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5000HP Compression Motor Starting Methods. Design Phase
2

5000HP Compression Motor Starting Methods. Design Phase

5000HP Compression Motor Starting Methods. Design Phase

(OP)
Hello All,

We are in the process of designing an air compressor which uses a 5000HP Induction motor. The data about the motor is attached.

This motor is proposed to be fed from a 15MVA - 69KV/4160V transformer which is inturn supplied from a 125MVA - 230KV/69KV transformer from the UTILITY end. I am sure the 125MVA system is quiet strong for this load but i am concerned if the 15MVA transformer would be able to start it.

The electrical head from the contractor is claiming that the motor is started in the unloaded condition and that this motor can start Direct online without any problems.

Is that possible. Would this 15MVA transformer be able to handle the initial inrush of the 5000HP motor? Isnt that sized small? The contractor dont want to spend the money on a VFD and the motor which is already procurred is not insulated enough to be able to run on a soft starter.
What would a solution here.

Please comment

Thanks

gokul


 

RE: 5000HP Compression Motor Starting Methods. Design Phase

If it can start unloaded you may be able to bring it up to speed with a VFD on a smaller motor.

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

RE: 5000HP Compression Motor Starting Methods. Design Phase

At 4.16 KV, the stator winding will be form wound and probably VPI treated too. So, it can withstand VFD or soft start application.

I have just seen a 6.6 KV, 6.65 MW, 4 pole O2 compressor motor running off a 132/6.6 KV, 18 MVA trafo with a soft starter. I don't think  your DOL start (even with an unloaded compressor) with a 15 MVA trafo will work.

Muthu
www.edison.co.in

RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

The electrical head from the contractor is claiming that the motor is started in the unloaded condition and that this motor can start Direct online without any problems.
If you have motor characteristics (thermal limit curve, torque speed curve, (current vs speed curve at selected voltages) and power system characterisitics, this is a fairly standard calculation.  Results are often presented in graphical form (actual current vs time which should have margin below thermal liit curve). Has it been done?

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RE: 5000HP Compression Motor Starting Methods. Design Phase

I was addressing above whether the motor can handle the start. Another aspect is how the voltage transient would affect any loads fed from same transformer as I'm sure you already know.

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RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

... and the motor ... is not insulated enough to be able to run on a soft starter.
Eroneous statement, undoubtedly spawned from his ignorance of the technology and unwillingness to spend even a slight bit more than the cheapest thing he can get away with. It never ceases to amaze me that someone will spend that much on a machine, motor and power system, then at the last decision point go for the cheapest way out of controlling and protecting it. I liken it to people who buy expensive cars, then buy the cheapest gas they can find at the local mini-mart and drive them 10,000 miles without changing the oil, yetlater complain about the repair costs of expensive cars...

There is nothing about "insulated enough" that bears any meaning to using a soft starter! Starting with a soft starter puts no more voltage stress on the winding insulation that DOL starting, in fact arguably it may even be less. And most MV soft starters on the market now have built-in motor protection that can rival even the best Motor Protection Relays, saving on having to add those. When doing an initial cost analysis, always look at the whole picture.

Maybe a VFD could be considered more stressful to the insulation, but if you investigated, even then it's pretty much a non-issue now. Most MV VFDs have far superior filtering as a matter of course compared to low voltage versions. And it's the same thing; it just makes no sense to go that far and stop short of doing it right, so most manufacturers include it for MV applications.

I have applied and commissioned probably over 100 MV soft starters and a dozen or so MV VFDs in my career. The "rule-of-thumb" that I have been very successful with based on that experience is that unless you can accept a serious voltage drop (+15%), the supply transformer kVA must be at least 2.5X the motor HP for DOL starting, 1.5X for Soft Starting, and if you cannot do either, a VFD can usually start with as little as 100% FLC. So based upon that rule, I concur that you will not be successful with a 15MVA transformer doing a DOL start of a 5000HP motor, but a soft starter will work. No need for a VFD unless you can also benefit from speed control or something else a VFD brings to the party.

A proper TMS (Transient Motor Starting) software analysis with something like SKM or ETAP would be the definitive way to predict success and get an idea of the VD it will cause, and I highly recommend hiring someone to do that before purchasing any starting equipment. But I have yet to be proven wrong using my thumbnail method as a quick way to determine where to start looking.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

...the supply transformer kVA must be at least 2.5X the motor HP for DOL starting...

