My company just had a meeting to discuss purchasing a VFD for a 6000 hp, 1783 rpm, 4160V induction motor.  The problem is the motor is too powerful for our process.  It is used a booster motor which when the pressure in the system gets above a certain level, we open a recycle valve and cycle some material back into the system.  Apparently throttling a valve(ball valve) to reduce input to the compressor isn't an option (I don't know why) and this motor is costing us thousands of dollars a month in wasted energy.  The recycle wasn't designed to be run open full time either.

So my question is:  Is it feasible to run a 6000 hp motor on a VFD?  Are there any better options?  Can anyone make a recoomendation on who would make such a monster of a VFD?

There are also variable speed transmissions.  "Voith" comes to mind.

I mention this because my company has decided that for applications under 10,000 HP (7500KW)the variable frequency drive is more economical, but for above that range, the Voith transmission and a synchronous motor is the more economical.  You can run your own numbers, but if you already have the horsepower in place, then that variable speed transmission might be your answer.

I have three of the variable frequency drives mentioned in previous postings, driving motors in the 7000-9000 horsepower range, and I've had three mishaps directly attributable to the drives, but they've been running pretty good for several months now.

From the sounds of your application, a VFD would likely pay for itself in a relatively short time via energy savings. While it's true that a motor only consumes as much power as the load requires, the caveat to that is in the case of throttling systems in variable torque applications. There are energy losses across whatever the throttling system is, and if the motor speed is reduced into that point of the power curve where the losses are greatest, the differences add up to big numbers over time. Mind you, if the motor is going to run full speed all the time, the equation goes the other way because of losses in the VFD, plus if you reduce the motor speed too much the pump/fan/compressor may cease to pump/blow/compress. The "sweet spot" usually works out to be between 40 to 90% speed, but every application is different.

I was recently working on some 5kV chiller systems for large commercial buildings and the energy savings from reducing the motor speed with VFDs resulted in an ROI of 18 months on each system, even though they cost almost $750k each! This may be a bit extreme and again, every application is different, but the savings are real and demonstrable.

Your situation, being 4160V, fits nicely into the bread basket of what manufacturers offer. Not to be repetitive, but ABB, Siemens/Robicon and Allen Bradley (if you are in North America) are also joined by Toshiba, GE and Eaton as major suppliers of MV VFDs. If you are in another continent A-B may be a difficult choice, especially ow that they have pulled in their horns a bit. But you might gain Cegelec, who stopped supporting product in the US years ago but is probably still around elsewhere(?). All of those companies have specialist focussed on MV drive sales and all of them have software programs that can analyze your application to show you an expected ROI.

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