Hello everyone,

I've been reading this forum for some time trying to improve my knowledge base, thanks to everyone who posts answers, now I have a question of my own.

I have to set up a soft starter for a 30kW centrifugal pump. I have two options for the start curve, a flat current limit start with soft start turned off or the soft start curve that ramps the current up over time.

I'm trying to determine which type of start curve is best for a centrifugal pump with high resistive starting torque? Or how should I determine which start curve is best?

If I start the pump with a flat current limit start I imagine the pump will ramp up very rapidly after the initial resistance is overcome and if I start it with a current ramp I'm concerned I won't get the required starting torque.

Thanks, Leigh

Hello Leighc

Welcome to the forum.

In order to answer your question, it is important that you understand the speed torque curve of the pump and the motor and also the current speed curve of the motor.

The pump torque curve basically increases with the square of the speed.

Create the speed torque curve of the pump.
On this curve, add the full voltage start torque of the motor.
Next you can plot a series of start torque curves for the motor under a constant current start.
Each point on the curve can be calculated by multiplying the full voltage start torque at that speed by the square of the current reduction at that speed.
From these curves, you can determine the minimum start current (if the start torque curve crosses the load torque, the pump can not start) and you can also determine the relative acceleration. (gap between the curves is the acceleration torque.)

A current ramp will boost the current and therefore the torque with time.

The advantage of the current ramp, is that you can start with a lower start current and the current will ramp up to the value required to develop enough torque to start the pump. The disadvantage is that the torque towards the end of the start is higher.

Best regards,

Thanks for your help marke, I see how it works.

Best regards, Leighc