Autotransformer on large motors (+5000HP)??

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jstrick2

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Hey all,

I was curious if anyone has any experience with large motor soft starts. I am bouncing around a couple of different ideas.

I have a 7000HP motor that I need to get going and I am looking at either a VFD or a RVS autotransformer. I am trying to avoid across the line starting due to bus voltage drops. If I can coordinate a VFD or similar, I can downsize my transformer a bit, which would be nice.

Does anyone have any experience with motors this large?


The main factor would be reliability in this situation. I have an immediate worry with VFD's as these would obviously be VERY expensive VFD's and all of the low voltage experience I have with them has me wary. I cannot seem to find any mean time between failure data on VFD's this large.

I am leaning towards a reduced voltage starter because when I think transformer, I think much higher reliability (however this could be an incorrect assumption). However, I have not seen too many applications with autoformers of this size, and by that I mean none. :D


Any help/time is greatly appreciated, thanks.
 
080501-1504 EST

jstrick2:

No experience, but my off hand judgement would favor the auto-transformer.

From "Alternating-Machinery" by Bailey and Gault, McGraw-Hill, 1951, p 142.
"Reduced-voltage Starting"
"If full rated voltage is applied to the armature of a synchronous motor at rest , it usually takes from five to seven times full-load current from the line."

Then it goes on to show an auto-transformer circuit and mentions 50 to 80 percent of line voltage for starting.

Can you uncouple any mechanical load?

If the motor is a delta design can you start it as a Y and switch to delta?

.
 
jstrick2 said:
Hey all,

I was curious if anyone has any experience with large motor soft starts. I am bouncing around a couple of different ideas.

I have a 7000HP motor that I need to get going and I am looking at either a VFD or a RVS autotransformer. I am trying to avoid across the line starting due to bus voltage drops. If I can coordinate a VFD or similar, I can downsize my transformer a bit, which would be nice.

Does anyone have any experience with motors this large?


The main factor would be reliability in this situation. I have an immediate worry with VFD's as these would obviously be VERY expensive VFD's and all of the low voltage experience I have with them has me wary. I cannot seem to find any mean time between failure data on VFD's this large.

I am leaning towards a reduced voltage starter because when I think transformer, I think much higher reliability (however this could be an incorrect assumption). However, I have not seen too many applications with autoformers of this size, and by that I mean none. :D


Any help/time is greatly appreciated, thanks.
Click to expand...

What voltage?
 
6.9KV, which is actually part of the problem.


13.8KV and I would be fine.



I think the best option may be just a softstart. I dont need to vary any speed, just need the startup.


I am unsure how pricing is to compare softstarts, RVS autoformers, and VFDs (at this sizing). If I had to guess, maybe roughly:

VFD - highest
Autoformer - 1/5 VFD or less
Sofstart - similar to Auto? hopefully slightly less? :D (I have no gauge for this one)
 
Last edited:
you may have to do a start up study first to see what the voltage drop will be during the start up.
Autotransformer may work.
You sacrifice torque but the inrush current drops squared.
The formulas can be found on the internet.
Allow max 10% voltage drop on the SWGR bus during the start.
make sure that the motor can be started at 15% below nominal and calculatew the votlage drop on the cable which should not be an issue
Also the equipment torque curve must be below the motor torque curve during the start.
There are different ways of doing the auto transformer start. Adding one one contactor however makes it safer.

Arda
 
I don't have any experience with soft starters this large, but I just got involved with three similar projects - putting VSD's on 2250HP, 3000HP, and 4000HP motors. I have a couple soft start projects going as well, but on 200HP and 400HP motors. I don't have any figures to offer for comparison purposes, but I will next week. Please let us know what you figure out.
 
arda said:
you may have to do a start up study first to see what the voltage drop will be during the start up.
Autotransformer may work.
You sacrifice torque but the inrush current drops squared.
The formulas can be found on the internet.
Allow max 10% voltage drop on the SWGR bus during the start.
make sure that the motor can be started at 15% below nominal and calculatew the votlage drop on the cable which should not be an issue
Also the equipment torque curve must be below the motor torque curve during the start.
There are different ways of doing the auto transformer start. Adding one one contactor however makes it safer.

Arda
Click to expand...

Thanks for the info, Arda. Only item I noticed is that I believe the current drop on an auto is proportional. I thought there was a squared drop on reactor starting, but not auto. However, I may be mistaken.
 
jdsmith said:
I don't have any experience with soft starters this large, but I just got involved with three similar projects - putting VSD's on 2250HP, 3000HP, and 4000HP motors. I have a couple soft start projects going as well, but on 200HP and 400HP motors. I don't have any figures to offer for comparison purposes, but I will next week. Please let us know what you figure out.
Click to expand...


I will definitely pass along what the solution becomes, a few meetings next week should set that.

Cheers.
 
please find my own application notes attached.
The LINE current and the motor torque and proportional.
that is not too bad:)
 
jstrick2 said:
Thanks for the info, Arda. Only item I noticed is that I believe the current drop on an auto is proportional. I thought there was a squared drop on reactor starting, but not auto. However, I may be mistaken.
Click to expand...

you are right. I meant that the torque and LINE current are proportional as I posted in my own notes.
 
I have lots of experience on starting large MV motors. There are a number of factors to consider;

In order of cost, highest to lowest:
  1. VFD (more with harmonics mitigation)
  2. RVSS - Soild State Soft Starter (at least 1/5th of VFD, up to 1/10th if VFD has harmonics mitigation)
  3. RVAT (slightly less than solid state)*
  4. Reactor starter (slightly less than RVAT)

In order of negative effects on voltage drop (lowest to highest):
  1. VFD, able to start most loads with FLA
  2. RVAT, able to start most loads with 150-300% FLA (line current).
  3. RVSS, typically 250-450% FLA
  4. Reactor Starter, typically 300-350%, but may not start very heavy loads.

In order of negative impact (aside from VD) on power system (most negative to least negative):
  1. VFD, creates severe harmonics that must be dealt with in most cases ($$$, size)
  2. RVSS, slight disturbance, but only during starting, no mitigation usually needed
  3. RVAT and Reactor, slightly less disturbance than RVSS, but again only during starting so no mitigation needed

In order of flexibility to match field conditions (highest to lowest)
  1. VFD, completely programmable
  2. RVSS, completely programmable, within limits
  3. RVAT, can only change taps on the transformer, limited starting time
  4. Reactor, no flexibility at all, it either works or it doesn't.
In order of floor space (smallest to largest):
  1. RVSS and Reactor are typically the same floor space.
  2. RVAT is typically 2-3 times RVSS
  3. VFD is typically 2-3 time RVSS, more if harmonic mitigation is necessary.

* RVATs tend to be more expensive than RVSS in low voltage systems, but are still slightly less in Medium Voltage.
 
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