Soft Motor Starting
Soft Motor Starting
(OP)
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.






RE: Soft Motor Starting
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
RE: Soft Motor Starting
RE: Soft Motor Starting
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
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RE: Soft Motor Starting
Keith Cress
kcress - http://www.flaminsystems.com
RE: Soft Motor Starting
RE: Soft Motor Starting
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.
RE: Soft Motor Starting
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: Eng-Tips.com Forum Policies
RE: Soft Motor Starting
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.