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.

 

[SamuelB]

Application is wood chipper. Lots of load swings. But nothing special.

The whys for VFD:
Motors are identical, but if one fails or needs maintenance, they will replace with whatever is in the shelf.
The chipper may have a heavy or jammed start every now and then. They will not care how many starts per hour the SS/contactor does.

Contactor cost is 20. SS is 30. VFD is 100. The system has a PLC and we can program the load share.

300kW/380V/550A/4pole motors. Yaskawa GA800 720A VFD. SSW900 820A SS. Profinet Siemens PLC.

 

[LionelHutz]

Soft-starter doesn't care, it will start with the spinning motor no problem.

The VFD would have to be setup for flying start if you start it while the motor is still spinning.

 

[waross]

Application is wood chipper. Lots of load swings. But nothing special.

The whys for VFD:
Motors are identical, but if one fails or needs maintenance, they will replace with whatever is in the shelf.

Not quite so.
A properly designed chipper will have a high inertia flywheel and a design "D" motor.
A normal, design "B" motor on the same shaft as a design "D" motor may be at 100% full load with as little as 20% or 30% load on the design "D" motor.
Torque sharing will be challenging, particularly for the frequent short time overloads that chippers are prone to suffer.

I would probably set a VFD driving a design "B" motor at 100% torque limit and run the design "D" motor DOL.
The design "B" motor will "HOG" the load until it reaches the torque limit at 100% of rated load.
Once the design "B" motor is at 100% the design "D" motor will come from behind and pick up any further load increases.
When the load increases beyond 100% on both motors.
The design "B" motor has been hogging the load and will be hotter and less able to acept added heat.
The design "D" motor, with the help of the flywheel, will pick up all of overload.
The design "D" motor has been loafing and will be cooler and more able to accept added heat.

The simplest way to set it up is to set the VFD for torque limit and use it for starting.
Once up to speed, go DOL on both design "D" motors.
If a design "B" motor must be switched in, drive it with the VFD on torque limit and leave the design "D" motor on DOL.
If someone insisted on full load sharing between a design "D" and a design "B", I may be able to do it.
More likely, I would say;
"Go for it", and quietly slip out the door.

I value your comments, jraef.