VFD and Soft Starter

[Eduardo Maun]

One of our project they did not put a VFD instead a Soft starter directly connected to the motor. In other station they provide both a Soft starter and then a VFD connected to the motor, I'm not sure if this was requested by the client and part of the contract, But the station are just the same the difference is one station have Soft Starter and VFD the other is Soft starter only. With safety concern is it more safe having a Soft starter and VFD? What is the difference of having a soft starter with a VFD?

Thank you.

God Bless...

 

[ron]

I would imagine that a VFD would not like to be fed from a softstarter. You should not have both.

 

[Besoeker]

I would imagine that a VFD would not like to be fed from a softstarter. You should not have both.

Maybe it is a main/standby arrangement with the motor being fed by the VFD or the Softstarter.

 

[Jraef]

Maybe it is a main/standby arrangement with the motor being fed by the VFD or the Softstarter.

That is the most likely scenario. It could be that in the one station with the soft starter only, it will always run at full speed. If in the other there is a need to vary the speed on a normal basis, but if the VFD fails, or the pump must run at full speed for a long time, they want to switch to using the soft starter.

If a VFD is run at full speed, there is an energy loss associated with it; typically around 3% or more. If a soft starter is bypassed when ramping is complete, there is no more energy loss than f it were run across the line. So if they have a VFD and they want the ability to save that 3% if the motor has to run for a long time at full speed, they will either select the Soft Starter to begin with, or change over to it after a period of time on the VFD. In that last case, since the VFD and Soft Starter cannot coexist on the circuit at the same time, you must have isolation contactors separating them. When you open up bth contactors for even a second while transfering power from one to the other, the motor slows down slightly and there s a chance for a serious spike. Using the soft starter can prevent that.