusting voltage during starting, the current drawn by the motor and the torque produced by the motor can be reduced and controlled. 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. The result of phasing back the SCR’s is a non-sinusoidal reduced voltage at the terminals of the motor. Since the motor is inductive and the current lags the voltage, the SCR stays turned on and conduct until the current goes to zero. This is after the voltage has gone negative.
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