Running Two 3500HP Induction Motors for Load Sharing? 4

[Laplacian]

For a hypothetical situation, would it be possible to connect two 3500HP induction motors (drive end into non-drive end on dual shafted motors) both with identical nameplate ratings to load share a 7000HP load?

Assume the final output shaft can handle the torque requirements. I've heard of this being done on certain applications, but smaller motors were used (500HP).

 

[Warpspeed]

I rather liked it too, it is the demonstration of an honest and sincere thinker.

 

[itsmoked]

I was amazed by the full multi-sided conversation... held.. by himself!

Multiple personalities discussing a problem.. A potent, effective mutation.

But I didn't like the "start one then the other" conclusion arrived at after the starting inertia point.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

Thank you guys for your valuable input. It is so nice to bounce ideas around in a forum with so much knowledge available.

 

[DaveScott]

What's the overload rating of the motors, and what is the running characteristic of the load (extruder)?

For example, a rolling mill motor will have up to 250% frequent overload rating. The shaft and coupling must be able to take this, with a safety factor.

I would suggest that you consider starting by using only the motor which is nearest to the load, so that you limit the starting torque that is applied to the final motor coupling.

 

[ozmosis]

Just a general question: is it typical to start a 7000hp motor DOL(or 2 x 3500hp), even on a 13.2kV network?

 

[Skogsgurra]

Yes. If the grid isn't too weak.

Gunnar Englund

http://www.gke.org

 

[Laplacian]

DaveScott,

There are multiple levels of overload protection for the load. First, the controlling PLC trips the entire process if load is above 125% instantly. Second, there is a Desch friction coupling between the motor and gearbox that will break loose at 150% torque instantly, also tripping the process.
The motor overloads are much slower, but are typical of class 10 curves. The motors have a 1.15SF and the load is smooth and easy to control.

sed2developer,

Yes, it is typical in a large ground based industrial facility. For smaller plants, offshore, or temporary sites, not so typical.

 

[electricpete]

I’m glad to provide some entertainment.

Good point Keith – there are disadvantages of starting motors sequentially as well (much tougher starting duty for the first motor). Whatever the starting approach, it needs to be determined up front and associated requirements incorporated into the moto specification.

It sounds like the coupling will provide some mechanical protection for locked rotor condition during starting. (By the way that scenario is not all that remote - we had a 7000hp 2-pole motor that bound and tripped during starting after having been idle for a long period without manual rotation.)

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[DaveScott]

Laplacian,
It looks like you don't have much to work with. I assume that you aim to replace the friction coupling, because it won't take the torque from two motors?
The motors themselves are only designed for 115% short term, but nowhere near 200%, which is what the shaft must transmit.

I don't know the mechanical layout, which I suppose must be such that the motors can be placed in series. Have you considered putting the motors in parallel, with a gearbox? You could use two pinions with one gear (Perhaps the gearbox would cost more than a replacement motor!). Just a thought.

 

[FordF350]

I used to work at a fossil fired power plant at a large government owned utility which had a start up boiler feed water pump on a 500MW unit using this configuration. Two 3500HP motors were connected in tandem to the pump. The pump it self was only about one half the physical size of one of the motors. The motors were 4160V. This unit was built in ~1970. The unit aux. system was weak. While I was there one of the two unit auxilary xfmrs failed a second time during starting of this pump.

 

[aolalde]

In Florida at least two different fossil plants have two 3500 HP, 3580 rpm induction motors coupled in tandem to move a 7000 bhp boiler feed water pump. The motors are started one at a time under no load and engaged later to the load by a fluid clutch drive.
To share the load both motors must have a close speed-torque curve, for that reason the manufacturer should know the application to provide the matching curves, shaft extensions and strength to handle the torque.