motor starting study 1

[niulun]

Hi

I'm doing a motor starting for the firs time in my life and I'm using ETAP.

The first problem I have is to choose the best calculation method for my purpose. I'm preparing a document as a preliminary issue so I don't have any kind of specific data of the involved machines.
I think a Static motor staring could be enough but I'm not sure about that (for dynamic motor starting I should use data of similar machines from other projects and I think it could leads me out of scope, that is to give some numbers to Vendors for the bid stage).

Does anyone can help me also to understand what ETAP do during Static motor starting? I know ETAP calculates the actual LRC on the basis of the voltage drop, but how it calculates the voltage drop? I'm quite sure that starting load is involved but I can't understand in which way.

Thanks

 

[DanDel]

For a Static Motor Starting study, ETAP runs a preliminary Load Flow with the existing loads running. You pick a motor to start, and then at the time of start, ETAP runs another LF study using that motor starting current instead of running current. Instantaneous flows and voltage drops are then calculated for each bus. There is no other results for the 'Static' study. It is very useful for determining the 'worst-case' voltage drops in the system quickly.

The Dynamic study models the actual changes over time from motor start(LRA) to normal running (FLA) currents. For this study, detailed motor and load parameters are needed. This is usually not necessary if you just want to see what the instantaneous voltage drops are at the time of starting.

 

[jraef]

Correct, it should be OK at a preliminary stage.
However, the Load Flow study using a Static Motor Starting model will not tell you whether the motor and system dynamic will allow the load to fully accelerate given those parameters, so the usefulness is limited in that aspect. Using a Dynamic Model will allow you to anticipate starting time and current to see if your system can hold up, especially if using any kind of reduced voltage starting method.

For the final bidding stage to vendors, I would use a Dynamic Model based on that typical data you have from other machines, but put a caveat in there that explains this. In a lot of cases where I was working for the vandor, we had to run several models ourselves because of varying motor and machine solutions proposed by the different sub-contractors. It makes it difficult, but not impossible and evey one of them was successful in the end because we took the time to do it right.

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