5000HP
x2.5
=
12,500kVA

Proposed transformer is 15 MVA?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 5000HP Compression Motor Starting Methods. Design Phase


Don't forget the transformer impedance. If the impedance of the 15 MVA transformer is low, the resulting voltage drop is low as well.

Wolf
www.hydropower-consult.com

RE: 5000HP Compression Motor Starting Methods. Design Phase

(OP)
Thank you all for your valuable inputs....

I'm not sure what the guy meant when he said the motor is not insulated for soft starter application but guessing that using a soft starter would have required more time to starting and thus was out of the damage curve of the motor at some point... But it is a reality that the motor manufacturer has given this feedback that a soft starter cannot be used.

Now if 2.5 times MVA of motor would be a rule of thumb then the 15 MVA transformer is sized properly......

We are requesting the contractor to hire an engineering firm to do the study and provide results which he accepted for.... Will let you guys know the results once I recieve it...

Again thanks for all your inputs......

Gokul

RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote (ScottyUK):

5000HP
x2.5
=
12,500kVA

Proposed transformer is 15 MVA?
Oh my... serious math brain fart... forgive me everyone...


"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: Eng-Tips.com Forum Policies  

RE: 5000HP Compression Motor Starting Methods. Design Phase

Well you got me.. I don't see how:

2.5 x 5000 x 746 / 1000 comes anywhere near 12,500.


Assumed PF?

Keith Cress
kcress - http://www.flaminsystems.com

RE: 5000HP Compression Motor Starting Methods. Design Phase

Keith,

Re-read Jeff's rule of thumb and forgive the horrible mixing of units.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 5000HP Compression Motor Starting Methods. Design Phase

Hi Keith. If you take the rated full load current of the motor which includes the overall efficiency including PF and adjust by 480/460 (Motors are rated at 460V, Transformer KVA is rated at 480V.) you get within 3% to 5% of 1 KVA per HP.

 

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

RE: 5000HP Compression Motor Starting Methods. Design Phase

Oh Kool.  Thought it was something like 1kVA/hp.

Thanks Bill.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 5000HP Compression Motor Starting Methods. Design Phase

gokulkrish

I just found out that the "soft start" of a 6.65 MW motor I had mentioned earlier was not an electronic SS but is actually a 3 phase inductor in series with the stator winding (for which you need the 3 neutral ends of the winding to be brought out). This reduced the motor voltage and the inrush current and when the motor gets up to speed, those inductors are bypassed.

Muthu
www.edison.co.in

RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

I'm not sure what the guy meant when he said the motor is not insulated for soft starter application but guessing that using a soft starter would have required more time to starting and thus was out of the damage curve of the motor at some point... But it is a reality that the motor manufacturer has given this feedback that a soft starter cannot be used.
I know of no reason why any particular motor design is not suitable for a soft start. The time / damage curve issue is often misunderstood. The energy it takes to start a motor from a dead stop is always the same regardless of the method. Soft starting puts no more stress on the motor that DOL. Where people get confused is when they fail to understand that the damage curve, i.e. the starting time, is based on DOL current. But with a soft starter you are using less current so the time is extended. In fact, one could argue that extending the time can allow a more even distribution of heat in the rotor and hot spots in the stator are mitigated, so soft starting may actually be better from that standpoint.

But if your motor supplier is willing to stand by his ignorance, there's not much you can do about that.


"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: Eng-Tips.com Forum Policies  

RE: 5000HP Compression Motor Starting Methods. Design Phase

jraef,

A soft starter can put more stress on a motor than DOL if the starting torque is too low to get the load up to speed in a reasonable time.  For example, if the load requires 50% rated motor torque to get up to full speed, setting the soft just above this level will extend the startup time by several times that of the DOL method, while current output might be about 70% of the DOL method.  In this case the motor will be much hotter after startup than with a DOL starter.

The motor will also experience more heating and insulation stress when starting due to harmonics of the voltage output, but if a bypass contactor is used after startup these effects will be minimal.

RE: 5000HP Compression Motor Starting Methods. Design Phase

I agree with X49, in the case that load torque is applied during start (not the case for the original post), as you decrease the terminal voltage, the total heat added to the rotor (integrated over the duration of the start) increases.  In the limit that the motor torque drops to load torque at some speed, the total heating goes to infinity (trip obviously occurs)
 

=====================================
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RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

A soft starter can put more stress on a motor than DOL if the starting torque is too low to get the load up to speed in a reasonable time.
I understand where you are coming from and this is the most common argument made. But think it through a little more; if the reduced starting torque that results from using a soft starter is insufficient to accelerate the load, then the starter application was not valid in the first place! One cannot make an argument that a particular technology is going to damage something else by using an example of misapplication.

