VFD Driven Helper Motor?

[Laplacian]

Background:
We have a 3500HP extruder motor that has been re-rated with a 1.40 service factor. The stator windings run cool, but is rotor limited beyond 1.40 loading.

A project is coming along to marginally increase the rates this machine must run. 10% increase in loading is expected after the project.

Management wants to couple a 350HP motor to the non-drive end of the 3500HP motor (this motor has a shaft extension on the NDE) fed from a VFD to boost HP of the application.

Question:
Is this possible from a purely technical standpoint?


The VFD would be set to run at a fixed speed around 70Hz (60Hz mains) with a current/torque limitation set to 100%. There would be no deceleration requirement for the VFD; it would always try to accelerate to an impossible speed target given the torque limitation constraint. I know the VFD can catch an already spinning motor and accelerate it. The 3500HP motor would already be running and fully loaded before running the VFD.

Management justification is that the 350HP motor and VFD is << than new a 5000HP motor and starter from a cost perspective.

Any input is welcome and appreciated. I know this is not an ideal setup. This is a cost saving measure to keep production in North America.

 

[CJCPE]

A VFD with good torque limit performance should be able to serve as a helper drive as you describe. However, I would recommend a sensorless vector VFD that is equipped with a torque regulating function. A drive of that type would be specifically designed for the application that you describe.

 

[itsmoked]

I would ask the big motor's maker about an additional 350HP running thru the shaft. They will probably say "no problem" or "it cannot work". Either answer would be helpful.

It sounds like a great idea and could probably be used to increase the life of the big motor on lower loadings.

What about using the 350 as a starter motor for the big one? Big one on a VFD?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

I think it would be plausible, but Keith's point is right on. The motor shaft was designed to handle a certain amount of torque. This motor has already been "re-rated" by 40% from that original design, now you want to add even more! When the original motor shaft was designed, that engineer probably had a "fudge factor" in the shaft strength. The first re-rate may have already consumed that, leaving you fudgeless or even exceeded it, leaving you in a negative fudge situation.

And in the immortal words of E.L. Fudge, "Fudgeless is not a good thing and negative fudge is just a fudging disaster waiting to happen".

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

I bet "cost saving"... is not a twisted off output shaft on a 3500HP anything..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

Thank you for your comments.

There is no VFD on the 3500HP motor. The 350HP motor will not have enough starting torque to move the load.

The 3500HP motor has a breakdown torque of 260%. I should not even start to use any fudge until this point. Even with the helper motor at full load, this is 150% net loading on the 3500HP motor.

 

[waross]

If the %torque/speed characteristics of the motors are similar, you may be able to go direct on-line with each motor and save the price of a VFD.
If the %torque/speed curves of the motors are not a good match into the moderate overload range, forget this idea.
respectfully

 

[cbarn24050]

Hi, over the years Iv'e seen many attempts to uprate extruders and it allways leads to unreliability. Even when you get it to work something expensive will break sometime soon.

 

[Skogsgurra]

Yes, cbarn has a valid point. The screw or the tool will probably wear a lot quicker than before. The shaft is designed to take the starting torque for short periods. It may or may not be able to take the same torque continuously. It is something the motor manufacturer should decide. Otherwise (from motor/drive standpoint) the idea is sound.

Gunnar Englund

http://www.gke.org

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