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Motor Starting

Motor Starting

Motor Starting

(OP)
We have LV motors on 12.47kV utilty. Motors started at no load. Is 10% voltage dip at utility bus generally acceptable when motors are started?

Also if soft starter is used with ramp set to 10 seconds and inrush current limited to 400%, the voltage dip at utility bus is improved by only 2%. Is it worth it to use soft starters in this case?

RE: Motor Starting

The motor could care less either way as long as it starts. What does the site and utility think about those voltage drop levels?

RE: Motor Starting

12.47kV indicates you are in North America. If you have that much of a drop at the primary side of the transformer, you likely are at the end of a long transmission line, or you are on too small of a feed for your connected load. If you have no neighbors connected to that feed, it could be as LionelHutz mentioned that nobody will care if you don't. But if there are others on that same feed, 10% is well beyond acceptable limits of voltage drop. You may also want to add power conditioning or UPS systems to any sensitive electronic loads in your facility.

As to the soft starters being worth it for a 2% improvement, in my opinion soft starters are "worth it" for a variety of reasons beyond reducing voltage drop. But 8% VD is still beyond acceptable if that is your only consideration. However, will your motors start at 350% or maybe even 300% and possibly bring that down to 5% or less? In your situation, turn the ramp time to zero and just go into current limit immediately, ramping will not help you with this situation. Do you already have the soft starters or are you modeling this in SKM or ETAP? If modeling it, just base it on the current limit, forget the ramp time.

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RE: Motor Starting

In my opinion 10% could be permissible only if it is measured at low voltage terminal of your other motors which are already in operation.
In any case 5% it is maximum acceptable at common utility medium voltage network.
But I don't understand how reducing the start current from 600% to 400% only 2% difference in voltage drop is measured.

RE: Motor Starting

Quote (7anoter4)

But I don't understand how reducing the start current from 600% to 400% only 2% difference in voltage drop is measured.
That's what led me to believe he is at the end of a very long weak line, and/or the substation transformer providing the 12.47 is too small for the load. He did say 10% at the utility bus, which to me means the line side of the 12.47 kV site transformer. Not good.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)

For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies

RE: Motor Starting

(OP)
I am modeling it in ETAP. I cannot lower the inrush current below 400% because then it will cause more than 35% voltage drop at motor terminals which will not let the motor start at all. Yes it is true that reducing the current from 600% to 400% only improves voltage drop by 2%, it looks weird.

RE: Motor Starting

Quote (timm33333)

Motors started at no load.
If a completely unloaded motor is not starting at below 400% current limit, something is wrong with your model. Completely unloaded, a motor should not have any problem starting at 200-250% current limit. TMS programs have a difficult time with soft starters sometimes. I used SKM, it seemed to be OK and it was never wrong if it DID say it would start, but there were a couple of instances where it said it would not, yet we already knew that it did (we just were trying to get a little more out of it). Turned out that the load torque-speed model we were given was flawed. Double check your data, make sure your load torque-speed profile and motor torque-speed curve is not based on some sort of gross over estimation somewhere. If you don't have a load torque-speed profile, don't just make one up from whole cloth, look around for one from a similar machine and tweak that.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)

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RE: Motor Starting

(OP)
Actually I am using the static motor starting mode (no dynamic models selected).

RE: Motor Starting

tim33333,
Have entered the correct inertia data?

RE: Motor Starting

You really need to enter the correct data for the motor and load and use the dynamic motor starting.

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