Soft starters are meant to be used when the motor starting torque exceeds the load accelerating torque requirements, which covers a lot of applications. But it does not mean every application is suitable and if the reduced torque can't do the job, don't use a soft starter!


"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: Eng-Tips.com Forum Policies  

RE: 5000HP Compression Motor Starting Methods. Design Phase

Quote:

A soft starter can put more stress on a motor than DOL if the starting torque is too low to get the load up to speed in a reasonable time.

This could be true for any reduced-voltage starting method, not just a soft starter.  

David Castor
www.cvoes.com

RE: 5000HP Compression Motor Starting Methods. Design Phase

Jeff - I'm sure you know much more about soft starting than me.  (We have only a handful at our plant and only because they were supplied by a vendor... not needed at a power plant).  But the discussion does not sound quite balanced to me. Please consider my comments below.
Respectfully

Quote:

I know of no reason why any particular motor design is not suitable for a soft start. The time / damage curve issue is often misunderstood. The energy it takes to start a motor from a dead stop is always the same regardless of the method. Soft starting puts no more stress on the motor that DOL. Where people get confused is ....
In the context of this thread (unloaded start) it is true.   However this logic would be incorrect if applied to loaded start.  The heat energy dissipated in the motor while going from rest to full speed  increases as voltage decreases if load torque is applied during the start.   I can post the math proof (which is also a claculation recipe) if anyone is interested.

Quote:

But think it through a little more; if the reduced starting torque that results from using a soft starter is insufficient to accelerate the load, then the starter application was not valid in the first place!
It sounds like circular logic to me.   How do you know what is insufficient (*) until you or the OEM has done the calcs.   NEMA MG-1 says starting the motor must safely start the specified load at 90% terminal voltage.  Most people specify 80% terminal voltage (we do). If it's not specified, you shouldn't bet on it imo.  (* checking for safe start involves not just checking load torque remains below reduced motor torque, but also plotting simluated starting time/current curve against motor thermal limit curve and verifying enough room to fit protection curve in between).

Maybe it is ok for most applications and I gather it is usually that way in your experience.  The troublesome ones would by large motors with heavy load during start.   At our plant we have 8000 hp 1200rpm motors driving single stage radial flow pumps located in a fluid loop that has no valves...  must be started under equivalent of "valve wide open" conditions.  It also has flywheel and takes 12 seconds to start at 100% voltage and 21 seconds to start at 80% voltage.   The total heating during that 20-second 80% voltage start is 140% of the total heating added during the 12 second 100% voltage start.   We also have 3500hp 324rpm motors driving single stage axial flow pumps that must be started against closed valve for water hammer prevention... Closed valve means maximum starting load torque for these pumps.  The time current curves on log/log scale come much closer to the protection for 80% start for these curves.  Of course "close" is a little ambigous on a time current curve especially log scale.

Another question: why would NEMA even bother to specify a minimum voltage for starting if it is somehow irrelevant to motor starting performance ?   

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RE: 5000HP Compression Motor Starting Methods. Design Phase

I think all these contradicting comments all boils down to the differences in motor application and types of motors! jraef maybe talking about a different scenario, and maybe epete's case is another application/ scenario.
Heat developed in the motor does not always result to high temp (which is a problem). The ability of a certain motor to cool-off is one factor. Motor speed affects the temp rise as most motors have vent fans driven by motor shaft in the same way as the method of starting employed. The point I see is that the posters ahead of me explained their cases very well, and these are very helpful in finding solutions to design problems. Kudos to all.

RE: 5000HP Compression Motor Starting Methods. Design Phase

Thanks Keith. I agree 100% it depends on application.  That was in fact my point. I interpreted (misinterpretted?) some previous comments to be offered as general rules... whereas I thought clarification of the assumption/application (no-load start) would be helpful.

At the risk of beating a dead horse, I think Richard Nailen has expressed similar viewpoint in Electrical Apparatus magazine:

http://findarticles.com/p/articles/mi_qa3726/is_200603/ai_n17176681/

Quote (Richard Nailen):

One might suppose that a reduced-voltage or "soft" start would ease the thermal stress on an accelerating motor. After all, heating within the motor can only result from current flow. That's the only energy source during acceleration. Lower current should mean less heat-doesn't that follow from the I^2*R relationship?

That simplistic view fails to take into account another relationship that is more important here: the rise in internal motor temperature is caused by total heat input, which involves time, and is proportional to I^2*t, not I^2*R. When motor voltage is reduced during starting, the associated torque reduction causes acceleration time to increase. The consequent increase in heat developed within the rotor and stator may far outweigh any reduction resulting from reduced accelerating current.

