VFD to slow a 6000 hp motor? 4

[TurbineGen]

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 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?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[edison123]

I don't understand what you mean by "the motor is too powerful for our process". The motor just reacts to the load demand from your compressor. So, actually, the compressor is too big for the job.

Having said that, I am looking for the drives aces to chip in on such a monster of a drive.

How about speed reduction by way of V-belts and pulleys ? ... On the other hand, may be not a good idea given the power.

 

[dpc]

You can certainly buy 6000 hp drives and larger. Check with ABB or Siemens/Robicon.

http://www.automation.siemens.com/ld/ac-umrichter/html_76/perfectharmony/perfectharmony-1.html

Whether or not this would actually be a solution or make economic sense, I'll leave to the experts.



"The perfect is the enemy of the good." -- Voltaire

 

[edison123]

BTW, a few days back I was called to look at a failed "VFD rated" transformer in the biggest oil company in India. The set up was

6.6 KV supply => step down to 600 V through this VFD rated trafo => Feed to VFD => Step up VFD output to 6.6 KV through another trafo => Feed 6.6 KV, 540 KW, 3000 RPM motor driving a pump that pumps out the petro-products over long distances (something like 500 kms).

Both the VFD trafos were rated at 710 KVA.

Why not direct 6.6 KV, 540 KW VFD to motor ?

 

[CJCPE]

If the compressor is a centrifugal machine, slowing the VFD should work fine to reduce the flow. If it is a positive displacement machine it will probably work, but you may need to consult the manufacturer.

Manufacturers 4160 volt, 6000 Hp VFDs include ABB, Siemens,Allen-Bradley and others.

 

[oldfieldguy]

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.

old field guy

 

[edison123]

Good point old field guy. Voith is the best there is in fluid drives.

 

[jraef]

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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[TurbineGen]

Thanks for the responses. I'm a little weary about the VFD because this motor is in a hazardous (class 1 Div 2) area and also is not rated for inverter duty. Should I be concerned running this motor on a VFD when it's not rated for it? We'd be running it around 60-80% power (3500-5000 hp). The motor is a Siemens CGII in an 8012 frame.

Should I be considering any features for this drive such as special braking functions or an active front end drive? It has it's own 13.8/4160 transformer. I know VFDs tend to be rich in harmonics, but I don't know if that would matter at all when this unit has it's own transformer.

Oldfieldguy, thanks for the advice on a variable speed transmission. I'll have to look into that.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[itsmoked]

The price on a correctly sized motor should be checked too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

You are correct that a motor with the correct rating on the nameplate for the explosive hazard it is in is still not suitable for variable speed. That nameplate assumes constant speed across the AC line.

To operate a motor variable speed, an additional nameplate is needed stating "PWM power, the speed range in hz, and an absolute current limit". That makes the motor suitable for variable speed usage.

The fact that your existing motor has the XP rating but not the extra variable speed data does not mean the motor must be changed out. Check with the manufacturer. They often can send you a new nameplate with the variable speed data added. That's a lot cheaper than a new motor!

If the manufacturer can not or will not send a new nameplate, do not be tempted to run that motor on a drive. Whoever commissions a drive/motor under those circumstances is, in my opinion, a fool and is putting his reputation, his family, and his customer at huge risk.

Especially with XP, do it right or don't do it!

 

[waross]

I would consider trimming the impeller to reduce flow and pressure. If you need a fixed operating point this may be cheaper by thousands of dollars. If it's a piston type machine, consider capacity reducing unloaders.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

I agree with waross. It's a matter of doing some calcs (pump affinity laws) and you should be able to hit the desired operating point of your pump (if it is a centrifugal pump)!
Please check this out:

http://www.engineeringtoolbox.com/affinity-laws-d_408.html

 

[electricpete]

I think it's a compressor. Sometimes compressors have adjustable vanes or unloader valves or various means to reduce load depending on the design.

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[CJCPE]

Per NEC 501.125(B):
"In Class I, Division 2 locations, the installation of open or nonexplosionproof enclosed motors, such as squirrel-cage induction motors without brushes, switching mechanisms, or similar arc-producing devices, shall be permitted."

However, it is still a good idea to check with the motor manufacturer about the suitability of the motor for VFD use.

 

[jraef]

Just to be the voice of experience here, it would be highly unusual for an engineer, such as whomever designed this system in the first place, to have picked out a motor that is grossly larger than was needed, at least for some part of the process. Often times the reason for the throttling system is that the peak performance is an occasional capacity issue, so for the majority of the time, the reduced capacity is used. Running with a smaller motor or trimmed impeller however can end up eliminating the system's peak response capability. If the process has changed since the original design, that's another story. But if it has not, and you don't know why the extra capacity was installed, using a VFD means that should you suddenly discover the reason some day in the future, you can still adapt to it.

Other issues brought up by TurbineGen;

MV VFDs are a whole different world and provide standard benefits that you normally don't see offered, or are grossly expensive, in Low Voltage drives. Several of the drives mentioned, Robicon in particular, address the harmonics issue very very well, in fact so well that they are specifically known for that ability. Their competitors all have to work hard to meet those performance specs, so the end result is that users benefit from a stable of very good choices from which to select. If you make that a tender requirement, they all have ways to mitigate it as part of the package. That goes for the transformer issue as well by the way.

In the same vein, unlike LV drive / motor systems, the vast majority of MV motor applications involve custom motors that cannot be easily or inexpensively replaced. Therefore the MV VFDs are specifically designed to NOT need special motors.

The Class 1 Div. 2 issue is another matter, but I know for a fact that this has been addressed many many times. I have seen Robicon drives used in a LOT of oil-field equipment, both new and retrofit, so I'm positive this issue is not one that they will be unfamiliar with. The same I'm sure holds true for their other competitors mentioned above (I just have indirect experience only with Robicon through business associates in Alberta Canada, but they were always competing against the others). Again, make it a requirement in your tender offer and it will be addressed.

These things are not off-the-shelf, they are all built to spec. You spec, they build.


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[TurbineGen]

Yes this is a compressor and it was designed to allow for plant expansion. Unfortunately there is no way to throttle it back via vanes or other method. All we have is the recycle. Next meeting I'll ask about ordering a reduced rate compressor, as that might be cheaper. I'll contact Siemens about the VFD duty and ask them about a drive suitable for that motor.

Thanks again

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Drivesrock]

Further to the Medium Voltage drive manufacturers:

Cegelec was incorporated in to Alstom and then the power conversion business which includes motors, drives and industrial automation was spun-off a few years ago and is now called 'Converteam'. You'll find them around the globe and in the US. Their LV and MV drive offerings are quite extensive and the new MV7000 multi-level medium voltage inverter has sold, I understand, many GW of power for a variety of applications from industrial to marine propulsion.

 

[jraef]

Thanks, good to know what happened to Cegelec / Alstom. They were a major player worldwide at one time, but got caught up in the French Government a while ago and seemed to disappear.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376