Soft Motor Starting 3

[kingi]

When the start of a 250HP 480Volt pump motor is limited to 350KVA soft start, what would be the starting current? Should I calculate this current as the current for a 350KVAR inductive load, or there is a power factor involved? For how long would this current last? I want to select a fuse to protect this motor at the 12.5kV side of a 300KVA transformer.

 

[itsmoked]

Generally you size the motor protection in the normal way you would for the motor at load since that is the 'continuous' running current.

The soft start is then adjusted to a limiting current needed to assure that the motor will start. Typical settings are 200% to 300% of running current. I've not seen pf even come into it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[kingi]

Thank you for the answer. Would you care to comment how long this reduced starting current will exist at rated motor load?

 

[jraef]

Very difficult to determine without a lot more information. It is almost totally dependent upon the load torque requirements and that is difficult to ascertain unless the pump supplier has already calculated it for you, making sure they included the motor inertia in their calcs. If you can get that, then you need to plot the load torque-speed requirement curve against the motor torque-speed output curve at the reduced voltage to ensure that they don't cross, then you can apply the basic acceleration formula to it.

There are software packages that will do this for you, they are typically called "Transient Motor Starting (TMS) Analysis" reports. SKM and ETAP are the best known. Another alternative is available from one of our forum members, Marke, at his website

http://www.lmphotonics.com/busbarexe.htm.

Not quite as comprehensive because it doesn't relate everything back to the system bus in terms of voltage drop, but in your case you have already been told what your maximum limit is, so you should be able to make it work. Besides, it costs thousands of dollars less!

It still needs the motor and load torque-speed curves though. If you download the manual and go to the section on Motor Starting, it shows you screen shots of the type of data you will need.

http://www.lmphotonics.com/busbar.pdf

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[itsmoked]

The short unqualified answer is about 10~15 seconds on a motor of that size. I recently did a 50HP pump motor on an SS. We had no real starting issues to worry about but we didn't want a big voltage dip. I set the SS to 280% limit and it starts in about 6 seconds. Probably a little more than double the time of a DOL start in my case. Call it 2.5x longer than DOL.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[kingi]

Thank you "jraef" and "itsmoked" for the valuable answers. I think they have a problem with the 350 KVA soft start limit, because the company observed about 900 Amp staring current. Of course this might be due to the motor trying to drive a stuck pump, and the rotor was completely stopped until the protecting fuse melted. Had it been a successful start, the starting current probably would have been limited to the equivalent current for a 350 KVA load.

 

[LionelHutz]

The big problem I see with your question is that you should not be selecting a fuse on the primary side of a 12.5kV:480V transformer to protect a motor connected on the secondary side. There really is no fuse only solution available at 12.5kV to do what you are suggesting. I'm not even sure a fuse and protection relay on the 12.5kV side can do what you suggest but that combination would at least have a chance of working.

I actually was just searching for some pictures I had of a motor control cabinet that is completely burned down because of doing what you are suggesting. I couldn't find the pictures though.

It would appear the soft-start is not limiting the power like you want. 900A is about 750kVA. If you want 350kVA then limit the starter to 420A. However, I would not count on the motor starting because I typically see 2-2.5 times FLA current being the minimum required to start a motor and pump.

 

[jraef]

I agree with Lionel on all points, especially about the line side protection only. In fact I think the transformer is probably too small as well (sorry for not picking up on that earlier). The "quick and dirty" rule I use for sizing transformers to accommodate motor starting is for the xfmr kVA to be 2.5-3X the motor HP for Across-the-Line starting, and at least 1.5X the HP for soft starting. So by that rule, the minimum transformer size would be at least 375kVA, taking nothing into account in the load profile. So what will happen is that you will bury that transformer and get a severe voltage drop above and beyond what the soft starter will do. If it continues without accelerating the motor, the soft starter may end up going to full voltage and then you have lost control of the current, which eventually becomes a severe spike as the transformer saturates, which pops the fuses (if you are lucky). I have seen this several times.

I too have never seen a pump start with less than 2X FLA, so that will be around 600A at some point in the acceleration curve. I'm not sure your 300kVA transformer is capable of delivering that for long enough. Your only viable option for accelerating that motor with that kind of kVA restriction behind that transformer will be a VFD or maybe a pony motor. If you didn't have the kVA restriction you might be able to allow the starter to go ahead and bury the transformer, letting it become essentially a saturating reactor during start. But that causes a voltage drop on the primary side which is probably what the utility wants to avoid by restricting your kVA.


"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

 

[kingi]

Thank you again Lionel and jraef. I didn't receive the necessary information on motor protection. It is obvious that the motor protection should be on the 480 volt side. I assumed the motor was protected because the utility's problem was fuse coordination on the 12.5 kV side. The 300 KVA transformer’s 20 A fuse survived, but the 10 A fuse upstream the feeder melted.

I am familiar with motor protection, but this wasn't a motor protection problem. I set motor protection relays but there was substantial power supply available to run the motors. Those motors started across-the-line, and only protecting the motors from overheating, short circuits, unbalanced currents etc. had to be provided. Transformer size was never a problem, and thank you that this was brought to my attention. I will have to learn more about soft motor starting.

If they can limit the motor current to not more than 2 times FLA (560 amp), a 10 A transformer fuse on the 12.5 kV side will survive this current for 20-25 seconds, and the utility's problem is solved when they use a larger fuse upstream the feeder.