Some argue that reduced voltage spreads out heat generation over a longer time, thereby allowing more of the heat to dissipate without raising motor temperature. That's true-but heat transfer to the surroundings is seldom rapid enough to offset the increased heat production.
I wouldn't go as far as he does in his last paragraph ("seldom").  I suspect as Jeff suggests most motor applications benefit thermally from soft start vs DOL start.  And there is no question for some high inertia, low-torque load such as centrifuge that it is a huge benefit.  My spin is just that there are competing effects when load is present (increased total heating disadvantage vs increased dissipation time advantage), so softstart is not universally beneficial in terms of motor thermal performance.   Ideally we'd like to apply motors based on specifications, calculations,  or thumbrules that demonstrate suitability.  As far as I can tell soft start application is beyond specification requirements for most motors.   If there is some thumbrule that allows easy evaluation of whether a soft start will adversely affect the thermal performance for loaded start in a given application, I'd be interested to hear it.

Sorry if I am taking the thread a little off-track.   
 

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RE: 5000HP Compression Motor Starting Methods. Design Phase

(OP)
edison,

Thanks for the update. I have seen similar kind of inline reactors used with softstarters and bypass switches. But those are for capacitance issues of long cables.

Also,

Is someone here from a motor manufacturing company. Do you guys think it would be worthwhile to call those guys and ask for an explanation of why a soft starter cannot be used for the motor we purchased? would i be welcomed with a question like that?

I have had good and bad times with US motors in different situations. It all depends on who u get when u dial that 800 number :)

gokul

RE: 5000HP Compression Motor Starting Methods. Design Phase


Gokul:

Whether you are welcomed or not: You as a user/customer are entitled to ask questions regarding the application of products you've purchased. If you are not welcomed, you have the option to change the OEM for your next procurement.

Pete and jraef made valuable contributions. All main aspects were covered and on your behalf I give a star to both gentlemen.

Wolf
www.hydropower-consult.com

RE: 5000HP Compression Motor Starting Methods. Design Phase

Thanks Wolf I appreciate it, although I am aware I am not contributing to original poster's question.  It was just an interesting sidetrack that we have discussed before that I wanted to follow up.

As a matter of clarification of my earlier comments:  my pumps examples were not good examples since they are centrifugal-like pumps.  Even though valve position was not optimal, they still have roughly Torque^speed^2 torque-speed curves.   Most of heat is added at lower end of the speed range so centrifugal pump curves are relatively forgiving in this respect since load torque does not slow them much accelerating through lower speed range.

In contrast certain compressor torque speed curves can be more challenging.  Example linked at bottom of page 2/47 and top of page 2/48 below shows evaluation of a compressor which turns out not suitable for starting even at 80% voltage (even though motor torque remains above load torque throughout)

http://books.google.com/books?id=V1SXEnhCN88C&pg=SA2-PA47&dq=This+motor+is+therefore+suitable+for+only+one+cold+or+one+hot&hl=en&ei=9_RFTNruLsG78gbnsYDWBA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCgQ6AEwAA#v=onepage&q=This%20motor%20is%20therefore%20suitable%20for%20only%20one%20cold%20or%20one%20hot&f=false

I'm done now winky smile

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RE: 5000HP Compression Motor Starting Methods. Design Phase

I would say with a full voltage start and a transformer impedance of 5% you could see around a 10% voltage drop. This does depends on the transformer impedance. I would hope for <5% as opposed to an impedance around 7% to 10% I sometimes see.

I would question them about reduced voltage starting if reduced voltage starting is a route you want to persue. An unloaded compressor should be an easy soft-starter application. The only time it could be an issue is if the motor was specifically designed for full-voltage starting with your source and load, but I doubt you have this case. I have seen cases where the motor had atypical torque and current curves which matched very well to it's application.

In theory, the I^2*t heating of the motor is always increased when using a soft-starter or reduced voltage starter. The increase is minimal if the torque is not reduced so much that the motor comes close to stalling. But the heating will become excessive if you lower the voltage so much the motor and load torques almost meet. If the motor actually suffering or not from an increased heating over a longer period of time is the debateable part.

 

RE: 5000HP Compression Motor Starting Methods. Design Phase

Gokul - The inductive reactors in the case I cited are specifically for starting purpose and not for some cable capacitance issues. The system has been working very well since it was installed about 12 years back.

Muthu
www.edison.co.in

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