DickDV
Member
There are three important issues about softstarts that haven't been covered in this thread.
First, the ability to soften the start is entirely determined by the load, not the motor or the electronics. A fan on ball bearings, for example, can be started with very little torque but a rock quarry conveyor covered in dirt, etc, might require all the torque a motor can deliver to get started. Softstarting would not be possible in that case.
Second, since soft starts simple reduce the voltage but not the frequency, they only reduce the starting and accelerating torque. As a result, the accel ramp is not enforced---on a light load the motor will accellerate quickly to full speed and on a heavy load it will take longer to reach full speed.
Third, a softstart will only reduce inrush current under the best of conditions to around 40% of full voltage starting. If cutting inrush current is the primary purpose for softstarting, then an inverter beats a softstart every time. An inverter can develop far more torque per amp for starting and usually can softstart a load at or under nameplate FLA----ie. no inrush at all. And the ramp will be enforced regardless of load. Of course, at 100hp the softstart is about 1/4 the price of an inverter with the price differential dropping to nothing at around 10hp.
First, the ability to soften the start is entirely determined by the load, not the motor or the electronics. A fan on ball bearings, for example, can be started with very little torque but a rock quarry conveyor covered in dirt, etc, might require all the torque a motor can deliver to get started. Softstarting would not be possible in that case.
Second, since soft starts simple reduce the voltage but not the frequency, they only reduce the starting and accelerating torque. As a result, the accel ramp is not enforced---on a light load the motor will accellerate quickly to full speed and on a heavy load it will take longer to reach full speed.
Third, a softstart will only reduce inrush current under the best of conditions to around 40% of full voltage starting. If cutting inrush current is the primary purpose for softstarting, then an inverter beats a softstart every time. An inverter can develop far more torque per amp for starting and usually can softstart a load at or under nameplate FLA----ie. no inrush at all. And the ramp will be enforced regardless of load. Of course, at 100hp the softstart is about 1/4 the price of an inverter with the price differential dropping to nothing at around 10hp.
