Solid State Starting and Starting Torque

In many cases soft starts are used to limit the inrush current and the resulting voltage drop on the line for relatively large motors running on relatively small power systems.

If this is the case for your installation, you would want settings that achieve the fastest possible start while limiting the voltage drop on the line to acceptable levels. In my opinion the most direct way to do this would be using 'current limit' starting. You can determine the settings by trial and error or by estimating the value of voltage drop for a given inrush current based on the upstream conductor sizing, length of run, and size of the upstream transformer.

If your soft start is being used for some other reason such as limiting accelerating torque due to mechanical limits in the system, then 'torque limit' starting may be a more intuitive way to do this. Even so, you would still want to achieve the fastest start possible without exceeding the mechanical torque limit and breaking something.

In its simplest form, the "softest" start is determined solely be the break-away torque of the load. Whatever torque and current that is required to accomplish that will be your lowest limit.

In general, due to the reduced torques and currents, you can accelerate up to speed in as long a time as you please. For NEMA size motors, the motor thermal limits will rarely be exceeded. It doesn't hurt to keep those thermal limits in mind, however, particularly for very large motors and very long accel times. In these larger motors, the limits will be specific to the particular motor and you will need to consult with the manufacturer for help.

Most well pumps I have started with soft starters will require at least 350% current limit to get the water up to the top; less than that and it may dead head. Most submersibles are fine with 350% for as long as 30 seconds, but it will likely start much faster than that. Initial torque is relatively meaningless in a pump, might as well set it as high as you like since the Current Limit will take over almost immediately anyway.

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Thanks for your help guys.

From my research on the motor and the pump, (Its a 300 HP vertical induction motor with a Floway 14 DKM 8 stage pump) it looks as if by reducing the current limit/torque limit this reduces the Speed-Torque curve of the motor. And from what I understand the Speed-Torque curve of the pump cannot exceed the curve for the motor before it reaches the rated speed of 1770 rpm. If this happens the motor will never reach its rated speed.

Does this sound correct? If the motor never reaches its rated speed it could possible create a thermal overload. Is this correct as well?

The whole purpose of this is that I am sizing a diesel generator for the site. If I can reduce the current limit/torque limit on the soft starter, I can reduce the size of the generator, which is optimal.

That's why I like reduced voltage softstarters. As jraef mentions, if you try to do it with current limitations, you run into trouble as you get up to full speed and full load.

At least with voltage control, under near-stall conditions, the rising voltage will eventually force progression thru the stalling point up to full speed.

If you require more current to get thru the stall point than the source can provide, you've got a fundamental problem with sizing that needs a different solution than softstarting.

I guess that is where the ramp up time on the soft starter comes into play. The ramp up time has a default where if the motor stalls or clips at a point where it will not ramp up anymore once the time limit is reached on the soft starter the SCR's open into bypass and full voltage is applied to the motor.