Starting and load sharing between VFD and soft-starter

[SamuelB]

Two 300kW motors on same shaft. One with soft-stater. Another with VFD. The intention is to start #1 with VFD and then #2 with soft-starter.

Questions:
Are there any issues flying start a soft-starter? It can ramp voltage and/or current.
Load share will be done by limiting current or torque on VFD (vector control) based on soft-stater current. Any recommendations?
If the start sequence would be soft-starter and then VFD, would flying start function be mandatory for VFD in case of vector control? What if current limit was used as a ramp?

 

[waross]

If you can come up to speed on the VFD you may be able to go DOL on the second motor.
I hope that the motors are identical or at least have the same rated full load speed.
When both motors are up to speed, close bypass contactors on both the VFD and the soft start.
The motors will sort out the load sharing.

 

[jraef]

The way load sharing is usually done with two VFDs is to make one of them a speed follower, the the second drive is set as a torque follower to the first drive. In your case you have no control of the one on the soft starter so there will be no speed control, only torque control on the one that is on a VFD. So if you feel the need for actual torque control of the second motor, you will need an ACCURATE torque value from the started motor. I know of one brand that provides that data (AB SMC-50) , but I don’t have knowledge of the accuracy of that data. How close would you need to be?

Since you can’t do speed control, if the purpose of the VFD is to just gain some accuracy of the torque sharing, the cost to do this may not be worth it. As Bill said, you might just want to soft start one, go across the line on the other, and let them fight it out.

 

[waross]

If one is a 1760 RPM and the other a 1740 RPM, you will need a VFD to balance the torque.
I would look for a circuit that monitored the line current of both motors and adjusted the VFD to equalize the line currents.