5000HP Compression Motor Starting Methods. Design Phase 2

[gokulkrish2]

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

 

[waross]

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

 

[edison123]

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

http://www.edison.co.in

 

[electricpete]

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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[electricpete]

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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[jraef]

... 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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[ScottyUK]

...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?


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

 

[wolf39]

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

http://www.hydropower-consult.com

 

[gokulkrish2]

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

 

[jraef]

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
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[itsmoked]

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

 

[ScottyUK]

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!

 

[waross]

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

 

[itsmoked]

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

Thanks Bill.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

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

http://www.edison.co.in

 

[jraef]

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.


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[X49]

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.

 

[electricpete]

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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[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.

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

 

[dpc]

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

http://www.cvoes.com